Crop responses to climatic variability and extremes
The yield productivity of many crops has risen over the past 40 years. Rates of yield increase in Europe have ranged from 0.8% (oats; Avena sativa) to 2.6% (triticale; Triticosecale) per year (Ewert et al. 2005). Rates of increase in wheat yields differ between European countries, but regional variation about the linear trend is less clear. More southerly European countries have lower rates of wheat yield increase than more northern ones, suggesting that weather factors such as temperature and precipitation play a more determining role in yield than in the north.. The fact that there have been lower rates of increase in yields in areas of Europe with more extreme conditions and that deviations from a linear trend have increased, points to the conclusion that warming since the start of the 1990s (Schär et al. 2004) has started to affect European wheat yields.
For crops, both changes in the mean and variability of temperature can affect crop processes, but not necessarily the same processes. Some crop processes, mostly related to growth such as photosynthesis and respiration, show continuous and mainly nonlinear changes in their rates as temperature increases. Rates of development and progression through a crop life cycle more often show linear responses to temperature. Both growth and developmental processes show temperature optima, whereby process rates increase over a range but thereafter flatten and decrease. For example, the light-saturated photosynthesis rate of C3 crops such as wheat and rice is at a maximum for temperatures from about 20–32 °C; total crop respiration, the sum of the growth and maintenance components, shows a steep nonlinear increase for temperatures from 15–40 °C followed by a rapid and nearly linear decline. The threshold developmental responses of crops to temperature are often well defined, changing direction over a narrow temperature range, as will be seen later.
An experimental study of climatic variability and wheat
Physiological responses to temperature changes in plants may occur at short or long time-scales (Wollenweber et al. 2003). Rapid changes in enzymatic reactions caused by differential thermosensitivity of various enzymes can deplete or result in accumulation of key metabolites. In addition, short-term effects involving altered gene expression, such as heat-shock protein synthesis, are likely to occur. Longer-term responses include alterations in the rate of carbon dioxide (CO2) assimilation and electron transport per unit leaf area, and impaired cell anaplerotic carbon metabolism, sucrose synthesis and carbon (C) and nitrogen (N) partitioning within and between organs (Jagtap et al. 1998). Altered carbon availability brought about by these events will affect uptake, transport and assimilation of other nutrients, disturb lipid metabolism and injure cell membranes (Maheswari et al. 1999), resulting in changes in growth rates and grain yield (Al Khatib & Paulsen 1999). However, temperature responses for specific physiological processes do not always relate directly to growth, because the latter is an integration of the effects of temperature on total metabolism (Bowes 1991).
The developmental stage of the crop exposed to increased temperatures has an important effect on the damage experienced by the plant (Slafer & Rawson 1995), but experimental studies of the effects of temperature variability on crop productivity are rare. This is mainly because of the difficulties in establishing and maintaining a temperature regime where a mean climatic value can be held constant between treatments that vary the amplitude of temperature (Moot et al. 1996). A solution to this is to examine the effects of extreme conditions at particular developmental stages (Ferris et al. 1998), in which the extreme conditions are defined with reference to literature (Porter & Gawith 1999). Wollenweber et al. (2003) tested the null hypothesis that wheat plants react to two separate periods of high temperature as if they were independent of each other. The chosen stages were the double-ridge stage of the apical meristem, which is close in time to the transition from vegetative to reproductive development of the apical meristem, and anthesis when extreme temperature events interfere with the development of fertile grains, as meiosis and pollen growth are affected (Wallwork et al. 1998). The experimental design, and the extreme temperature conditions were defined as a heat period of eight days of 25 °C at the double-ridge stage and/or a heat event of 35 °C at anthesis. Biomass accumulation, photosynthesis and the components of grain yield were analysed. While a high temperature event of 25 °C at the double-ridge stage is not a stress event sensu strictu for wheat, reproductive spikelet initiation can be impaired (Porter & Gawith 1999) and 25 °C is 13 °C higher than mean daily temperatures measured over 30 years at the experimental site in Denmark.
Grain yields were significantly lower in the treatments with high temperatures at anthesis and at both developmental stages. The major yield component reduced by the treatments was the harvest index; that is, the proportion of total dry matter invested in grain. The harvest index was lower in plants experiencing heat periods because their grain number per plant was reduced by 60% . However, there was no significant difference in the grain yield of plants as between those warmed at anthesis and those at double ridges and anthesis, meaning that the plants experienced the warming periods as independent and that critical temperatures of 35 °C for a short-period around anthesis had severe yield reducing effects. The conclusions from such results for climate change are that yield damaging weather signals for cereals such as wheat are in the form of absolute temperature thresholds, are linked to particular developmental stages and can be effective over short time-periods. This means that yield damage estimates of coupled crop–climate models need to have a maximum temporal resolution of a few days and incorporate models of crop phenology to deal with the overlap between such extreme weather events and crop sensitivity to them.
In contrast to the effects on developmentally linked processes, no significant differences were seen in the relation between light-saturated photosynthesis and leaf internal CO2 concentration for the heat treatments. A heat episode during DR increased the rates of light-saturated photosynthesis (Asat) in green leaves slightly. There were no significant differences in Asat and carboxylation efficiency, reinforcing the conclusion that the principal effects of high temperatures are on developmental processes, such as flowering and the formation of sinks for assimilated carbon, which in itself either is stimulated or is little affected by short-term warming. An extreme heat episode during vegetative development does not seem subsequently to affect the growth and developmental response of wheat to a second heat event at anthesis, and high-temperature episodes seem to operate independently of each other.
Crop temperature thresholds
In addition to the linear and nonlinear responses of crop growth and development processes described above, short-term extreme temperatures can have large yield-reducing effects on major crops. These effects were reviewed for wheat by Porter & Gawith (1999) and, for annual crops in general, by Wheeler et al. (2000). A general point arising from these reviews were that temperature thresholds are well defined as absolute threshold temperatures above which particularly the formation of reproductive sinks, such as seeds and fruits, are adversely affected, as seen in the experiment described above.
The largest standard error found was 5.0 °C for the maximum temperature for root growth, followed by 3.7 °C for the optimum temperature of root growth. Others, such as the base and optimum temperatures for shoot growth, the optimum temperature for leaf initiation and base temperature for anthesis have standard errors of less than 0.5 °C. Thus, the consensus is that functionally important temperatures for wheat are conservative when compared between different studies.
A crop that is important in the developing world is groundnut (Arachis hypogaea L.). This is an important food crop of the semi-arid tropics, including Africa, and can experience temperatures above 40 °C for periods during the growing season (Vara Prasad et al. 2000). The harvestable seeds of groundnut are formed following flowering and fruiting periods. When exposed for short-periods at high temperatures of up to 42 °C just after flowering, a clear relationship between fruit set and mean floral temperature was found (Vara Prasad et al. 2000). From between 32 and 36 °C and up to 42 °C, the percentage fruit set fell from 50% of flowers to zero and the decline in rate was linear, illustrating once more the sharpness of response of crop plants to temperatures between 30 and 35 °C during the flowering and fruiting periods.
Various literature sources have identified similar patterns for other important food crops such as maize and rice. For example, maize exhibits reduced pollen viability for temperatures above 36 °C; rice grain sterility is brought on by temperatures in the mid-30s °C and similar temperatures can lead to a reverse of the vernalizing effects of cold temperatures in wheat. What is perhaps more surprising than the consistent damaging effects of high temperatures in food crops is that cold-blooded animals also exhibit threshold temperature responses for various activities. As with plants, the lethal limits are the widest, followed by activity limits, development and growth with the reproductive limits being the narrowest from 24 to 30 °C, the upper value interestingly close to the limits seen for many crop plants, but this is presumably a coincidence. It would be very useful to have equivalent diagrams for the major crop plants in the world and thereby provide specific quantitative information on the probability and consequences, in other words the risk, from crop damaging climate change at the regional or country level. This would further be the linkage between crop physiology, crop agronomy and climate science (Porter 2005).
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