Effects of [CO2 and temperature on crop development

It is difficult to disentangle with certainty direct (as CO2 per se) and indirect (temperature increases due to smaller gs) effects of [CO2] on crop development. However, there is little evidence of any direct CO2 effect on the rate of development in wheat at any stage. Slightly faster rates of leaf, ear, spike and tiller development are frequently observed in wheat grown at elevated [CO2] and are probably indirect. The complete absence of an effect of elevated [CO2] on wheat phenology in conditions of relatively low radiation also supports this conclusion (Mitchell et al., 1995; Batts etal., 1996, 1997; Wheeler et al., 1996b). In the FACE experiments, N fertilizer altered crop architecture (Brooks et al., 1996), but elevated [CO2] had a minimal effect on rates of overall canopy development and senescence (Wall et al., 1997). This was in contrast to accelerated development seen in earlier studies when temperatures differed (Garcia et al., 1998). This emphasizes the great importance of small increases in temperature for wheat development and, ultimately, yield.

The rates of development of wheat are sigmoidal rather than linear functions of temperature (Shaykewich, 1995). Development, while dependent on variety, begins above a base temperature (1—5°C) and the rate rises slowly as temperature increases, then more rapidly to a maximum at about 30°C and rapidly slows thereafter. The effect of constant temperature increment on the duration of phenological phases is, therefore, dependent on absolute background temperature. However, the effects can often be modelled accurately with the thermal time approach, as shown for data from temperature gradient studies. These experiments provide a good range of temperature around the ambient. They indicate that a 1 °C warming would reduce crop duration by about 21 days (8%) and the reproductive period by about 8 days (6%). The period from anthesis to maturity is shortened by about 3 days and the duration of grain-fill by about 2 days (5%) per °C. Similar results were obtained for an experiment with a single temperature treatment with a 4°C increment above ambient (Mitchell et al., 1995). However, these responses show considerable differences among varieties (Batts et al., 1998a,b). Rawson (1988) presented data for rapidly developing wheat, suggesting that, with increased radiation, ample water and nutrients, high temperatures may not be detrimental to production. Improved understanding of such responses may be of value for exploitation of hot environments.

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