Aidan D Farrell, University of the West Indies, Trinidad
Exposure to excessive temperatures during development limits the yield of many of the world’s major crops, especially in the tropics. Increasing global temperatures over the last three decades have resulted in significantly reduced yields in many crops. In addition to the general warming, a predicted increase in the occurrence of heatwaves is likely to result in further yield losses (Long and Ort 2010). Increasing global temperatures and increasingly frequent heatwaves are likely to have similarly negative effects on natural systems in the tropics and subtropics.
In daylight hours leaf temperatures are often higher than that of the surrounding air, as the canopy absorbs incident solar radiation. Overheating occurs when heat dissipation from the canopy is unable to keep pace with the thermal energy absorbed (for plant energy budget see Section 14.1.3). This typically occurs when incident radiation is high and transpirational cooling is low. Even at temperate latitudes, such conditions often develop at midday when solar radiation peaks and soil water reserves are depleted. In warm, dry environments heat stress can persist for prolonged periods. As heat stress is frequently encountered in combination with water deficit and excess irradiance, it can be difficult to disentangle the effects of the three factors. Nonetheless, there is a distinct set of injuries and plant responses that are associated with heat stress. These are detailed in the following sections.
The effect of heat stress on staple crops like wheat can be severe. The impact varies depending on the developmental stage of the plants, with the most vulnerable stage being flowering. High temperatures shorten the duration of growth of both the leaves and the grains, accelerating their development and thus limiting the ability of the plant to accumulate the carbohydrate necessary for grain growth. In addidtion, heat stress before flowering can cause floret sterility, causing yield losses due to reduced grain number. This effect is most acute when heat occurs at or just after pollen meiosis, when carbohydrate supply to the developing pollen grains appears most critical. Grain size in heat stressed plants can be severely reduced, predominately from a reduction in starch, which makes up most of the mass of the grain.
Wheat (a temperate C3 species) produces its most grain at temperatures below 26°C; and its yield is reduced at higher temperatures. Yet in most grain-growing areas in the southern hemisphere, and in Meditteranean climates in the northern hemisphere, temperatures increase steadily during the growing season, and brief periods above 30 °C often occur during the grain-filling period. In many arid countries mean day temperatures can easily exceed wheat’s high temperature threshold during much of the growing season and so heat stress can significantly reduce crop yield by accelerating plant senescence, diminishing seed number and final seed weight.
Where vegetation is sparse, maximum daytime temperatures occur at the soil surface where exposed soil absorbs solar radiation and quickly warms above ambient. Under such conditions soil temperatures can exceed 50oC. Exposed soil is a particular problem when planting crops in warm regions where the dark, moist soil surface can reach high temperatures and severely inhibit germination and seedling emergence (http://www.plantstress.com/Articles/heat_i/heat_i.htm). A similar phenomenon has been seen in temperate climates when plastic mulch is used to artificially insulate the soil surface during planting (Farrell and Gilliland 2011).