Elevated air temperature accelerates various aspects of plant metabolism (Larcher 2003), and can affect crops through changing the timing and rate of physiological processes, including the rate of organ development, respiration, senescence and their source-sink relations (Farrar and Williams 1991). Plant development can be accelerated by high temperatures, which lead to earlier shifts to the next ontogenetic stage (Badeck et al. 2004).
Crop responses to temperature depend on the specific optimum temperature for photosynthesis, growth and yield (Conroy et al. 1994). If the temperature is below optimum for photosynthesis, a slight increase in temperature may lead to increased plant growth and development, but if the temperature is close to maximum, a small increase in temperature can negatively affect crop growth and in turn, decrease yield (Baker and Allen 1993). Also, the character of temperature increase and the developmental stage of the crop determine crop responses to higher temperatures (Porter and Gawith 1999).
High temperatures reduce plant biomass and adversely affect crop reproductive efforts (Polowick and Sawhney 1988; Ferris et al. 1998), particularly flowering and fertilization (Morison and Lawlor 1999). As shown in a number of species, including bean (Phaseolus vulgaris L.), linseed (Linum usitatissimum L.), tomato, corn, wheat and canola, high temperature stress reduced seed yields (Polowick and Sawhney 1988; Young et al. 2004). Also, Brassica species that have vigorous growth between 10° and 30°C, with the optimum temperature of ~20°C, are very sensitive to high temperatures at the blooming time, even with ample moisture availability. Extended periods of high temperature (over 30°C) can result in severe sterility and high yield losses as well as in low oil content and poor seed quality if it happens during the period of seed filling (Sovero 1993). Young et al. (2004) showed that, in canola, high temperature reduced pollen viability and germinability, induced flower and fruit abortion and as a result, negatively affected seed production. In annual crops, such as peanut (Arachis hypogaea L.) (Challinor et al. 2005) and wheat (Ferris et al. 1998; Wheeler et al. 2000), even a brief period of high temperature near to the flowering stage may lead to severe yield reduction due to pollen sterilization. Mitchell et al. (1993) indicated that reduced yield in wheat could be due to shortened phenological stages and in turn, less time for the accumulation of resources for grain formation. High temperature can also affect yield quality. As reported in cereal grains, high temperature reduced protein content and altered types and proportions of proteins and lipids (Lawlor 2005).
High temperatures decrease plant biomass and yield by decreasing photosynthesis and increasing transpiration and stomatal conductance (Nobel 2005). A sharp increase in the basal chlorophyll fluorescence indicates a blockage of the photochemical reaction centre of photosystem II under high temperature that affect photosynthesis (Hallgren et al. 1991). Plants tend to produce smaller leaves and more extensive root systems under higher temperature to offset leaf water loss and to increase water intake (Gliessman 1998). Also, plants mitigate overheating by leaf rolling and drooping and vertical leaf orientation (Larcher 2003; Nobel 2005) or by transient wilting (Chiariello et al. 1987; Nobel 2005). Such adaptive mechanisms likely reduce leaf exposure to incident light and in turn, may lead to decreased photosynthesis. Reduced net photosynthesis due to increased temperature can in part be related to Rubisco deactivation (Crafts-Brandner and Salvucci 2000). In contrast to elevated CO2, high temperatures lead to increased photorespiration by decreasing the affinity of the enzyme Rubisco to CO2 relative to oxygen (Lawlor 1998).
Plant hormones, such as abscisic acid (ABA), indole-3-acetic acid (IAA) and ethylene, are involved in the responses of plants to high temperature (Nilsen and Orcutt 1996) by balancing transpiration through affecting stomatal conductance (Dodd and Davies 2004). High temperatures increase ABA, but decrease IAA (Nilsen and Orcutt 1996) and ethylene evolution (Yu et al. 1980). IAA is a growth promoting hormone and its lower concentration under higher temperature can, in canola, negatively affect plant growth and that results in lower dry matter (Qaderi et al. 2006).
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