Introduction

As summarized by Rosenzweig and Hillel (1998), based on a doubling of effective greenhouse gases (which include [CO2], methane, nitrous oxide and chlorofluorocarbons), most global circulation models predict an increase in global temperature of 2-4°C. The projected temperature increase may already be well underway. Preliminary data (NOAA, 1999) indicate that the average temperature in the USA in 1998 was 12.6°C. Thus, based on records dating back to 1895, 1998 tied with 1934 as the warmest year in a 103-year period. Ten out of the last 13 years have averaged from nearly as warm as to much warmer than the long-term mean. High-temperature records were also set worldwide in 1998 (Fig. 10.1), which was the warmest year on record. The second warmest year was 1997, and 7 of the 10 warmest years have occurred in the 1990s. While anomalously warm temperatures are found throughout the tropics, the warmest anomalies occurred over North America and northern Asia (Fig. 10.2).

The predicted temperature rise may not be evenly distributed between day and night and between summer and winter. Most theories, models and observations suggest that night-time minima will increase more than daytime maxima (Karl et al. ,1991) and winter temperatures will increase more than summer temperatures. Although the absolute amount of the temperature increase may be small, Mearns et al. (1984) suggested that relatively small changes in mean temperature can result in disproportionately large changes in

Fig. 10.1. Combined global land and ocean temperature anomalies from 1880 to 1998 relative to an 1880-1997 base period. National Climatic Data Center/NESDIS/NOAA.

Fig. 10.2. A map of surface temperature anomalies for 1998, derived by merging both sea and land surface temperatures (in situ and satellite observations, 1992-1998), to show the extent of the anomalous warm temperatures. (NOAA, 1999.)

Fig. 10.2. A map of surface temperature anomalies for 1998, derived by merging both sea and land surface temperatures (in situ and satellite observations, 1992-1998), to show the extent of the anomalous warm temperatures. (NOAA, 1999.)

the frequency of extreme events; i.e. even if the variance of maximum daily temperatures does not change, the probability of strings of successive days with high temperatures increases substantially.

While considerable attention has been focused on the effects of higher [CO2] on crops, less research has been directed to crop responses to predicted temperature increases. Rosenzweig and Hillel (1998) point out that high-temperature injuries commonly reduce productivity in crops grown in tropical and temperate regions, and that temperature stress is among the least well understood of all plant processes. If mean warming reaches the upper end of the predicted range (a temperature rise of approximately 4°C), developing heat-tolerant varieties of major crops will become a vital task for plant breeders.

On the other hand, potential beneficial effects of global warming include longer growing seasons, multiple cropping, better seed germination and emergence and more rapid crop growth.

Seedling germination and emergence

COOL-SEASON VEGETABLES Table 10.2 groups vegetable crops on the basis of their temperature requirements for seed germination. For cool-season vegetables, which include most leafy vegetables, brassica (cole) crops, some legumes and most root crops, the temperature range for germination is 3—17°C. Direct-seeded vegetable crops should benefit more than transplanted crops from warmer soil temperature in the spring. Since most leafy vegetables are direct-seeded and since uniform maturity (which is highly desirable) requires uniform germination, having optimal soil temperatures for germination is

Table 10.2. Minimum germination temperature (Tmin) and heat sum (S) in degree-days for seedling emergence, and the applicable temperature (T) range for germination of various vegetables. Crops are ranked within groups by heat sum (S) in degree-days. (From Taylor, 1997.)

Tmin S (degree

Table 10.2. Minimum germination temperature (Tmin) and heat sum (S) in degree-days for seedling emergence, and the applicable temperature (T) range for germination of various vegetables. Crops are ranked within groups by heat sum (S) in degree-days. (From Taylor, 1997.)

Tmin S (degree

Group

Crop

Genus and species

(°C)

days)

T (0C)

Leaf vegetables

Purslane

Portulaca oleracea

11.0

48

15

25

and brassica

Cress

Lepidium sativum

1.0

64

B

1Z

crops

Lettuce

Lactuca sativa

B.5

Z1

6

21

Witloof, chicory

Cichorium sativa

5.B

85

9

25

Endive

Cichorium endiva

2.2

9B

B

1Z

Savoy cabbage

B. oleracea var. sabauda

1.9

95

B

1Z

Turnip

B. campestris var. rapa

1.4

9Z

B

1Z

Borecole, kale

B. oleracea var. acephala

1.2

10B

B

1Z

Red cabbage

B. oleracea var. purpurea

1.B

104

B

1Z

White cabbage

B. oleracea var. capitata

1.0

106

B

1Z

Brussels sprouts

B. oleracea var. gemmifera

1.1

108

B

1Z

Spinach

Spinacea oleracea

0.1

111

B

1Z

Cauliflower

B. oleracea var. botrytis

1.B

112

B

1Z

Corn salad

Valerianella olitoria

0.0

161

B

1Z

Leek

Allium porrum

1.Z

222

B

1Z

Celery

Apium graveolens

4.6

2BZ

9

1Z

Parsley

Petroselinum crispum

0.0

268

B

1Z

Fruit

Tomato

Lycopersicon esculentum

8.Z

88

1B

25

vegetables

Aubergine

Solanum melongena

12.1

9B

15

25

Gherkin

Cucumis sativus

12.1

108

15

25

Melon

Cucumis melo

12.2

108

15

25

Sweet pepper

Capsicum annuum

10.9

182

15

25

Leguminous

Garden pea

Pisum sativum

B.2

86

B

1Z

crops

French sugar pea

P. sativum var. sacharatum

1.6

96

B

1Z

Bean (French)

Phaseolus vulgaris

Z.Z

1B0

1B

25

Broad bean

Vicia faba

0.4

148

B

1Z

Root crops

Radish

Raphanus sativus

1.2

Z5

B

1Z

Scorzonera

Scorzonera hispanica

2.0

90

B

1Z

Beet

Beta vulgaris

2.1

119

B

1Z

Carrot

Daucus carota

1.B

1Z0

B

1Z

Onion

Allium cepa

1.4

219

B

1Z

critical. At both the lower and upper ends of this range, germination is inhibited. With global warming, seed germination could be improved for spring crops, but autumn soil temperatures could become too high in some areas for good germination. For example, germination of celery (Apium graveolens L. var. dulce (Mill.) Pers.) requires a daily temperature fluctuation with night temperatures falling below 16°C, and is inhibited by temperatures above 30°C (Pressman, 1997). The rate of germination may increase more than the total percentage germination (onion data are given in Fig. 10.3), but rapid emergence makes the seedlings more competitive against diseases and insects,

Fig. 10.3. (a) Relationship between temperature and the rate and percentage of germination of onion seeds on moist paper. Rates are reciprocals of the number of days for 50% of viable seeds to germinate (data of Harrington, 1962). (b) Relationship between temperature and rate of cotyledon elongation before hook formation for newly germinated onion seedlings cv. White Lisbon (Wheeler and Ellis, 1991). (c) Relationship between temperature and relative rate of cotyledon elongation after hook formation for the same seedlings as in (b) (Brewster, 1997).

Fig. 10.3. (a) Relationship between temperature and the rate and percentage of germination of onion seeds on moist paper. Rates are reciprocals of the number of days for 50% of viable seeds to germinate (data of Harrington, 1962). (b) Relationship between temperature and rate of cotyledon elongation before hook formation for newly germinated onion seedlings cv. White Lisbon (Wheeler and Ellis, 1991). (c) Relationship between temperature and relative rate of cotyledon elongation after hook formation for the same seedlings as in (b) (Brewster, 1997).

and possibly against weeds. Thus, where both spring and autumn crops are grown, the spring crop could be seeded earlier, but planting of the autumn crop might need to be delayed. Warmer winter temperatures could allow the autumn crop to be grown farther into the winter, but higher summer temperatures could restrict the production of spring crops.

WARM-SEASON VEGETABLES. For solanaceous fruit vegetables (tomato, aubergine and pepper) and cucurbits - cucumber, squash (Cucurbitapepo L.) and melon (Cucumis melo L. Reticulatus group) - the temperature range for germination (13-25°C) is much higher than for cool-season vegetables (Table 10.2). Any soil warming would be advantageous for cucurbits, which are generally direct-seeded and have a high heat requirement. However, solanaceous fruit crops grown for the fresh market are generally seeded in heated greenhouses and not transplanted into the field until the danger of frost is past. Theoretically, these crops could be direct-seeded if soils warmed earlier, but soil temperatures are unlikely to rise sufficiently to make this practical. Also, current practices for these crops, including black plastic mulch, drip irrigation and fumigation, comprise a system that is highly efficient in terms of productivity, fruit quality and weed control. Thus, growers of fresh-market tomato and pepper are unlikely to switch to direct seeding but, because some processing tomato and pepper crops are direct-seeded, warmer soil temperatures would be beneficial. Sweet-corn, with an optimum germination temperature of 35°C (Maynard and Hochmuth, 1997), is also direct-seeded, and sh2 supersweet lines in particular would benefit from higher soil temperatures in the field (Wolfe et al, 1997).

Growth rates

For most vegetables, growth is more rapid as temperatures increase, at least up to about 25°C (Table 10.3). Figure 10.4 illustrates onion growth rate as a function of temperature. Bulbing in onions is induced by photoperiod; once induced, it occurs more rapidly at higher temperatures (Brewster, 1997). Maize growth rates increase linearly between 10 and 30°C (Wolfe et al., 1997).

Even at temperatures above 25°C, plants sustain some growth through heat adaptation. In heat-adapted plants, changes in the lipid composition of chloroplast membranes raise the temperature at which the photosynthetic electron transport systems are disrupted (Fitter and Hay, 1987). Another protective mechanism in plants is the production of heat-shock proteins after sudden exposure to high temperature. These proteins may help crops to acquire tolerance to temperature stress, maintain cell integrity, prevent protein denaturation and protect the photosystem II centre. However, their exact role remains unknown (Paulsen, 1994).

Flower induction and dormancy

A greater increase in winter temperatures than in summer temperatures should reduce the potential for summer heat stress, but may lead to a lack of

Temperature (°C)

Fig. 10.4. (a) Effect of temperature on relative growth rate of whole plant dry weight (solid symbols) and of leaf area (open symbols) of onion cv. Relative growth rate (RGR) is rate of increase in dry weight per unit fexisting dry weight. RGR = 1/W.d W/dt, where W = dry weight and t = time. Similarly, relative growth rate of leaf area is rate of increase of leaf area per unit of existing leaf area. (b) Effect of temperature on rate of initiation of leaves by main shoot apex (i.e. not counting leaves on side shoots) of cvs Hygro, Hyton and Rijnsburger, all 'Rijnsburger types', growing in controlled environments. (From Brewster, 1997.)

Temperature (°C)

Fig. 10.4. (a) Effect of temperature on relative growth rate of whole plant dry weight (solid symbols) and of leaf area (open symbols) of onion cv. Relative growth rate (RGR) is rate of increase in dry weight per unit fexisting dry weight. RGR = 1/W.d W/dt, where W = dry weight and t = time. Similarly, relative growth rate of leaf area is rate of increase of leaf area per unit of existing leaf area. (b) Effect of temperature on rate of initiation of leaves by main shoot apex (i.e. not counting leaves on side shoots) of cvs Hygro, Hyton and Rijnsburger, all 'Rijnsburger types', growing in controlled environments. (From Brewster, 1997.)

vernalization (induction of flowering through low temperatures) for some crops. Flowering in brassicas, celery and onion is affected by interactions among winter temperatures, day length, and seedling age and nutritional status. Cultivar selection and planting dates are directed toward either suppressing flower initiation, in the case of celery, onion and cabbage, or delaying it, in the case of broccoli (Brassica oleracea L. var. italica Plenck.) and cauliflower (Brassica oleracea var. botrytis), until the seedling is big enough to support formation of a large head. Thus, if winters become milder, different planting dates and cultivars may be required.

In bean, high temperatures delay flowering because they enhance the short-day photoperiod requirement (Davis, 1997). In cucumber, sex expression is affected, with low temperatures leading to more female flowers (generally desirable) and high temperatures leading to production of more male flowers (Wien, 1997b).

In lettuce and spinach (Spinacia oleracea L.) high temperatures and long days induce flowering. Once the seed stalk starts to develop (referred to as bolting), crop quality declines significantly. Head lettuce cannot be sold at all, and leaf lettuce and spinach become tough and strong tasting. Thus, cultivars with greater resistance to bolting may need to be selected or production areas moved north as the climate warms.

Some seed production and perennial vegetable production locations may need to be moved farther north. Biennial vegetables, which include some root crops and many cole crops, require specific periods of chilling during the winter to produce a seed crop the following season. Celery requires a cold period to produce seed the following season, and high temperatures during seed development may reduce seed quality (Pressman, 1997). In perennial crops, such as chive (Allium schoenoprasum L.), asparagus (Asparagus officinalis L.), and rhubarb (Rheum rhabarbarum L.), low temperatures over the winter are required before new growth is initiated in the spring (Krug, 1997). Assuming dormancy requirements are fulfilled at the proper time, however, a longer growing season might increase production in perennials.

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