Review: Annual crop adaptation to abiotic stress on the Canadian prairies: Six case studies

Canadian prairies: Six case studies

Rosalind A. Bueckert and John M. Clarke

Department of Plant Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, Saskatchewan, Canada S7N 5A8 (e-mail: rosalind.bueckert@usask.ca and j.clarke@usask.ca).

Received 25 July 2012, accepted 8 January 2013.

Bueckert, R. A. and Clarke, J. M. 2013. Review: Annual crop adaptation to abiotic stress on the Canadian prairies: Six case studies. Can. J. Plant Sci. 93: 375385. More than half of Canada’s grain crop production comes from the Canadian prairies, a region that experiences short growing seasons characterized by temperature and moisture stress. Historically, the region was dominated by temperate cereal production, but in recent decades crops have included canola (Brassica species) and pulses (chickpea, Cicer arietinum L.; dry bean, Phaseolus vulgaris L.; pea, Pisum sativum L.; lentil, Lens culinaris L.). Here we describe climatic conditions and the resulting abiotic stresses that are common in prairie crop production. We also showcase how specific cultivars have been successfully adapted to fit a short growing season of 95 to 120 d, and examine current strategies to improve crop performance on the Canadian prairies. Durum wheat (Triticum turgidumL. var. durum) production has been increased by incorporating stress escape through early flowering, and stress avoidance through increased seasonal water extraction, water use efficiency and reduced loss from leaves. Dry bean, a warm-season crop, has been improved by selecting for rapid emergence in cool soils. The indeterminate crops chickpea, lentil, and canola (Brassica juncea L.) have been improved through breeding for early flowering, double podding (chickpea), high harvest index, and a longer reproductive duration (lentil and canola). Enhanced drought tolerance in chickpea is in progress using early flowering for drought escape, and rooting traits that improve water extraction and canopy transpiration to avoid water and heat stress. Crops grown on the Canadian prairies have superior quality profiles and two crops, durum and lentil, have become dominant in global exports.

Key words: Canada, prairies, crop adaptation, cultivar, stress, temperature, drought, wheat, pulse, canola Bueckert, R. A. et Clarke, J. M. 2013. Adaptation des cultures annuelles au stress abiotique dans les prairies canadiennes : six e´tudes de cas. Can. J. Plant Sci. 93: 375385. Plus de la moitie´ des ce´re´ales produites au Canada le sont dans les Prairies, re´gion qui connaıˆt une bre`ve pe´riode ve´ge´tative marque´e par le stress thermique et hydrique. Sur le plan historique, les ce´re´ales a` climat tempe´re´ ont toujours domine´ dans la re´gion, cependant depuis quelques de´cennies, s’y sont ajoute´s le canola (espe`ces du genre Brassica) et les le´gumineuses (pois chiche, Cicer arietinum L.; haricot sec, Phaseolus vulgaris L.; pois, Pisum sativum L.; lentille, Lens culinaris L.). Les auteurs de´crivent les conditions climatiques et le stress abiotique re´sultant que subissent couramment les cultures dans les Prairies. Ils montrent aussi comme certains cultivars se sont adapte´s a` une bre`ve saison de croissance, de 95 a` 120 jours, et examinent les strate´gies actuelles visant a` hausser le rendement dans la re´gion. On a accru la production de ble´ dur (Triticum turgidum L. var. durum) en permettant a` cette culture soit d’e´chapper au stress par une floraison haˆtive, soit de l’e´viter par une plus grande extraction de l’eau en saison, un meilleur usage de l’eau et la re´duction de la quantite´ d’eau s’e´vaporant par les feuilles. Le haricot sec, culture de saison chaude, a e´te´ ame´liore´ par se´lection de varie´te´s qui le`vent rapidement dans les sols plus frais. Les cultures a` inflorescence inde´finie que sont le pois chiche, la lentille et le canola (Brassica juncea L.) ont e´te´ ame´liore´es par hybridation  floraison haˆtive, double fructification (pois chiche), indice de re´colte e´leve´, pe´riode de reproduction prolonge´e (lentille et canola). Enfin, on accroıˆt actuellement la tole´rance du pois chiche a` la se´cheresse en recourant a` la floraison haˆtive pour aider la culture a` e´chapper a` ce stress, ainsi qu’a` des caracte`res permettant aux racines de mieux extraire l’eau du sol et au feuillage de moins transpirer pour l’aider a` e´viter le stress thermique et hydrique. Les cultures re´colte´es dans les prairies canadiennes se de´marquent par leur qualite´ supe´rieure, et le ble´ dur et la lentille dominent sur le plan des exportations mondiales.

Mots cle´s: Canada, prairies, adaptation des cultures, cultivar, stress, tempe´rature, se´cheresse, ble´, le´gumineuses, canola

Throughout this country everything is in extremes 

unparalleled cold and excessive heat; long droughts balanced by drenching rain and destructive hail.

Barley generally yielded a fair return; but wheat was almost sure to be destroyed by the early frosts.

Sir George Simpson, Governor-in-Chief of the Hudson’s Bay Company’s territories in North America, 1847.

The cropping region of the Canadian prairies is at the northern edge of the Great Plains of North America, and is classified as a cool continental climate or semi-arid desert steppe. The region has long cold winters where the ground is frozen for at least 5 months with limited snowfall. Increasing summer temperatures with mostly spring and summer rainfall are followed by a return to

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decreasing temperatures with less frequent precipitation in fall (Environment Canada Climate Normals 1971 2000; Phillips 1990). Before European settlement, early explorers, sent out to discover western Canada and find passages to the Pacific coast, were stunned by the immensity of the plains and the extremes of the weather. As a result they concluded that the present cropping regions of Manitoba, Saskatchewan and Alberta were predominantly pastoral and unsuitable for agricultural production (e.g., David Thompson’s Narrative from 1784 to 1812, Glover 1962). Land use has changed with settlement: the Canadian prairies produce more than half of Canada’s grain crops and now dominate global exports in grain crops like durum wheat, lentil and pea. Climate remains a serious force in crop production. Producers have tried and continue to try new crops, and much of their success has been due to crop adaptation to abiotic stress. In this short review paper we look at the key abiotic stresses, temperature and water-deficit stress (drought), and analyze physiological strategies and mechanisms that have been used to adapt annual crops to the prairie climate. We include examples of cultivars that have risen to elite status based on their adaption to a wide geographical area and duration of production. Climate of the Canadian Prairies

Climatic features associated with crop production at a wide range of locations in the Northern Great Plains have been reviewed previously (Campbell et al. 1990; Miller et al. 2002; Padbury et al. 2002). This continental region has large diurnal temperature variations, limited spring and summer precipitation with infrequent rainfall events, and total precipitation in the range of 300 to 450 mm annually (Padbury et al. 2002; Sauchyn 2010). Long-term station records list Edmonton, Alberta, having an annual average temperature of 3.18C, and both Winnipeg, Manitoba, and Regina, Saskatchewan, as 2.28C. The Saskatoon and Swift Current Climate Normals for growing season months illustrate two regions of Saskatchewan in more detail (Table 1). We define the growing season as the frost-free period, as used in Padbury et al. (2002). Saskatoon has fewer than 64 d with rain between May and September, a cumula-tive annual rainfall of 265 mm, a total precipitation of 350 mm, and high evaporative demand from dry air and

wind (the average wind speed is 16 km h1). Swift

Current is situated in a drier windier region (average

wind speed of 20 km h1), it receives 52 d of rain in a

growing season, has an annual rainfall of 261 mm and total precipitation amounting to 350 mm, although surrounding regions have less precipitation.

Across the cropping regions of the Canadian prairies, the soil zones have formed as a result of climate (Padbury et al. 2002). Broadly speaking, the Black and Gray soil zones associated with parkland and the edge of the boreal forest are in regions that are wetter ( 400 mm annual precipitation) and cooler (1400 heat units, 58C base temperature), with about 95 frost free-days

resulting in less crop choice (Zentner et al. 2002). These regions include the Peace River of north central Alberta and north central Saskatchewan. In the Dark Brown, Brown or Dry Brown soil zones of Saskatchewan, cropping regions are drier (around 360 mm annual precipitation), warmer (1700 heat units), with a longer crop season (up to 120 frost-free days in the Brown soil zone), allowing for a greater choice of crops but having an increased risk of erosion. Southern cropping regions in Alberta and Manitoba are warmer with greater amounts of heat units (2300) and the frost free period is longer, typically 125 d. In southern Manitoba regions are also wetter with 450 to 520 mm annual precipitation. The original ecosystem before crop culti-vation was predominantly tall- and short-grass prairie so annual temperate grain crops with short-season habits are best suited to dryland agricultural production. From a cropping perspective, the low amount of precipitation and heat limit the growth and maturation of warm-season crops. All crops are likely to experience cool temperatures and even frost either early in estab-lishment or during late grain-fill. Summer daytime temperatures can exceed 328C in some years and heat stress during flowering and reproductive growth is likely. Water-deficit stress in the Brown soil zones in most years is guaranteed, and in some years flooded and anaerobic soils are problematic in low lying areas of fields or cropping regions on flood plains. Due to the widespread adaptation of zero tillage in the 1980s and 1990s, stubble has modified the impact of soil moisture deficit resulting in greater moisture retention, enabling an increase in crop intensity (one crop a year) over a longer period (more years) of cropping (Zentner et al. 2002). By virtue of soil genesis some regions have mineral deficiencies and toxicities, notable toxicities being salinity in higher pH areas and the heavy metal cadmium.

Crop production has to fit within the narrow confines of the available growing season, usually less than 115 d. Cereal crops and pea are sown early, after snowmelt and when the ground has thawed, in early May or late April. In wetter and cooler regions, canola (Brassica napus) is seeded when there is less risk of frost, and in warmer drier regions pea is displaced by lentil. Crops requiring warm soils [soybean (Glycine max (L.) Merr.], dry bean, maize (Zea mays L.) are seeded in May, and even as late as early June. Delaying seeding into late May and early June places crop production at risk of delayed maturity or premature death (prior to physiological maturity) from frost. Most crops are harvested in late August and September, depending on how hot and dry the summer has been. In early to mid September, any remaining crop growth is curtailed by the onset of fall frosts. Additional information on crop diversity tailored to specific re-gions, soil zones, and water availability can be found in Miller et al. (2002), Johnston et al. (2002) and Zentner et al. (2002).

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CROP ADAPTATION AND ABIOTIC STRESS Crop research over the past 115 yr has improved the diversity, yield and quality of crops grown in Canada. The history of most crops currently grown follows the pattern:

1. Production of a newly introduced cultivar.

2. Evaluation of the cultivar’s adaptation constraints.

3. Discovery of genetic diversity to overcome the

constraint.

4. Breeding of adapted cultivars and the development

of agronomic systems.

This pattern illustrates that crops recently moved into a new region are not always satisfactorily adapted, and their production, yield and market quality can be limited from a scale of slight reduction to total crop loss. The cultivars or accessions that are tested when a new crop is introduced to a new agroclimatic region are not necessarily adapted to these specific conditions. Because of this, the initial performance of a new crop can range from promising to disastrous, and may determine its acceptance or rejection by producers in this area. In the event that the new crop appears promising, constraints to full adaption are identified and options examined to overcome them. The fastest and least expensive solution to overcome a constraint is by conventional crop breeding, which involves searching through germplasm accessions of the crop species originating from other regions. The aim is to find genotypes with superior performance under this constraint, and then use this germplasm as parents to breed a new and better adapted cultivar. As a crop achieves a minimum level of resistance to abiotic stress, the agronomic system can be developed as a result of crop improvement or concomitantly with crop improvement. Finally, the result of successful and stable crop production is based on elite cultivar characteristics as well as sophisticated

agronomic practices that are tailored to a crop species. Elite cultivar characteristics not only ensure crop survival and stable yield production, but they include superior quality traits that ensure the crop is desirable and marketable for both producers and consumers.

In cases where crops are poorly adapted to a new region with known climatic constraints, the crop fails to fit the environment by developing too slowly, or too rapidly, or it yields poorly. Four stresses associated with temperature and water supply and their effects on crops are summarized in Table 2. All stresses will lower yield in a crop by reducing seed number per unit area, and often they reduce seed size. On the Canadian prairies and other dry regions drought and heat have one positive benefit, producing high protein grain, which results in market premiums in wheat for bread making. Plants have a range of lifecycle and physiological strategies to cope with stress as part of the natural adaptation process. These have been artificially but usefully categorized based on ecological principles that date back to the 1970s (Levitt 1972), referred to as resistance and avoidance (Larcher 1975; Arnon 1992). Crops can escape from stress, or they can resist it by avoiding or tolerating it. For example, in a hot summer, the escape mechanism would be early flowering, so the crop would flower and set seed before the onset of the stress, or it could be grown in a different and cooler region. In resistance by avoidance, the crop would have some physiological mechanism whereby it could avoid the stress. In resistance by tolerance, the crop could exist or survive under extreme stress because it has several morphological or physiological mechanisms to cope with the stress. In tolerance, the mechanisms are expensive metabolically and reduce yield under optimal conditions.

These ideas have been better developed for water-deficit stress resistance by Arnon (1992), and we have included a range of crop and plant traits in Fig. 1.

Table 1. Climate features of the crop season for two locations in Saskatchewan, Canada, using data from the Climate Normals (1971



2000), Environment Canada

Month of the cropping season

Climate parameter May June July August September

Saskatoon (lat. 52810.2?N, long. 106843.2?W, 504 m elevation)

Nights with frost ( B18C) 5.2 0.1 0 0.1 4.5

Days with temperatures308C 0.8 2 3.3 4.4 0.9

Mean daily minimum temperature (8C) 4.5 9.4 11.4 10.2 4.4

Mean daily maximum temperature (8C) 18.4 22.6 24.9 24.4 18

Monthly precipitation (mm) 50 61 60 40 31

Days with precipitation events 4 mm 2.9 3.5 3.6 2.5 2.1

Swift Current (lat. 50816.2?N, long. 107843.8?W, 825 m elevation)

Nights with frost ( B18C) 5.4 0.2 0 0.1 3.7

Days with temperatures308C 0.5 1.7 4.3 5.6 1.4

Mean daily minimum temperature (8C) 4.5 9.2 11.3 10.8 5.3

Mean daily maximum temperature (8C) 17.6 22 24.8 25 18.3

Monthly precipitation (mm) 44 66 52 40 28

Days with precipitation events 4 mm 2.9 4.5 3.1 2.2 1.9

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The most effective strategy for a crop subjected to water-deficit stress is to escape the stress, as in the left-hand upper box, emphasized by the oval ring. Typically, crops escape stress with a shortened vegetative period, early flowering and early maturity. However, the short life cycle results in reduced yield potential when compared with later-flowering crops in a less-stressful environment. The next best strategy is for the crop to resist the stress by avoidance. In avoidance, the crop maintains a favourable water balance by either water conservation or increasing water uptake. For a given amount of water, mechanisms that conserve water reduce the amount a crop uses, or they limit the amount a crop loses per unit time, or they maximize the amount of growth per unit of water so the crop is efficient. The mechanisms that increase seasonal water uptake differ in that they enable a plant to extract more water than

before, or they force the plant to ration its water uptake so it can take up water for longer and at greater depth during the season, or they enable the plant to operate for longer when it contains less water (Passioura 1983, 2006). Many crops can have both avoidance mechan-isms at play (i.e., conserving water and improving water uptake), and, generally, at moderate stress levels, con-servation works well with least reduction in growth and yield. As stress becomes increasingly severe, the positive impact of traits imparting superior water uptake under restricted supply is progressively reduced. The crop will have continued growth and function but efficiency of growth and yield is lowered. Breeding for drought resistance is not an easy task, and success often rests on multiple trait selection (Clarke 1987; Richards 2006) under carefully chosen stress levels (Clarke et al. 1992). Native plant species that have evolved to fit niches with

Adaptations to dry growing conditions

ESCAPING DROUGHT Ephemerals Early-maturing cultivars Early flowering DROUGHT RESISTANCE AVOIDING STRESS Maintaining favorable Water balance TOLERATING STRESS Tolerating desiccation CONSERVING WATER Water savers Stomatal closure Inc’d photos. efficiency Inc’d transpiration efficiency Low cuticular transpiration Leaf wax, hair

Small narrow leaves Efficient water use (WUE) Water growth into yield (HI)

IMPROVING WATER UPTAKE

Water spenders, rationing Efficient root systems Greater water use (WU) Increased osmotic potential

LOW YIELD SURVIVAL STRATEGY

Fig. 1. Crop adaptations to dry growing conditions (drought), modified from ideas and Fig. 6.1 of Arnon (1992), and featuring traits from Clarke (1987), Richards (2006) and Passioura (2006). Inc’d photos. efficiency is increased photosynthetic efficiency, WUE is water use efficiency, WU is water used by the crop, and HI is harvest index (ratio of grain or harvested yield to standing biomass). Table 2. Effects of four stresses on crop growth and yield

Stress Crop establishment Crop life cycle Yield formation Market quality

Cold Delayed Delayed Reproductive organ abortion

Less and smaller seed

Immature seed

Heat Advanced Advanced Reproductive organ abortion

Less and smaller seed

High protein seed,

Lower carbohydrate

Drought Delayed or poor Advanced Reproductive organ abortion

Less and smaller seed

High protein seed,

Lower carbohydrate

Too wet Delayed or poor Advanced, or delayed with late-season stress

Reproductive organ abortion

Less and smaller seed

Reduced seed vigor due to premature sprouting

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extreme stress have a range of mechanisms suited to low productivity and survival, but these strategies carry an increased metabolic cost, low yield, and have limited use in main stream crop production. Examples include high root to shoot ratios, Crassulacian acid metabolism photosynthesis in bromeliads, periodic flowering in a decade, reducing leaves to thorns, and stem succulence (Arnon 1992; Larcher 1975).

The same stress categorization can be used for temperature stress (Fig. 2). Escape from adverse tem-perature includes a short life cycle (the ephemeral trait). Crops that flower early can escape most of the seasonal heat stress, early maturity can help them escape frost at the end of the growing season, or they can have seed dormancy mechanisms to delay germination in cool or hot soils. In temperature stress avoidance, crops can maintain favourable temperature with a wealth of mechanisms designed to reduce temperature extremes, reflect excess heat, insulate organs, or guard sensitive tissue from harsh temperature (Larcher 1975; Arnon 1992; Hall 1992). With prolonged or repeated exposure to adverse temperatures, crops can be adapted to stress by improving their growth at these more marginal

temperatures. Mechanisms include indeterminate

growth habits with a long duration of flowering, pollen viability and dehiscence synchronized with stigma receptivity for successful fertilization in reproduction, maintenance of transpirational cooling, increased radia-tion reflecradia-tion through leaf orientaradia-tion and leaf mor-phology, alternate metabolic pathways, cell protectants and increased membrane stability (Hall 1992, 2004; Singh et al. 2007; Wahid et al. 2007). Tolerance to

extreme temperatures requires more extensive trait adaptation, and such species rely on secondary and deep dormancy, highly specialized metabolism, specia-lized organ structure, and unusual life-cycle adaptations (Larcher 1975; Arnon 1992). Like drought tolerance, many of these high temperature tolerance mechanisms are more suited to native plant survival than crop production.

On the Canadian prairies, various abiotic stresses are likely to occur every growing season (Table 2). Cold stress is possible when plants are germinating and emerging in spring, when early-flowering crops (winter wheat) coincide with a late spring frost, or it can occur close to crop maturity with an early fall frost. To withstand cold stress, crops require the ability to emerge in cool soil and air temperatures, and have early or timely maturity. Winter wheat and perennial crops additionally require the ability to survive winter when soils are frozen for many months. Heat stress is possible mid-season, from flowering to the middle of grain filling. Crops with first flowering dates that are early enough can avoid the main onset of heat, and improved pollination and fruit retention are additional ways to reduce losses to heat stress (Hall 2004; Salem et al. 2007). Water-deficit stress is possible at any time in the growing season, but is more likely after flowering. Trait selection for drought resistance is difficult because water-deficit response is complex and no single trait confers superior resistance (Clarke 1987). Stress resis-tance traits include early flowering, along with moderate to high harvest index, indeterminate habits for addi-tional yield formation after stress periods, root systems

Adaptations to adverse temperature conditions

ESCAPING HEAT/COLD Ephemerals Seed dormancy Early-maturing cultivars Early flowering TEMPERATURE RESISTANCE AVOIDING STRESS Maintaining favorable growth & development

TOLERATING STRESS Secondary / deep dormancy CAM metabolism

Organ succulence

CONSERVING TEMPERATURE BALANCE

Leaf & stem hair Wax and cuticle Protected meristems

Stomatal opening (transpirational cooling) Volatile metabolites (for cooling) Anthocyanin and osmoprotectants Leaf rolling, flipping (radiation reflection) Metabolic cycling, energy dissipation

IMPROVING GROWTH IN MARGINAL TEMPERATURES Indeterminate habit

Long duration of flowering

Pollen viability, synchrony with stigma Flavonoid metabolism

Alternate energy metabolism Membrane stability/fluidity Cell protectants(small molecules) Supercooling, freezing point depression

LOW YIELD SURVIVAL

Fig. 2. Crop adaptation to adverse temperature conditions (heat and cold), modified from Fig. 130 of Larcher (1975) and featuring traits from Hall (1992, 2004), Singh et al. (2007) and Wahid et al. (2007).

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that can optimally use the available soil moisture, glaucous (waxy) leaves for reduced leaf water loss, delayed wilting and high water use efficiency. In the next section we will examine the strategies that were used to improve the adaptation of six annual crops to abiotic stress: water-deficit stress and cool temperature.

SUCCESS



SIX CASES OF IMPROVED

ADAPTATION

1. Strongfield Durum and Drought Tolerance Strongfield is an example of an elite durum wheat cultivar that is well adapted to the dry areas of the prairies; the Brown and Dry Brown soil zones. These areas were first categorized by explorers as a desert unfit for agriculture. Strongfield has superior performance in water stress and heat, and its grain quality has met strict protein and quality criteria (Clarke et al. 2005, 2006). Saskatchewan grows 80% of Canada’s durum, and most is exported with such elite cultivars commanding about 50% of global durum trade. Since 1970, durum yield has improved at a rate of 1.4% per year over the cultivar Hercules, registered in 1969 (Clarke et al. 2010). Half of that improvement was through plant breeding and the other half was through crop management. The ideotype for spring durum on the Canadian prairies is a plant with a main stem plus two tillers, usually long day sensitive (but day neutral is possible), 90 to 105 d to maturity with about 55 d of vegetative growth (to heading), and from 35 to 50 d of reproductive growth depending on water availability. Measured traits for water-deficit stress include leaf wax, leaf size and angle, and delayed wilting measured by excised leaf water retention (Clarke and McCaig 1982; Clarke 1987). For registration, grain quality measurements include high protein, gluten strength, yellow pigment, and low cadmium concentration. The high protein (14.9%) results in yield drag. The low cadmium allele reduces grain cadmium concentration to 50% of the checks, in

the order of 0.011mg g1for marketability to countries

with low heavy metal thresholds in grain (Clarke et al. 1997). The cultivar has high yield and high harvest index (35%) for its environment (Wang et al. 2009).

With the shift to zero tillage, one could argue that seasonal water availability has increased slightly in these regions. However, Strongfield’s parents have only slightly more water extraction at depth, therefore these cultivars have succeeded through improved water use efficiency (Wang et al. 2007). Strongfield will also likely

have high water use efficiency. Strongfield’s parents are AC Avonlea and DT665. DT665 has the parents Kyle and Nile, Nile coming from ICARDA. The grain yield of Strongfield is greater than Kyle and AC Avonlea (Clarke et al. 2005). In Table 3 the parents Kyle and AC Avonlea have increased water extraction over Marquis (the first adapted and extensively grown wheat cultivar on the prairies), and specifically higher water use efficiency (Wang et al. 2007).

2. Improving Dry Bean Emergence

Dry bean is a warm season crop. The literature reports its base temperature as 128C (Otubo et al. 1996), and as low as 88C in a few genotypes (Dickson 1971; White and Montes 1993; Zaiter et al. 1994). Generally, germination proceeds at temperatures of 98C or higher (Hucl 1993; Nleya et al. 2005). On the prairies, dry bean is restricted to areas with longer growing seasons because the crop is usually late seeded (June) when soil has sufficiently warmed. The main problem with delayed seeding is that the growing season becomes too short for a profitable crop because maturation coincides with declining fall temperature (Balasubramanian et al. 2004). One goal to improve adaptation to the constraints of a cool climate was to increase germination and emergence of dry bean in cool soil so this crop can be grown earlier and in wider areas.

A subset of genotypes from Balasubramanian et al. (2004) was chosen for a cool temperature tolerance study under controlled conditions in petri dishes, and observations compared with stand emergence in soil (Nleya et al. 2005). Although germination was severely limited, two genotypes, Isstrot (G8823), Sanilac (G9345) germinated at the lowest temperature, 108C day and 68C night, suggesting that the genotypes identified from a petri-dish test would have the best cool temperature tolerance (Table 4). At 108C day and 88C night, four genotypes were able to show a level of germination, these being Isstrot, Sanilac, CDC Nighthawk and AC Polaris. Of this group, Isstrot and AC Polaris had

significantly higher germination. When

fungicide-treated genotypes were sown in pots of soil having soil moisture similar to the field, the two genotypes with superior emergence were two selected in Canada, CDC Polar Bear and CDC Nighthawk. Despite differences in temperature buffering capacity between petri dishes and soil, the best two genotypes that emerged in cool soil were not the best two that germinated in petri

Table 3. Features of water use of two elite durum cultivars (Kyle, AC Avonlea) compared with an earlier durum (Hercules) and Marquis spring wheat. Data from Wang et al. (2007)

Cultivar Year of release Yield t ha1 Water use (mm) Water use efficiency (kg yield ha1mm1)

Marquis 1907 2.80 380 7.4

Hercules 1969 3.45 386 8.9

Kyle 1984 3.58 392 9.1

AC Avonlea 1997 3.91 387 10.1

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dishes incubated at low temperature. CDC Nighthawk and other black bean genotypes were identified by Balasubramanien et al. (2004) as having cold tempera-ture tolerance in field nurseries located on undulating land (Rosthern: lat. 52854?N, long. 106820?W, Black soil zone), which has greater risk of frost exposure. The fact that Isstrot and Sanilac were respectively developed under cool wet northern soils in the United States of America and the Netherlands may help explain this discrepancy. On the other hand, AC Polaris was selected in Alberta (Mundel et al. 2001). Therefore, what appears to be the cooler temperature tolerance for emergence of CDC Polar Bear and CDC Nighthawk is more likely attributable to superior growth during the warm part of the day, not a lowering of the base temperature per se for germination and growth. Lowering a plant’s base temperature to avoid cold stress appears to be difficult to achieve, and selection for rapid germination above the base temperature may be an easier strategy. 3. Enhancing Earlier Chickpea Maturity

Chickpea is a recent crop for the Canadian prairies, and to date all introductions and cultivars are highly indeterminate. Although a cool season crop, chickpea prefers warmer regions and is grown in the Brown soil zones. Its indeterminate habit leads to delayed maturity and the likelihood of yield loss and low quality problems from early fall frost. The goal is to better adapt chickpea and its yield performance through early flowering and earlier maturity. Three traits were assessed to achieve this goal (Anbessa et al. 2007): double podding, early flowering, and short internode length. Double podding is two flowers per node or leaf, as opposed to the default of one (Srinivasan et al. 2006), and the short internode trait could be associated with a bushy plant habit. Bushy plant habits are associated with determinacy and earlier maturity in soybean (Kilgore-Norquest and Sneller 2000), and double podding is more likely to compete with vegetative growth because a single leaf is the main source of assimilate for its subtended pod/s (Li et al. 2006). Double podding was associated with 1 wk earlier crop maturity compared with single podded genotypes (Anbessa et al. 2007). Early flowering, controlled by two major genes and polygenes (Anbessa et al. 2007), resulted in 10 d earlier maturity. The short internode

trait produced very short plants with diminutive leaves and was unsuccessful in achieving satisfactory earliness and yield. Double podding plus early flowering together worked by a strategy of escaping late-season conditions that exacerbate continual vegetative growth, plus it produced high harvest index, although true determinacy was not achieved.

Chickpea and Drought Tolerance

Despite an extensive root system (Kashiwagi et al. 2005), chickpea always yields less under water-deficit stress compared with moisture sufficient soils (Leport et al. 1999; Gaur et al. 2008). Due to its rooting ability, chickpea is often touted as drought tolerant and a crop of choice in regions that experience terminal water-deficit stress (Kashiwagi et al. 2006). Chickpea landraces originating from arid areas exhibit large root length densities as a mechanism to extract maximum soil water (Kashiwagi et al. 2005). In a joint project with ICARDA, the Crop Development Center at University of Saskatchewan (CDC) assessed a subset of germplasm that ICARDA had previously identified as having improved drought tolerance in the field (Stoddard et al. 2006). Rehman et al. (2011) measured a set of recombinant inbred lines derived from the cross of drought tolerant ILC 588 and drought susceptible ILC 3279. Genotypes with higher yield under terminal water-deficit stress had early flowering, early maturity, high harvest index, higher stomatal conductance and main-tained cooler crop canopies in the field. The strategy of drought tolerance was both escape and resistance by avoidance. Genotypes having the deeper root systems had access to more water during early reproductive growth, and the crop also had improved water use efficiency and harvest index. Chickpea is a water-spender in that it maintained stomatal conductance in a heat stress to cool its canopy. Therefore, a combina-tion of a vegetative phase of moderate length coupled to a shorter reproductive phase, similar to Strongfield durum, allowed high water use efficiency and har-vest index (Rehman 2011). In the highly stressed environments of the Brown soil zone where heat and water-deficit stress started early in mid-vegetative growth, we have seen chickpea harvest index plummet to B20% in 2002 and 2003, very severe drought years

Table 4. Maximum germination (%) and emergence (%) of four bean genotypes in controlled temperature regimes, adapted from Nleya et al. (2005). Growth chamber temperature regimes were day temperature for 8 h/night temperature for 16 h

Regime Germination in petri dishes Emergence from soil

Genotype 228C/168C 168C/108C 128C/128C 108C/88C 108C/68C 228C/168C 168C/108C

Isstrot 100a 90a 15bc 7a 3a 82cd 59e

Sanilac 100a 85a 23b 5ab 2ab 91ab 77cd

CDC Polar Bear 97a 42b 2c 0b 0b 88bc 82ab

CDC Nighthawk 97a 27c 23b 5ab 0b 93ab 92a

AC Polaris 98a 85a 53a 7a 0b 80d 72cd

ae Means within a column followed by the same letter do not differ at P B0.05.

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(Vanderpuye 2010). Germplasm from the ICARDA drought project has been incorporated into the breeding program at CDC, and cultivar development is also in progress for Middle East regions. Although drought tolerance of chickpea is being improved, the major barrier to its wider cultivation on the Canadian prairies is ascochyta blight, a fungal disease that thrives in the cooler climatic conditions of the region.

5. Adapting Oilseeds

Oilseed crop production has expanded since the 1960s with the advent of the spring habit of oilseed rape (Brassica species), and the shifting of B. napus and B.

rapaoil profiles to become an edible food crop, canola,

through lower erucic acid and glucosinolates. Canola production area has increased tremendously in the last decade due to higher prices generated from increased global demand, its incorporation into biofuels, and the widespread adoption of hybrids and agronomic traits (herbicide resistance). Canola now includes the species

Brassica juncea (Woods et al. 1991), and improvement

of this crop illustrates adaptation to fit the Canadian prairies’ climate.

Yields of the spring habits of Brassica crops rank from high to low for B. napus canola, B. rapa canola, and B. juncea canola, respectively (Gan et al. 2007).

Brassica napuscanola has a longer lifecycle (96 d) than

B. rapa canola by 14 d, making B. napus canola the

higher-yielding and more widely produced crop. While

B. junceahas evolved in warmer drier regions, the short

life cycle of early and poorly adapted cultivars reduced yield potential and limited wider uptake of the crop. The rapid improvement of B. juncea canola’s flowering date and flowering pattern has brought B. juncea canola to the yield range of B. napus hybrids. Brassica juncea canola now has a longer life cycle of 95 d, being longer than B. rapa by 13 d through breeding advances (Gan et al. 2007). Brassica juncea canola is suited to the dry soil zones because it flowers 4 d earlier than B. napus canola (B. napus flowers at about 49 d), which allows for a longer flowering duration. Brassica napus canola has a flowering duration of about 23 d, B. rapa canola and B.

juncea mustard (juncea that has not been improved as

canola) both flower for 26 d, and B. juncea canola flowers for 30 d. The earlier flowering date (Kirkland and Johnson 2000) permits avoidance of water-deficit stress and heat, and the longer flowering duration enables the crop to set successive flushes of flowers, an ‘‘indeterminate’’ type risk strategy that a crop uses to abort flowers in adverse weather and set yield when weather conditions improve (e.g., chickpea; Berger et al. 2006). Brassica napus canola still has higher yield potential across a diverse set of environments, and is more uniform in flowering duration and maturity (Gan et al. 2007).

Adapting new B. juncea canola cultivars by lengthen-ing flowerlengthen-ing duration compared with B. juncea mustard has improved yield, along with maintained and perhaps

greater drought tolerance via its matched phenology to water availability. In crop physiology, ‘‘phenology’’ describes the response of vegetative and reproductive growth to environmental conditions via the number of vegetative nodes produced by the plant, the flowering transition, and the number of reproductive nodes. Early flowering limits vegetative growth, enables reproductive growth to occur before terminal stress, and usually correlates with early maturity (Serraj et al. 2004). Early flowering/early maturity is a drought escape mechanism, but can limit yield in more favourable growing condi-tions. Indeterminate crops have a long flowering dura-tion and the ability to capitalize on any favourable weather conditions during a longer period of reproduc-tive growth (Berger et al. 2006). Surprisingly, actual heat tolerance of yield formation in B. juncea remains similar to other B. rapa and B. napus oil crops at 29.58C (Morrison and Stewart 2002; Gan et al. 2004), so its better performance compared with the other canola crops in warm areas is via phenology, specifically the timing of its reproductive duration to climatic condi-tions. Harvest index has not greatly improved, being in the range of 31 to 17% (Angadi et al. 2000; Gan et al. 2007) compared with less-advanced cultivars in the late 1970s (31 to 24%; Degenhardt and Kondra 1981). Further adaptation of B. juncea canola is therefore needed to improve yield stability over diverse environ-ments, along with greater yield to compete with the highest-yielding canola hybrid cultivars.

6. Lentil Maturity and Agronomy

Canada is the largest global exporter of lentil, with the majority of Canadian production located in Saskatch-ewan. The major climate constraints are a short growing season and cool, wet end-of-season conditions, which can delay maturity in this crop. Lentil is an indetermi-nate crop, with no known determiindetermi-nate gene. Breeding goals have been first to use early flowering to shorten the lentil lifecycle in small- and large-seeded market classes, and then to select for slightly longer reproduc-tive duration and higher yield to better fit cultivars to production areas. Other goals have been to add agronomic traits for crop management, and to enhance grain quality traits for overseas markets. Since the introduction of early cultivars like Laird in 1978 from the large green-seeded market class (Slinkard and Bhatty 1979), earlier flowering (Tullu et al. 2008) and small-seeded types (Slinkard and Bhatty 1981) have been selected for improved agronomic production and deliberate targeting of global markets. The goal to achieve early flowering and early maturity ( B100 d) was first associated with lower yield. The large green market class has high yield potential but very late maturity of at least 110 d, compared with small-seeded types maturing in B110 d (Saskatchewan Ministry of Agriculture 2012; Vandenberg et al. 2001, 2006). The long lifecycle of lentil restricted lentil production to the Dry Brown, Brown and Dark Brown soil zones where

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annual precipitation is less and terminal water stress more likely. In Saskatchewan adapted lentil crops start flowering in late June, with a vegetative phase of 48 to 54 d in yield trials (Saskatchewan Ministry of Agricul-ture 2012), which is just under halfway through the growing season. The CDC has released cultivars more recently that show a shift to medium (105 d) and medium late (105 to 110 d) maturity for higher yield potential via a slightly longer reproductive duration, along with an improvement in yield for small-seeded lentil (Hanlan et al. 2006; Vandenberg et al. 2006; Saskatchewan Ministry of Agriculture 2012).

In the past decade, large green-seeded market class cultivars have commanded a greater production area, with a lesser area of small red-seeded lentil, despite red lentil selling at higher prices. The 2010 to 2011 area of production had a greater area of small-seeded lentil compared with the larger green-seeded lentil due to a change in cultivar choice and an expansion of the growing area into wetter soil zones. Earliness is a very attractive crop feature for growers in the Dark Brown and Black soil zones, where increased late season moisture can unduly delay crop maturity. Incorporation of non-transgenic herbicide tolerance into cultivars in the small red-seeded market class (cultivars similar to or better than CDC Blaze), as well as other market classes, has further contributed to the expansion of production (Slinkard et al. 2007; Muehlbauer et al. 2009). Lentil is a small stature crop and weed control was previously limited by few registered herbicides, poor competitive-ness against weeds, and lentil’s sensitivity to herbicide residues from the previous rotation. Therefore, expan-sion of lentil production is being driven by a suitably early and less risky life-cycle with higher yield in the Dark Brown and Black soil zones, or a slightly longer lifecycle with higher yield for drier soil zones, plus the additional benefit of sophisticated agronomy. In addi-tion, the lentil breeding program in the CDC has a strong emphasis on visual and nutritional seed char-acteristics (e.g., Thavarajah et al. 2009; Vandenberg 2009; Ubayasena 2011), quality features that set the crop apart in a highly desirable category for production and global trade.

CONCLUSIONS

Adaptation of crop cultivars for abiotic stresses com-mon to the Canadian prairies requires the refinement of flowering date to limit the life cycle length to a short-season cultivar. The best matched flowering date also permits a crop to escape the worst of water-deficit and heat stress. Stress resistance by avoidance of moderate stress is more important for yield than traits that are related to stress tolerance. But the strategy of early flowering and early maturity can limit the growth and productivity of cultivars, particularly when they are grown in regions or years with slightly longer growing seasons. Water-deficit stress remains the major limita-tion of crop produclimita-tion, and a vegetative phase that is

just over 50% of the growing season fits elite cultivars of durum, and just under 50% of the growing season for indeterminate crops like lentil and canola. This enables sufficient vegetative growth and the filling of yield with a moderate to good harvest index for production on less than 400 mm of water. Elite durum cultivars also have improved water extraction and high water use efficiency by selecting for delayed wilting, wax and size of leaves. Yield and stress tolerance of juncea canola has been improved by early flowering and a longer flowering duration so it can set yield over a longer period in its lifecycle. Limiting the life cycle in indeterminate crops in some years remains a major breeding goal to improve yield in the absence of determinate growth-habit genes, but some success has been achieved through early flowering and focusing on reproductive duration through double podding (chickpea) and high harvest index (lentil). Wider adaptation of warm-season crops, unless they are very short-season, remains a challenge because the Canadian prairies have limited seasonal heat units and shifting the crop base temperature is difficult to achieve. In dry bean, emergence has been improved by selecting for rapid germination in cool soil. Elite cultivars in two crops, durum and lentil, have greatly contributed to production volumes because they have been adapted through careful attention to life-cycle, yield, end use quality and agronomic management. Similar strategies are used in oilseeds and other pulse crops.

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