Yields

Effects of high temperatures on agronomic crops have been reviewed by Boote et al. (1997). Declines in harvest index, seed size and mass and seed-growth rate are rapid as temperatures increase above a critical level. Krug (1997) pointed out that increasing temperature generally increases the developmental rate, but the growth rate is not necessarily stimulated to the same degree. In an early-maturing pea cultivar, increasing the temperature from 16 to 24°C increased the differentiation rate of nodes from emergence to the appearance of the first flower, but the growth rate was reduced, which reduced total dry matter and yields. Cultivars with a proportional increase in the rate of differentiation and the rate of growth as temperature increases should be less sensitive to temperature. In onion, warmer temperatures shortened the duration of growth, accounting for a negative correlation between elevated temperatures and crop yields (Daymond et al., 1997).

Predicting which vegetables will be most affected by high temperatures is difficult. It has been suggested (Hall and Allen, 1993) that indeterminate crops

Fig. 10.6. Effect on (a) percentage fruit set, (b) fruit number, (c) fruit weight (g) and (d) seed number of exposing tomato pollen (male-fertile parent) and tomato pistils to mean daily temperatures of 25, 27 and 29°C. Pollen was collected from male-fertiles maintained in separate growth chambers and hand-applied to the stigmas of male-steriles maintained at 25°C (open triangle), 27°C (+) or 29°C (closed circle). (Peet et al, 1998.)

Fig. 10.6. Effect on (a) percentage fruit set, (b) fruit number, (c) fruit weight (g) and (d) seed number of exposing tomato pollen (male-fertile parent) and tomato pistils to mean daily temperatures of 25, 27 and 29°C. Pollen was collected from male-fertiles maintained in separate growth chambers and hand-applied to the stigmas of male-steriles maintained at 25°C (open triangle), 27°C (+) or 29°C (closed circle). (Peet et al, 1998.)

are less sensitive to periods of heat stress because the time of flowering is extended compared with determinate crops. This is probably true for crops harvested at seed maturity. In fact, soybean, having no especially critical period in its development, is considered to have a great ability to recover from stress (Acock and Acock, 1993). Similarly, dry bean, winter squash (Cucurbita sp.) and pumpkin (Cucurbita sp.), which are harvested at maturity, should be relatively unaffected by temporary heat stress. These crops will usually keep flowering and setting fruit until a certain number of fruit develop on the vine. However, most vegetable crops that flower over an extended period, such as cucumber, snapbean, tomato and pepper, are harvested well before seed maturity. In bush snapbean and processing tomato, plants are mechanically harvested once and any pods or fruit that are too small or too large are discarded. Thus a period of heat that reduces seed set may cause considerable economic loss by making the crop less uniform in harvest maturity, even if recovery occurs later. Even in crops that are hand-harvested several times, a spread-out period of fruit set increases labour and packing costs, thus decreasing efficiency and possibly moving the crop out of its market 'window' of high demand. Thus, it could be argued that indeterminate crops also have a longer window of vulnerability to high temperatures, compared with crops such as maize, which are relatively insensitive to high temperatures except during the early stages of reproductive development (pollination and early seed development).

Cool-season crops, which are currently widely grown in the tropics, are most likely to be negatively affected by global warming. Cabbage grown at 25°C has a lower dry-matter content (and thus lower quality), a reduced growth rate, and lower water-use efficiency than cabbage grown at 20°C (Hara and Sonoda, 1982). Brussels sprouts require monthly temperatures of 17—21°C for 4 months, followed by 2 months of 12°C temperatures, during which sprouts develop. Thus not only cool weather, but also long, continuously cool weather is required (Wien and Wurr, 1997).

Since night-time temperature minima may increase more than daytime maxima (Karl et al., 1991), additional night-time heat stress may be more of a factor than additional daytime heat stress. Cowpeas have been described as more sensitive to night-time than to daytime heat stress (Mutters and Hall, 1992) because they can tolerate temperatures of 30°C during the day, but not at night. Ahmed et al. (1992) noted premature degeneration of the tapetal layer and lack of endothecial development in cowpea, which they felt was responsible for the low pollen viability, low anther dehiscence and low pod set under 30°C night temperatures. Pepper fruit set has also been described as being more sensitive to high night temperatures than to high day temperatures (Wien, 1997c).

Went (1944) suggested that night-time heat stress most limits fruit set in tomato. In a recent re-examination of this question, Peet et al. (1997) reported that when heat stress was applied to male sterile tomatoes provided with pollen from non-stressed plants, mean daily temperature, rather than high night temperature, was the main factor affecting fruit and seed yield. Similarly, Peet and Bartholomew (1996) did not observe a disproportionately large heat-stress response to night temperatures ranging from 18 to 26°C when daytime temperatures were the same (26°C).

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