In contrast to bulb onions and garlic, leek is harvested as a growing shoot. The objective in crop production is to produce shoots of marketable size before the plants bolt. Therefore, it is useful to understand what controls flower stalk initiation and development relative to vegetative growth. The rate of elongation of flower stalks, once initiated, clearly shows some seasonality, and visible bolting is concentrated in the long photoperiods of spring and summer.
However, the main variable in the ‘race’ to produce a marketable-sized leek before the plant visibly bolts is not the rate of flowering, but the rate of growth.
In fertile soil, growth rates are primarily determined by temperatures and light levels. Premature bolting is a problem from very early plantings, which grow slowly because of low temperatures in the early spring, and from late summer and autumn plantings, where the low temperature and light levels of winter restrict growth rates, and the plants fail to achieve marketable size before bolting occurs in the spring.
Early vegetative growth can be described by fairly simple rules involving temperature, light and leaf number. For spring-sown leeks in field conditions, both the number of leaves initiated at the shoot apex, the number of leaves
emerged from the top of the pseudostem, the number of fully expanded leaves (on which the ligule at the base of the leaf lamina can be seen) and the number of senescent leaves show straight line increases when plotted against accumulated day-degrees above 0°C (see Fig. 4.48).
For cv. ‘Autumn Mammoth’, Fig. 4.48 shows that leaves were initiated at a constant rate of one per 92°C days (base temperature 0°C) and that leaves appeared at one per 135°C days. Leaves took longer to appear than to initiate, not because leaf initials accumulated at the shoot apex, as with cereal plants, but because the pseudostem increased in length by a constant amount for every leaf that emerged. Leaf extension growth occurs in the first 3–4 cm above the apex and the rate of extension was almost the same for successive leaves. Hence, although the rate of elongation of successive leaves was the same (per degree-day), every leaf had to grow further than its predecessor to appear.
The process is summarized in simple mathematical terms by the equation:
DD per leaf appearance = DD per leaf initiation + Additional
pseudostem length per leaf/Rate of leaf elongation per DD (Eqn 4.23) Here, DD stands for day-degrees above 0°C (Hay and Kemp, 1992).
Fig. 4.48. Leaf development in leek cv. ‘Autumn Mammoth’ grown from a mid-April sowing in Scotland. Changes with accumulated thermal time (base 0°C) in the total number of leaf structures initiated (s), the number of leaf tips visible above the leaf sheath (t), the number of fully extended leaves with visible ligules (l), the number of leaves that were at least 50% senescent (d) and the number of primordial leaves at the shoot apex (p). The lines were fitted by linear regressions on data from 20 plants per sample date (from Hay and Kemp, 1992. Courtesy of Annals of Applied Biology).
The rate of ligule appearance (which indicates full expansion of a leaf blade) was slower than leaf tip appearance, showing that the duration of leaf expansion progressively increases for the first eight to ten leaves. As a consequence, in cv. ‘Autumn Mammoth’, each leaf was about 6 cm longer than its predecessor, and also about 0.45 cm wider (see Fig. 4.49). Cultivars differ in rate of leaf initiation, appearance and elongation per degree-day. Also, these rates tend to be higher in bright, sunny years than in dull seasons, suggesting a possible influence of light as well as temperature on these relationships. This would not be surprising, since the growth rate of leek seedlings is more sensitive to daily light income than are other crops (see below).
The growth in weight of leek seedlings has been shown to be well modelled by Eqn 4.18. The constant p in this equation is the relative growth rate per
‘effective day-degree’ (EDD). Effective day-degrees are day-degrees above the base temperature, TB, modified by an effect of daily photosynthetic radiation income, R, using the constant f (Scaife et al., 1987) (see Eqn 4.19b). Leek cv.
‘Winterreuzen’ has values of TB = 5.9°C, p = 0.015/EDD and f = 0.146 MJ/m2/DD. The f value is higher than for any of nine other crops studied, the base temperature for growth TBis typical of many temperate vegetable species and p is low compared with other species (Brewster and Sutherland, 1993).
The latter indicates an intrinsically low potential growth rate.
Hence leeks need a long growing season to produce plants of marketable size given their small seedling weight at emergence (about 0.002 g dry weight).
The high value of f shows leek seedling growth rate to be particularly sensitive to daily light income. Probably the narrow, upright leaf habit, which is
ill-Fig. 4.49. The dimensions of fully expanded leaf blades of leek plants cv. ‘Autumn Mammoth’ from three spring sowings (1, 2, 3) and one transplanting (t) in the same season in Scotland. The vertical bars indicate standard errors of means x 2 (from Hay and Brown, 1988. Courtesy of Annals of Applied Biology).
adapted to light interception, is the cause of this. Using this relationship it is possible to predict growth rates in conditions of varying temperature and light, such as may occur in the field or in a glasshouse used for plant raising, providing growth is not restricted by lack of water or nutrients. As the plants get larger they begin to shade each other, and competition for light will cause the growth rates to diverge below the prediction of this equation. The higher the plant density the sooner this will occur.
Crop growth can continue to a ceiling or potential yield that probably varies with both temperature and daily light income and, therefore, with time of year and region. Well-watered, well-fertilized, early-planted leek crops do indeed reach a plateau of total shoot dry matter yield of around 1200g/m2 (about 120 t/ha fresh weight) in high summer (see Fig. 4.50).
Later plantings plateau at a lower yield during autumn and winter, probably because growth rate is slowed by low temperatures, and also because the average daily light income is insufficient to sustain summer yield levels.
The fact that high-yielding, summer-grown crops tend to decline in yield when left in the field over winter, due to the rotting of some leaves and the death of the smaller plants, supports the latter view. As a consequence of these effects, leek yields decrease the later in the season they are planted. The yields attained correlate with both accumulated day-degrees and accumulated solar radiation (light) during the growing period (see Table 4.8).
The diameter of the pseudostem or ‘shaft’ is often the important criterion of marketability of leeks, and Wurr et al. (1999) took a diameter of 15 mm as the minimum for marketable early leeks in the UK. From field trials they found that shaft diameter in early leeks was related to effective day-degrees (EDD)
Fig. 4.50. Shoot dry weight yields of leek cv. ‘Autumn Mammoth Goliath’ grown at a density of 25 plants/m2with abundant irrigation and nutrients from a succession of plantings at Wellesbourne, central England (note log scale for crop dry weight).
accumulated since planting, using a base temperature of 6°C and an f value (Eqn 4.19) of 0.6 (see Fig. 4.51). Accumulated EDDs with these optimal constants were a better predictor of diameter than accumulated day-degrees, time since planting or radiation accumulated since planting.
Although bulbing is uncommon in leeks, ‘bulbiness’ can constitute a quality defect. Strong bulbing, with the formation of bladeless inner scale leaves, has been induced under 24 h photoperiods (Dragland, 1972). As with most features of leeks, there is wide plant-to-plant variation in the extent of bulbing in these conditions. Bulbs are also frequently seen at the base of the flower stalk (van der Meer and Hanelt, 1990). Low soil temperatures also favour bulbiness, possibly because of an accumulation of unused carbohydrate at the base of the pseudostem (Dragland, 1972).
Table 4.8. Aspects of leek cv. ‘Splendid’ as affected by planting date at Wellesbourne, UK (from Salter et al., 1986).
Planting date
Aspect 10 April 3 July
Yield 168 days after planting (t/ha) 69 26
Day-degrees > 6°C from planting to harvest 1403 1203 Accumulated solar radiation (MJ/m2) from 2938 1733
planting to harvest
Fig. 4.51. The relationship between leek sheath diameter and the accumulated effective day-degrees from planting derived from seven experiments at
Wellesbourne and in Cornwall, UK. The best-fitting Effective Day Degree (EDD) units were defined by: EDD1= DD1+ 0.6.R1, where DD are day-degrees with a base of 6°C and R is the total solar radiation in MJ/m2for each day (from Wurr et al., 1999. Courtesy of Journal of Horticultural Science and Biotechnology).
Flowering
Experiments using controlled environments indicate that flower induction in leeks occurs most rapidly between 2 and 8°C and that the percentage of plants induced increases with the duration of cold exposure up to at least 6 weeks (Wiebe, 1994; Fig. 4.52). Seedlings weighing 2 g fresh weight with four to five visible leaves were much more responsive than smaller plants, indicating a juvenile stage in young leeks when vernalizing temperatures are ineffective.
Exposure to 22°C immediately following a cold treatment reduced the percentage bolting and therefore had a devernalizing effect. Exposure to a 16 rather than a 10 h photoperiod during vernalization slightly reduced the percentage bolting, but the effect was small at the near-optimal inductive temperature of 5°C.
Fig. 4.52. The effects of vernalization temperature and the duration of vernalization on the percentage of plants bolting in leek cv. ‘Alma’. Following the treatments shown the plants were grown at 15°C for 2 weeks and then transplanted to the field, where the numbers subsequently bolting were recorded (from Wiebe, 1994.
Courtesy of Scientia Horticulturae).
Dragland (1972) also observed more and earlier bolting at cooler temperatures within the range 12–18°C. He also noted that flower stalks elongated faster the longer the photoperiod over the range 9–24 h, indicating that once inflorescences are initiated their rate of growth increases with photoperiod, as is the case with onion provided bulbing does not suppress the scape. Using a statistical analysis of temperatures and flowering from leeks growing from a range of planting dates at two sites, with and without protective crop covers, Wurr et al. (1999) concluded that 7°C was optimal for inducing flowers in cv. ‘Prelina’. The strongest influence on flowering was the duration of temperature in the 4–8°C range during the 42 days subsequent to the plants reaching the three visible leaf stage, suggesting an end of juvenility at an earlier stage than four to five leaves of Wiebe (1994).
In summary, the control of flowering in leek is similar to that in onion, with a juvenile stage before which vernalizing temperatures are ineffective, an optimum temperature for vernalization around 7°C falling off to zero flowering when plants are exposed to temperatures of 18–20°C, and devernalization possible from temperatures of 22°C and above. Elongation of scapes is promoted by both long photoperiods and warmer temperatures (Wurr et al., 1999).