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6. Effects of higher day and night temperatures on growth and yields of some crop plants

Division of Plant Physiology, Indian Agricultural Research Institute, New Delhi, India

University of Georgia, College of Agricultural and Environmental Sciences, Georgia Agricultural Experiment Station, Griffin, Georgia, USA

Mechanisms for heat tolerance
Crop growth and development
Extreme temperature effects on crops
Long-term effects of high temperatures on crops

Gaseous emissions from human activities are substantially increasing the concentrations of atmospheric greenhouse gases, particularly carbon dioxide, methane, chlorofluorocarbons and nitrous oxides. Global circulation models predict that these increased concentrations of greenhouse gases will increase average world temperature. Under the business-as-usual scenario of the Intergovernmental Panel on Climate Change (IPCC), global mean temperatures will rise 0.3°C per decade during the next century with an uncertainty of 0.2 to 0.5% (Houghton et al., 1990). Thus global mean temperatures should be 1°C above the present values by 2025 and 3°C above the present value by 2100. Although global circulation models do not all agree as to the magnitude, most predict greenhouse warming. There is also general agreement that global warming will be greater at higher latitudes than in the tropics. Different global circulation models have predicted that global warming effects will vary diurnally, seasonally and with altitude.

It is also possible that there will be an autocatalytic component to global warming. Photosynthesis and respiration of plants and microbes increase with temperature, especially in temperate latitudes. As respiration increases more with increased temperature than does photosynthesis, global warming is likely to increase the flux of carbon dioxide to the atmosphere which would constitute a positive feedback to global warming.

This paper describes the effects of higher day and night temperatures on crop growth and yield. Temperature effects at different levels of organization - biochemical, physiological, morphological, agronomic and systems - are considered. This is followed by identification of options for germplasm improvement and crop management that may mitigate the adverse effects of higher day and night temperatures. The main focus is on wheat (Triticum aestivum L.) and rice (Oryza saliva L.).

Mechanisms for heat tolerance

Crop plants are immobile. They must adapt to prevalent soil and weather conditions. Except for transpirational cooling, plants are unable to adjust their tissue temperatures to any significant extent. On the other hand, plants have evolved several mechanisms that enable them to tolerate higher temperatures. These adaptive thermotolerant mechanisms reflect the environment in which a species has evolved and they largely dictate the environment where a crop may be grown.

Four major aspects of thermotolerance have been studied: (1) thermal dependence at the biochemical and metabolic levels; (2) thermal tolerance in relation to membrane stability; (3) induced thermotolerance through gradual temperature increase vis-a-vis production of heat shock proteins; and (4) photosynthesis and productivity during high temperature stress.


Temperature effects on the rates of biochemical reactions may be modelled as the product of two functions, an exponentially increasing rate of the forward reaction and an exponential decay resulting from enzyme denaturation as temperatures increase (Figure 6. la). The greatest concern is whether it is possible to increase the upper limit of enzyme stability to prevent denaturation.

Failure of only one critical enzyme system can cause death of an organism. This fact may explain why most crop species survive sustained high temperatures up to a relatively narrow range, 40 to 45°C. The relationship between the thermal environment for an organism and the thermal dependence of enzymes has been well established (Senioniti et al., 1986).

The shape of this function also describes temperature effects on most biological functions, including plant growth and development. The function can be categorized by the three cardinal temperatures - minimum, optimum and maximum. Modellers frequently simplify the relationship into a stepwise linear function. The stepwise linear function has a plateau rather than an optimum temperature (Figure 6.1b).

The thermal dependence of the apparent reaction rate for selected enzymes may indicate the optimal thermal range for a plant. The range over which the apparent Michaelis-Menten constant for CO2 (Km) is minimal and stable is termed the thermal kinetic window (Mahan et al., 1987). For crop plants, the thermal kinetic window (TKW) is generally established as a result of thermally induced lipid phase changes, rubisco activity and the starch synthesis pathway in leaves and reproductive organs (Burke, 1990).

In cotton and wheat, the time during which foliage temperature remained within the TKW was related to dry matter accumulation (Burke et al., 1988). The cumulative time that rainfed crop foliage is outside the TKW provides an index of the degree of extreme temperature stress of the environment (Figure 6.2). Irrigation is one management option to reduce crop exposure to heat stress.

Temperatures that inhibit cellular metabolism and growth for a cool season C3 species such as wheat may not inhibit warm-season C3 species such as rice (Oryza sativa L.) and C4 species such as sorghum, maize (Zea mays L.) and sugar cane (Saccharum spontaneum spp.). The identification of TKWs for different species can aid in the interpretation of the differential temperature stress responses for crop growth and development among species (Burke, 1990).

Figure 6.1. (a) Exponential rate of reaction as a function of temperature.

Figure 6.1. (b) Stepwise increase in reaction rate as a function of temperature

Figure 6.2. Seasonal foliage temperatures of wheat (cv. Kanking) and cotton (cv. Paymaster 145) grown at Lubbock, Texas. The vertical lines represent the temperature range that comprises the species-specific thermal kinetic window as determined from the changes in the apparent Km of purified enzymes with temperature. Foliage temperatures were measured with a 50° field-of-view Teletemp Model 50 infrared thermometer (Teletemp Corp., Fullerton, CA) positioned at 1.5 m above the crop. Instruments were scanned at 1 min intervals with a 15 min average computed and stored. The infrared thermometer viewed an area of 0.75 m2, with the same area continuously sampled (from Burke et al., 1988; reproduced with permission)


The plasmalemma and membranes of cell organelles play a vital role in the functioning of cells. Any adverse effect of temperature stress on the membranes leads to disruption of cellular activity or death. Heat injury to the plasmalemma may be measured by ion leakage (Chaisompongpan et al., 1990; Hall, 1993). Injury to membranes from a sudden heat stress event may result from either denaturation of the membrane proteins or from melting of membrane lipids which leads to membrane rupture and loss of cellular contents (Ahrens and Ingram, 1988).

Heat stress may be an oxidative stress (Lee et al., 1983). Peroxidation of membrane lipids has been observed at high temperatures (Mishra and Singhal, 1992; Upadhyaya et al., 1990), which is a symptom of cellular injury. Enhanced synthesis of an anti-oxidant by plant tissues may increase cell tolerance to heat (Upadhyaya et al., 1990, 1991) but no such anti-oxidant has been positively identified.

A relationship between lipid composition and incubation temperature has been shown for algae, fungi and higher plants. In Arabiodopsis, exposed to high temperatures, total lipid content decreases to about one-half and the ratio of unsaturated to saturated fatty acids decreases to one-third of the levels at temperatures within the TKW (Somerville and Browse, 1991). Increase in saturated fatty acids of membranes increases their melting temperature and thus confers heat tolerance. An Arabiodopsis mutant, deficient in activity of chloroplast fatty acid W-9 desaturase, accumulates large amounts of 16:0 fatty acids, resulting in greater saturation of chloroplast lipids. This increases the optimum growth temperature (Kunst et al., 1989; Raison, 1986).

In cotton, however, heat tolerance does not correlate with degree of lipid saturation (Rikin et al., 1993) and similar differences in genotypic differences in heat tolerance have been unrelated to membrane lipid saturation in other species (Kee and Nobel, 1985). In such species, a factor other than membrane stability may be limiting growth at high temperature.


Synthesis and accumulation of proteins were ascertained during a rapid heat stress. These were designated as 'Heat Shock Proteins' (HSPs). Subsequently it was shown that increased production of these proteins also occurs when plants experience a gradual increase in temperature more typical of that experienced in a natural environment.

Three classes of proteins as distinguished by molecular weight account for most HSPs, namely HSP90, HSP70, and low molecular weight proteins of 15 to 30 kDa (LMW HSP). The proportions of the three classes differ among species. In general, heat shock proteins are induced by heat stress at any stage of development. Under maximum heat stress conditions, HSP70 and HSP90 mRNAs can increase ten-fold and LMW HSP increase as much as 200-fold. Three other proteins, though less important, are also considered to be heat shock proteins viz. 110 kDa polypeptides, ubiquitin, and GroEL proteins.

In arid and semi-arid regions, dryland crops may synthesize and accumulate substantial levels of heat shock proteins in response to elevated leaf temperatures. The induction temperature for synthesis and accumulation of heat shock proteins in laboratory-grown cotton ranged from 38 to 41°C (Burke et al., 1985). Soil water deficits resulting in midday canopy temperature of 40°C or greater for two to three weeks were used to study heat shock proteins in field-grown cotton (Figure 6.3). A comparison of polypeptide patterns of dryland and irrigated cotton leaves showed that at least eight new polypeptides accumulated in about half of the dryland leaves analysed. The polypeptides that accumulated in the dryland leaves but not irrigated cotton leaves had molecular weights of 100, 94, 89, 75, 60, 58 and 21 kDa. In a similar experiment with field-grown soybean (Glycine max (L.) Merr.), several heat shock proteins were observed in both irrigated and dryland treatments, although levels were greater in the non-irrigated treatments (Kimpel and Key, 1985).

Correlation between synthesis and accumulation of heat shock proteins and heat tolerance suggests, but does not prove, that the two are causally related. Further evidence for a causal relationship is that some cultivar differences in heat shock protein expression correlate with differences in thermotolerance. In genetic experiments, heat shock protein expression co-segregates with heat tolerance. Another evidence for the protective role of heat shock protein is that mutants unable to synthesize heat shock proteins, and cells in which HSP70 synthesis is blocked or inactivated, are more susceptible to heat injury.

Figure 6.3. Seasonal changes in the midday canopy temperatures of irrigated () and dryland () cotton. Arrow indicates the day on which dryland plots were irrigated with 10 to 12.5 cm of water (106 DAP). The cotton strain is T185. (From Burke et al., 1985: reproduced with permission)

The mechanism by which heat shock proteins contribute to heat tolerance is still not certain. One hypothesis is that HSP70 participates in ATP-dependent protein unfolding or assembly/disassembly reactions and that they prevent protein denaturation during stress (Pelham, 1986). If this mechanism is true, then heat shock proteins may provide a significant basis for increasing heat tolerance of crop plants in a global warming situation. The LMW HSPs may play a structural role in maintaining cell membrane integrity during stress. Other heat shock proteins have been associated with particular organelles such as chloroplasts, ribosomes and mitochondria. In tomato (Lycopersicon esculentum L.), heat shock proteins aggregate into a granular structure in the cytoplasm, possibly protecting the machinery of protein synthesis.

HSPs provide a significant opportunity to increase heat tolerance of crops. To elucidate their mechanisms of action and to exploit their potential contribution to increasing heat tolerance, four lines of investigations are suggested:

1. Establish the biochemical activities of individual HSPs as a preliminary step.

2. Characterize the genetic variability of specific heat shock proteins across a wide range of germplasm. Develop iso-population and near isogenic lines selected for production of low and high levels of HSP synthesis.

3. As HSPs appear to participate in maintaining the conformation or assembly of other protein structures, analyse the molecular details of these processes and establish all participating protein substrates. Such biochemical studies are needed to understand how these processes protect or allow recovery from heat stress.

4. Identify specific HSP mutants or create transgenic mutant plants to complement molecular and biochemical understanding with genetic approaches.


Variability in leaf photosynthetic rates within or between species is often unrelated to differences in productivity. Similarly, high photosynthetic rates at high temperatures do not necessarily support high rates of crop dry matter accumulation. The temperature optimum for photosynthesis is broad, presumably because crop plants have adapted to a relatively wide range of thermal environments. A 1 to 2°C increase in average temperature is not likely to have a substantial impact on leaf photosynthetic rates. Further, there is a possibility that photosynthesis of crop plants can adapt to a slow increase in global average temperatures. Thus, global warming is not likely to affect photosynthetic rates per unit leaf area gradually or on a closed canopy basis over the next century.

While photosynthetic rates were found to be temperature-sensitive in other crops, wheat and rice appear to be different. In wheat, no measurable differences were found in photosynthetic rates per unit flag leaf area or on a whole-plant basis in the temperature range from 15 to 35°C (Bagga and Rawson, 1977). In rice, there is little temperature effect on leaf carbon dioxide assimilation from 20 to 40°C (Egeh et al., 1994).

Recent research has shown significant variation among wheat cultivars with respect to reduction in photosynthesis at very high temperature. Photosynthesis of germplasm adapted to higher temperature environments was less sensitive to high temperature than was germplasm from cooler environments (Al-Khatib and Paulsen, 1990). When this germplasm was grown under moderate (22/17°C) and high (32/27°C) temperatures in the seedling stage or from anthesis to maturity, there was a highly significant correlation between photosynthesis rate and either seedling biomass (r=0.943***) or grain yield of mature plants (r=0.807**). Genotypes most tolerant to high temperatures had the most stable leaf photosynthetic rates across temperature regimes or they had the longest duration of leaf photosynthetic activity after anthesis and high grain weights. The above relationship was exemplified by 'Ventnor' from the high temperature area of Australia and 'Lancero' from the high altitude area of Chile (Table 6.1). See Al-Khatib and Paulsen (1990).

Despite observed negative effects of high temperature on leaf photosynthesis, the temperature optimum for net photosynthesis is likely to increase with elevated levels of atmospheric carbon dioxide. Several studies have concluded that CO2-induced increases in crop yields are much more probable in warm than in cool environments (Idso, 1987; Gifford, 1989; Rawson, 1992, 1995). Thus, global warming may not greatly affect overall net photosynthesis.


In tomato (Lycopersicon esculentum) (Behboudian and Lai, 1994) and cotton (Gossypium hirsutum) (Thomas et al., 1993), elevated CO2 increased dark respiration possibly because of increased carbohydrate accumulation in tissues. The latter has been shown to increase alternative pathway respiration as well (Amthor, 1991).

Apparent dark respiration may decline under elevated CO2 if there is dark CO2 fixation or if elevated CO2 directly inhibits or inactivates respiratory enzymes as may occur through increased formation or carbamate (Wullschleger et al., 1994).

Table 6.1. Mean weekly photosynthetic rate (fmol CO2/m2/s) and duration of photosynthetic activity (weeks, in parentheses), and grain biomass of two wheat genotypes grown at two temperature regimes


Seedlings after two weeks of treatment

From anthesis to maturity

Grain biomass (g/tiller)

22/17 °C

32/27 °C

22/17 °C


22/17 °C

32/27 °C





5.3 (7)






4.1 (10)




Figures in parentheses give duration of photosynthetic activity in weeks from anthesis to physiological maturity. (Modified from Tables 2 and 3 of Al-Khatib and Paulsen (1990); reproduced with permission.)

Few studies have successfully partitioned the effects of elevated CO2 on growth and maintenance respiration. Both components appear to decline, probably because of decrease in leaf protein levels which results in reduced construction and maintenance costs (Wullschleger et al., 1994).

Elevated CO2 reduced maintenance respiration of Medicago sativa and Dactylis glomerata at lower temperatures (15 to 20°C), whereas elevated CO2 reduced growth respiration of M. sativa at 20 to 30°C and D. glomerata at 15 to 25°C (Ziska and Bunce, 1993).

Crop growth and development

Wheat development is customarily divided into vegetative and reproductive phases, with either ear emergence or anthesis as the event that separates the two phases. In the past 30 to 40 years, the sequence of pre-anthesis phenological events has been critically assessed with respect to grain yield potential and sensitivity to weather variables, particularly prevailing temperature and day length. Several systems are used to classify the sequence of phenological events. We identify five such developmental stages:

(i) germination - seeding to seedling emergence;
(ii) canopy development - emergence to first spikelet initiation, the double ridge stage;
(iii) spikelet production - first spikelet initiation to terminal spikelet formation;
(iv) spikelet development - terminal spikelet formation to anthesis;
(v) grain development - anthesis to maturity.

These stages are generally based on early recognized features of the apical meristem. They mark significant changes in morphology or physiology of different crop organs. Numbers of leaf and tiller primordia are determined before spikelet initiation but their subsequent growth and development are controlled by temperature and day length during the differentiation of spikes into spikelets. Similarly, floret number within each spikelet is established by anthesis, at which time the potential grain number per spike is established (Figure 6.4).

Productivity of wheat and other crop species falls markedly at high temperatures. Wheat in India is invariably exposed to extreme temperatures during some stages of development (Abrol et al., 1991). In Australia, wheat is usually exposed to brief periods of heat stress during grain development.

All stages of development are sensitive to temperature. It is the main factor controlling the rate of crop development (Table 6.2). Development generally accelerates as temperature increases, a phenomenon that is often described as a linear function of daily average temperature. The growing degree day concept is a common example of a linear model of developmental response to temperature. While a linear model works well to describe wheat development as long as temperatures remain within 10 to 30°C, a non-linear model as in Figure 6.1 is needed to describe development when a crop is exposed to extreme temperature stress.

Figure 6.4. Schematic diagram of wheat growth and development, showing the stages of sowing (Sw), emergence (Em), first double ridge appearance (DR), terminal spikelet appearance (TS), heading (Hd), anthesis (At), beginning of the grain-filling period (BGF), physiological maturity (PM), and harvest (Hv). Patterned boxes indicate the period of differentiation or growth of specific organs. Bars represent the periods of development when different components of grain yield are produced (heavy lines refer to main shoots and light lines represent extension associated with tillers. (Adapted from Slafer and Rawson, 1994; reproduced with permission)


Several experiments have observed the effects of temperature on the duration from sowing or emergence to heading under controlled environment and field conditions. Unfortunately, however, few experiments have been conducted with enough cultivars to assess the genetic variability in this trait. The major conclusions from these studies are:

1. All genotypes are sensitive to temperature at one stage or another. Temperature sensitivity, however, varies greatly with genotype.

2. Phenological stages differ in sensitivity to temperature.

3. The duration of phase from sowing to first spikelet initiation is less sensitive to change in temperature than are other phases, although genotypes do differ in thermotolerance during this phase.

4. The stages during which environment has the greatest impact on yield are from first spikelet initiation or terminal spikelet formation until anthesis. Spikelet number and floral number (potential grain number), both dominant yield contributing attributes, are established during these phases. Grain weight, on the other hand, appears to be much less sensitive to heat stress than is grain number.

Table 6.2. Response of phasic development to temperature photoperiod and vernalization

Developmental phase








Emergence-double ridges




Double ridges-terminal spikelet




Terminal spikelet-heading












For the estimation of sensitivity, the total life span was divided into the stages shown in Figure 6.4. An arbitrary scale was used to show when the effects are strong (+++++), moderate (+++) or slight (+). 0 denotes that the factor does not affect the process and question marks refer to uncertainties in the literature. For each factor, genetic variation in response was considered.

(From Slafer and Rawson (1994); reproduced with permission.)


In experiments under controlled conditions from 25 to 35°C, mean grain weight declined 16% for each 5°C increase in temperature (Asana and Williams, 1965). In pot experiments, grain yield decreased by 17% for each 5°C rise (Wattal, 1965). For every 1°C rise in temperature, there is a depression in grain yield by 8 to 10%, mediated through 5 to 6% fewer grains and 3 to 4% smaller grain weight.

To elucidate the causal factor for reduced grain filling in wheat because of higher temperatures, Wardlaw (1974) studied the three main components of the plant system. The three components are: (a) source - flag leaf blade; (b) sink - ear; and (c) transport pathway - peduncle. He observed that photosynthesis had a broad temperature optimum from 20 to 30°C with photosynthesis declining rapidly at temperatures above 30°C. The rate of 14C assimilate movement out of the flag leaf, phloem loading, was optimum around 30°C; the rate of 14C assimilate movement through the stem was independent of temperature from 1 to 50°C. Thus, in wheat, temperature effects on translocation result indirectly from direct temperature effects on source and sink activities.

In a subsequent experiment with source-sink relationships altered through grain excision, defoliation and shading treatments, heat stress still reduced grain weight (Wardlaw et al., 1980). This result supports the earlier findings that temperature effects on grain weight are direct effects rather than assimilate availability (Bremner and Rawson, 1978; Ford et al., 1978; Spiertz, 1974). Furthermore, respiration effects do not appear to be the direct cause of decreased grain size in heat-stressed wheat (Wardlaw, 1974).

Reduction of grain weight by heat stress may be explained mostly by effects of temperature on rate and duration of grain growth. As temperature increased from 15/10°C to 21/16°C, duration of grain filling was reduced from 60 to 36 days and grain growth rate increased from 0.73 to 1.49 mg/grain/day with a result of minimal influence on grain weight at maturity. Further increase in temperature from 21/16°C to 30/25°C resulted in decline in grain filling during 36 to 22 days with a minimal increase in grain growth rate from 1.49 to 1.51 mg/grain/day. Thus, mature grain weight was significantly reduced at the highest temperature.

Research on the effects of brief periods of ear warming after anthesis on ear metabolism have identified differential responses of starch and nitrogen accumulation in grain of four wheat cultivars (Bhullar and Jenner, 1983, 1985, 1986; Hawker and Jenner, 1993, Jenner 1991a,b). Warming increased the rate of dry matter accumulation in all the cultivars but the increase was less in cv. Aus 22645 than in the other cultivars studied. Rate of increase in nitrogen accumulation was, however, higher than the increase in total dry matter accumulation (Table 6.3). Under long-term exposure to heat stress, increased grain nitrogen concentration is almost entirely as a result of decreased starch content rather than a change in total grain quality (Bhullar and Jenner, 1985). The conversion of sucrose to starch within the endosperm is decreased by elevated temperatures. Furthermore, heat stress effects on final grain weight were associated with reduced levels of soluble starch synthetase activity (Hawker and Jenner, 1993).

In summary, high temperature reduction of grain yield results from: (a) reduced numbers of grains formed; (b) shorter grain growth duration; and (c) inhibition of sucrose assimilation in grains.

Extreme temperature effects on crops

There are two major forms of extreme temperature stress on crops - heat and cold. An increase in global temperatures may have either or both of these two acute effects: more frequent high temperature stress and less frequent cold temperature stress.

Increase in temperature will lengthen the effective growing season in areas where agricultural potential is currently limited by cold temperature stress. Thus, increased temperature will cause a poleward shift of the thermal limits to agriculture. This poleward shift will be especially important for crops such as rice that have tropical centres of origin and adaptation but are also grown in temperate latitudes during warm seasons. Global warming impact will be greater in the northern than southern hemisphere because there is more high-latitude area cultivated in the northern hemisphere.

Table 6.3. Effect of whole-plant warming on the rate of total dry matter and nitrogen accumulation, between days 10 and 20 after anthesis, in the grains of four cultivars of wheat



Rate of increase (mg/grain/day)

Total dry matter

N content

AUS 22645




























Plants were grown at 21/16°C and some (W) were warmed, between 10 and 20 days after anthesis, to 33/25°C and then returned to 21/16°C where they stayed until maturity. Control plants (C) were grown at 21/16°C throughout. Values given are the means ±.e; values in parentheses are percentage of cotton values. (From Bhullar and Jennar (1985); reproduced with permission.)

Increased temperature would also affect the crop calendar in tropical regions. In the tropics, however, global warming, though predicted to be of only small magnitude, is likely to reduce the length of the effective growing season, particularly where more than one crop per year is grown. In semi-arid regions and other agro-ecological zones where there is wide diurnal temperature variation, relatively small changes in mean annual temperatures could markedly increase the frequency of highest temperature injury. For example, canopy temperature is 10 to 15°C higher in dryland cotton (Gossypium hirsutum L.) than in irrigated cotton (Figure 6.2). Thus, global warming would reduce dry matter accumulation in dryland cotton because of increased respiration, and reduced photosynthesis and cellular energy.

In India, the growing season for wheat is limited by high temperatures at sowing and during maturation. As wheat is grown over a wide range of latitudes, it is frequently exposed to temperatures above the threshold for heat stress. For example, rainfed wheat depends on soil moisture remaining after the monsoon rains recede in September. High maximum and minimum temperatures in September (about 34/20°C), which adversely affect seedling establishment, accelerate early vegetative development, reduce canopy cover, tillering, spike size and yield. Hence, sowing is typically delayed until after mid-October when seedbeds have cooled, though much of the residual soil moisture may be lost. High temperatures in the second half of February (25/10°C), March (30/13°C) and April (30/20°C) reduce the numbers of viable florets and the grain-filling duration. High temperature stress particularly reduces yield of wheat sown in December/January which is necessitated in some regions because of the multiple cropping system.

The situation is similar for sorghum (Sorghum bicolor (L.) Moench) and pearl millet (Pennisetum glaucum (L.) R.Br.) which are exposed to extreme high temperatures in Rajasthan, India. After sowing, air and soil temperatures often exceed 40°C and midday soil surface temperatures above 50°C are common (Figure 6.5).

Acute effects of high temperature are most striking when heat stress occurs during anthesis. In rice, heat stress at anthesis prevents anther dehiscence and pollen shed, to reduce pollination and grain numbers (Mackill et al., 1982; Zheng and Mackill, 1982).

Clearly, many crops in tropical areas are already subjected to heat stress. If temperatures increase further, crop failure in some traditional areas would become more commonplace.

Long-term effects of high temperatures on crops

More important than acute effects of extreme temperature stress are the chronic effects of continuously warmer temperatures on crop growth and development. Chronic effects of high temperature include effects on grain growth discussed above. Record crop yields clearly reflect the importance of season-long effects on crop yields: crops generally yield the most where temperatures are cool during growth of the harvested component.

Crop growth simulations show that rice yields decrease 9% for each 1°C increase in seasonal average temperature (Kropff et al., 1993). This chronic effect of high temperature differs significantly from the acute effect of short-term temperature events, because seasonal temperature effects are mostly a result of effects on crop development. For most grain crops, there is much greater genotypic variation in thermal requirements for vegetative than for reproductive development. As long-term temperatures increase, grain-filling periods decrease, and there appears to be little scope to manipulate this effect through existing genetic variation within species.

Figure 6.5. Diurnal temperature data recorded in Fatehpur, Rajasthan, India. (Latitude 27°C 37'N) in June 1989). Each measurement is the mean value from three thermocouples placed at either 5 cm depth of soil (): 0.5 cm depth of soil (): or 150 cm above the soil surface (). (From Howarth (1991); reproduced with permission.)


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