Tree Growth and Development

Light

In an orchard, light distribution within and between tree canopies can have a profound effect on growth and development of the fruit. We have previ-ously discussed the effect of light on photosynthesis and defi ned the opti-mum light levels required for mango leaves. When light levels fall below the

threshold required for light saturation of photosynthesis, the subsequent reduction in available photoassimilates will affect growth of the tree. In many tree fruit crops, fl ower-bud induction, fruit size and fruit colour are reduced when low light levels occur due to crowding within and between tree cano-pies (Jackson, 1980; Flore, 1994; Whiley and Schaffer, 1994). There is no pub-lished information on the effect of light levels on mango fruit size, although fruit are photosynthetically active and a reduction in size under low Q could be expected.

Fruit skin colour is an important feature of mango with fruit of many cultivars developing attractive pink to red coloration. Fruit colour is geneti-cally determined and the reddish blush is generally more developed in monoembryonic cultivars, while fruit from most polyembryonic cultivars remain green/yellow at maturity. Skin coloration of mature fruit is partly due to anthocyanins which develop when tissues are exposed to light. While this subject is well researched in other fruit crops (Proctor and Creasey, 1971), light levels required for skin coloration of mango fruit have not been quanti-fi ed. Studies in Australia with the polyembryonic cultivar ‘Kensington’, which develops a blush only on the exposed side of the fruit, indicated that the position of fruit on trees had a signifi cant effect on the development of colour due to differences in the penetration of light into the canopy during fruit ontogeny (Schaffer et al., 1994). The intensity of redness was greatest on fruit from the eastern side of the tree followed by fruit from the south-western

100

80

60

40

PLC

20

0

–4 –3 –2

Xylem pressure (MPa)

–1 0

2006 light 2006 shade 2005

Fig. 6.7. Vulnerability of xylem of 13-year-old ‘Cogshall’ mango trees to cavitation in twig segments from sun-exposed (light) or shaded sides of the tree. Cavitation is expressed as percentage loss of conductivity (PLC) with decreasing xylem water potential. Symbols represent means and error bars represent one standard error.

Vulnerability curves were obtained with the centrifuge technique (Source: Cochard et al, 2005; and from H. Cochard, unpublished data, with permission).

and northern sides of the tree. This information establishes an important con-cept with respect to the light regime but does not quantify the absolute light levels required for anthocyanin development. Further research is necessary to establish physiological parameters from which pruning and orchard man-agement strategies can be developed.

Temperature

Mango is a predominantly tropical species although the tree will usually grow and produce more successfully in frost-free subtropical latitudes with a marked dry season and high heat accumulation. Under optimum tempera-tures with non-limiting nutrients and water, the tree remains vegetative with growth fl ushes occurring at regular intervals. The large size and poor crop-ping of trees in the humid lowland tropics are well known, and there is a direct relationship between temperature and the frequency of vegetative fl ushes. Trees grown at 20°C days/15°C nights (20/15°C) required 20 weeks (mean of ten cultivars) to complete a growth/dormancy cycle while at 30/25°C the same cycle was completed in 6 weeks (Whiley et al., 1989). There are marked differences between cultivars with respect to their tendency towards vegetative growth. For instance, in controlled temperature studies over a 20-week period, at 30/25°C, ‘Irwin’ produced 2.0 growth fl ushes with approximately 45 days of dormancy between active growth periods while

‘Kensington’ produced 4.7 growth fl ushes with only 5 days of quiescence between fl ushes (Whiley et al., 1989). Dry matter accumulation over the 20 weeks was similar for the two cultivars; however, starch accumulation in the woody trunk tissues of ‘Irwin’ and ‘Kensington’ was 13 and 3.6% of dry mat-ter, respectively. The response differences between these two cultivars may be the contributing factor to their performance at tropical latitudes where temperature is non-limiting for growth. Under these conditions ‘Irwin’ has more reliable cropping than ‘Kensington’, suggesting that the genetically determined low-vigour trait is more sensitive to environmentally precipi-tated stresses that induce fl owering.

The number and size of leaves which develop on each growth fl ush are also infl uenced by temperature. Whiley et al. (1989) reported that on trees growing at 20/15°C an average of 7.1 leaves per fl ush were produced while at 30/25°C there were 13.6 leaves on each growth fl ush (data are mean values from ten cultivars). At 30/25°C the mean leaf size was 300% greater than those on trees growing at 20/15°C. Soil temperatures have also been reported to have a strong effect on the growth of mangoes. In studies with ‘Irwin’

grafted on ‘Turpentine’ rootstocks, episodic shoot growth occurred when soil temperatures were held at 27°C or 32°C for 120 days but an extended dormant period developed when soil temperatures were held at 21°C (Yusof et al., 1969). These results indicate that environmental control over shoot growth of mangoes may in part be related to soil temperatures.

From controlled temperature studies it has been calculated that the median daily temperature (mean of the maximum and minimum daily temperatures)

at which shoot growth ceases is approximately 15°C (mean value for ten cul-tivars) (Whiley et al., 1989). Subsequent studies (Issarakraisila et al., 1991) have confi rmed that 15°C is the critical minimum growth temperature for shoots of ‘Kensington’.

Stress-inducing temperatures which prevent shoot growth have been shown to promote fl oral induction in mangoes, but this is outside the scope of this discussion. For further information of the effects of temperature on pollination, fl oral initiation and fruit development, see Davenport, Chapter 5, this volume and Schaffer et al. (1994). We again emphasize that although mango is a ‘heat-loving’ crop well adapted to the hot, semi-arid subtropics and monsoonal tropics, in these environments it experiences extremes of heat, drought and evaporative demand that may cumulatively reduce potential production capacity.

Drought

Although mango is considered to be drought tolerant and may survive with-out rain or irrigation for > 8 months (Gandhi, 1955), water defi cits during the reproductive cycle can have severe effects on the retention and early growth of mango fruit. In studies with bearing, container-grown ‘Irwin’ trees, pre-dawn <_l levels were maintained at either less than –0.3 MPa (non-stressed) or –1.2 MPa (water stressed) for the fi rst 2 months after fruit set. For the fi rst 5 days following fruit set, all trees lost a similar percentage of fruit, but there-after fruit abscission was greater on water-stressed trees. After 1 month, drought-stressed trees had retained approximately 4% of their initial fruit set compared with approximately 8% on non-stressed trees. During the fi rst 30 days following fruit set, the rate of fruit growth for non-stressed trees was twice that of drought-stressed trees, and fi nal fruit size (measured 60 days after fruit set) of non-stressed trees was 20% greater than on water-stressed trees. In a separate study, another group of ‘Irwin’ trees was maintained stress-free (pre-dawn water potential above –0.3 MPa) during the fi rst month following fruit set with water-stress (–1.2 MPa) imposed on some trees dur-ing the second month of fruit development (Pongsomboon, 1991). There was no effect of drought on fruit retention but fi nal fruit size was 34% smaller for stressed compared with non-stressed trees.

In fi eld studies, Singh and Arora (1965) compared fruit drop of monoem-bryonic ‘Dashehari’ trees irrigated at 1-week intervals with trees irrigated at 3-week intervals. During the fi rst 6 weeks of fruit growth, weekly irrigation reduced fruit drop compared with the irrigation at 3-week intervals. During the latter stages of fruit development, these gains were lost as more fruit dropped from the weekly irrigated trees. In another study, fi eld-grown monoembryonic ‘Tommy Atkins’ trees were managed under different irriga-tion regimes from early fruit set until the start of the rainy season (approxi-mately 43 days) (Larson et al., 1989). Trees were irrigated on a 7- or 14-day schedule or received no irrigation. Pre-dawn <_l was –0.3 MPa for trees irri-gated on the 7-day schedule and decreased to –0.5 MPa for the non-irriirri-gated

trees. Irrigation at 7-day intervals resulted in the greatest yield with the larg-est fruit, especially during the early harvlarg-est period (Larson et al., 1989).

Irrigation, therefore, particularly during the fi rst 4–6 weeks following fruit set, increases individual fruit size and yield. This is a critical period of fruit development since it is when cell division is most rapid and cell walls are developed. Even slight reductions in plant water status during this period can have adverse effects on fruit growth and retention (Pongsomboon, 1991).

Although drought tolerance of the mango tree is well known, this comes at considerable cost to tree performance, particularly in areas with prolonged dry seasons that extend through fl owering and fruiting. Irrigation is there-fore one of the most powerful tools to alleviate non-lethal yet potentially yield-reducing drought stress.

Flooding

Studies with container-grown mango trees have reported variable responses with respect to tree survival. Larson et al. (1991c) observed that as many as 45% of trees died after their roots were submerged in water for 4–10 days, but in the surviving population no further mortality occurred when fl ooding was extended for up to 110 days. In other experiments, there was no tree mortality after container-grown mango trees were fl ooded from 1 to several months although tree growth was signifi cantly reduced (Larson et al., 1991c;

B. Schaffer unpublished data, Homestead, Florida, 1993).

The ability of mango trees to survive prolonged fl ooding appears to be dependent on the development of hypertrophic (swollen) stem lenticels immediately above the water line (Plate 42). The initial stages of lenticel hypertrophy are characterized by the development of intercellular spaces in the phellem tissue and production of additional phellem tissue by increased phellogen activity. Later stages of hypertrophy are characterized by the development of intercellular spaces in the phellem tissue and cortex (Larson et al., 1991a). Observations vary among studies whether or not trees devel-oped hypertrophic lenticels or how quickly after fl ooding they formed. These anomalies have been attributed to environmental differences at the time of root submersion (Larson et al., 1991c). In trees that died as a result of fl ooding stress there was no lenticel hypertrophy; however, stem lenticels hypertro-phied within 4–10 days on mango trees that survived fl ooding (Larson et al., 1991a, c, 1993). Sealing hypertrophic lenticels of mango trees with silicone grease or petroleum jelly resulted in tree death within 3 days of fl ooding, thereby demonstrating their necessity for tree survival. The role of hypertro-phic lenticels in fl ood-tolerant species is not clear, although they are thought to eliminate potentially toxic metabolites such as ethanol, acetaldehyde and ethylene which result from anaerobic respiration in the roots (Chirkova and Gutman, 1972; Larson et al., 1993). They may also confer fl ood tolerance by enhancing O₂ diffusion to the roots (Kozlowski, 1984).

In some instances, adventitious roots have developed above the water line when container-grown mango trees have been fl ooded for long periods

(Schaffer et al., 1994). It is likely that these roots facilitate the absorption and translocation of O₂ to submerged roots and their development is a common morphological response of many woody plants to root anoxia. The develop-ment of adventitious roots has not been reported for fl ooded, fi eld-grown trees and they may only form on young trunks after extended fl ooding peri-ods, which usually do not occur under normal production conditions.

Vegetative growth of mango trees generally declines when trees become fl ooded for > 2–3 days. When trees in a limestone soil in containers were fl ooded for > 110 days, there was a 94% reduction in shoot extension growth, while fl ooding for approximately 10 days resulted in a 57% reduction in shoot extension growth (Larson et al., 1991c). In a subsequent study, the stem radial growth (a more sensitive indicator of tree growth than shoot extension growth) of mango trees decreased 2 weeks after roots were submerged.

Flooding for > 14 days also signifi cantly reduced root dry weight, resulting in an increased shoot to root ratio (Larson et al., 1991c). These adverse effects of fl ooding on the growth of mango trees are expected as reduced net photo-synthesis and presumably higher root respiration limit the availability of carbon-based assimilates required for growth.

Wind

Most fruit trees benefi t from wind protection, particularly during the estab-lishment years when the disruption of physiological processes results in a signifi cant depression of growth in young trees. In addition, wind also causes abrasions to the skin of fruits, particularly when they are small, which develop into unsightly blemishes by the time they are fully grown thereby reducing quality and market value. However, the cost of windbreaks may not be offset by higher returns. In some mango-producing regions, winds are not suffi ciently strong to justify the cost of wind protection. Until recently, wind protection in South Africa was not recommended for mangoes due to the loss of potential cropping space by ‘living’ windbreaks, their potential to create frost pockets, and the likelihood of promoting the incidence of fl ower and fruit diseases through increased humidity (Van der Meulen et al., 1971);

however, the value of windbreaks is well appreciated today in South Africa (B.N Wolstenholme, personal communication, Pietermaritzburg, South Africa, 1995).

In studies with ‘Kensington’ mangoes in Australia, where artifi cial wind-breaks were constructed to shelter trees from the prevailing summer south-easterly winds, a 600% increase in yield was recorded in the fi rst year following wind protection (Mayers et al., 1984). This signifi cant improvement in tree performance was a result of better growth of trees which set and held more fruit per panicle, suffered less damage to leaves (cuticle fracturing) and had reduced fruit loss from bacterial black spot (caused by Xanthomonas camp-estris pv. mangiferaeindicae) compared to the wind-exposed trees. These results indicate that wind can have a signifi cant effect on mango productivity from the reduction of both growth and yield through undisclosed physiological

mechanisms, and a decreased level of bacterial black spot infection. The pro-vision of windbreaks in orchards is expensive with decisions to be made on the use of either ‘living’ or artifi cial shelters. Requirements for wind protec-tion will vary depending on site circumstances, and all factors pertaining to crop performance will require careful consideration.

Salinity

Salt stress in mango trees produces symptoms similar to those described for other species (Harding et al., 1958; Ehlig, 1960; Kadman, 1964; Bingham et al., 1968). Mild symptoms of chloride toxicity are scorched leaf tips and margins and leaf curling, while in more severe cases growth ceases, leaves abscise and trees die. Necrotic areas develop on leaves of trees exposed to high sodium levels (Jindal et al., 1976; Kadman et al., 1976; Gazit and Kadman, 1980). Irri-gation water concentrations of 20–60 mM sodium chloride (NaCl) or sodium sulfate (Na₂SO₄) reduced leaf area and changed the branching structure of container-grown mango trees, suggesting that salinity resulted in reduced leaf cell elongation, and affected the activity of the terminal meristem (Schmutz and Lüdders, 1993). As the duration of the exposure to saline con-ditions increased, transpiration decreased exponentially (Schmutz and Lüd-ders, 1993). In a later study, Schmutz (2000) found that following a gradual increase in salinity of the nutrient solution applied to potted polyembryonic

‘13-1’ rootstocks (from 0 to 120 mM NaCl over 15 days), A_max signifi cantly declined despite there being no visible leaf symptoms of salt toxicity. The decline in A_max occurred within 6 days of beginning the salinity treatment, which was 15 mM NaCl for 3 days followed by 30 mM NaCl for 3 days. This indicates that photoassimilation in mango is quite sensitive to exposure of trees to salinity.

There is considerable variation in salinity stress of mango, both within and between populations of mono- or polyembryonic mango ecotypes. Based on the results of limited studies, there appears to be greater salt tolerance in polyembryonic than in monoembryonic populations (Jindal et al., 1975; Kad-man et al., 1976). In seedling populations from mono- and polyembryonic cultivars irrigated for 2 years with water containing approximately 10 mM chloride, most plants developed leaf scorching after 6 months which gradu-ally became more severe, culminating in degeneration and death. However, some seedlings which had no damage or only slight toxicity symptoms were mostly of the polyembryonic ‘13-1’ rootstock cutivar or related types (Kad-man et al., 1976). Leaf analyses revealed that the chloride concentration in tolerant seedlings (0.68–0.77%) was greater than in susceptible seedlings (0.43–0.55%). In addition, tolerant plants had lower leaf concentrations of potassium, calcium and magnesium than saline-sensitive seedlings, possibly a result of comparative nutrient dilution since vegetative growth was greater for saline-tolerant than for saline-sensitive seedlings. Kadman et al. (1976) also suggested that the mechanism of chloride tolerance in ‘13-1’ was based on greater physiological tolerance of chloride concentrations in leaf tissues,

rather than ion exclusion or a selective uptake mechanism common in other species (Collander, 1941; Walker, 1986). However, the relative sodium toler-ance of ‘13-1’ was due to exclusion of sodium from shoots and its accumula-tion in root cell vacuoles (Schmutz and Lüdder, 1993). More recently Hoult et al. (1996) reported signifi cant cultivar differences within a population of 21 polyembryonic mango cultivars exposed to saline (480 mg/l NaCl) irrigation water for 10 months. Differences were measured in leaf Na (0.37–1.34%) and Cl (0.39–1.07%) concentrations but these were poorly correlated with toxicity symptoms on leaves.

Salinization of agricultural land is increasing and in many areas salinity management is critical. There appears to be suffi cient genetic diversity within Mangifera indica to enable the selection and development of saline-tolerant rootstocks (Kadman et al., 1976; Gazit and Kadman, 1980; Hoult et al., 1996).

However, quantitative data on the critical limits of soil and water salinity which mango trees will tolerate without reductions in yield and fruit quality are needed.

Elevated atmospheric CO₂ concentration

Growing ‘Kensington’ mango trees for 6 months in a controlled atmosphere glasshouse with an ambient CO₂ concentration of 600 Pmol/mol resulted in more dry matter partitioned to the roots compared to plants grown in an ambient CO₂ environment of 350 Pmol/mol (Schaffer et al., 1999) (Fig. 6.8).

Fruit dry weight was greater for mango trees grown in an atmospheric CO₂ concentration of 600 Pmol/mol compared to trees grown at 350 Pmol/mol (Schaffer et al., 1999) (Fig. 6.9). Most of the increased total fruit dry weight at

Old leaves

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Dry weight (g)

600 μmol CO2/mol 350 μmol CO₂/mol

Fig. 6.8. Partitioning of dry matter in ‘Kensington’ mango trees grown for 6 months in atmospheric CO₂ concentrations of 350 or 600 Pmol/mol. Bars represent means (n = 6 trees) ± standard error (Source: redrawn from Schaffer et al., 1999).

the higher atmospheric CO₂ concentration was a result of increased amount of pulp; whereas, there were no signifi cant effects of increased atmospheric CO₂ concentration on dry matter accumulation in the skin, testa or seed (Schaffer et al., 1999) (Fig. 6.9). Thus, increasing the atmospheric CO₂ assimi-lation rate increased growth and partitioning to the mesocarp. Therefore, increased atmospheric CO₂ concentration (at least to 600 Pmol/mol) as a result of global climate change may increase the economic yield of mango (Schaffer et al., 1997). However, under actual environmental conditions result-ing from global climate change, water and nutrient availability may be limit-ing and is likely to offset any increases in biomass or economic crop yield resulting from elevated ambient CO₂ concentrations (Gitay et al., 2001).