Abstract
We have tested whether thermal time can account for the effect of meristem temperature on leaf appearance rate (LAR) and leaf elongation rate (LER) of sorghum in a wide range of conditions without water or nutrient deficits. This requires that responses to temperature should be consistent in different locations and different seasons, and that responses of LER of different leaves of the plant should be similar. Sorghum plants ( Sorghum bicolor (L.) Moench., cv. E-35-1) were sown in the field in Bamako (Mali) from October 1995 to August 1996, in Montpellier (France) in summers 1995 and 1997, and in a growth chamber. Irradiance, air and meristem temperatures ( T m) and air humidity were measured together with LER in all experiments. Unique and tight relationships were observed between T m and LER of leaves located at different positions on the stem, for all experimental conditions when evaporative demand was low (meristem-to-air vapor pressure deficit, VPD ma, lower than 2.5 kPa). Relationships remained linear over the whole studied range of T m, i.e. 13–32°C ( R 2, from 0.7 to 0.87). With high evaporative demand, LER was lower than the LER expected at the same temperature but with low VPD ma (LER reg); the normalized difference between LER measured on a given day (LER a) and LER reg was linearly related to VPD ma ( R 2=0.52). A linear relationship was also observed between T m and LAR measured before the beginning of stem elongation when T m was lower than 27°C. The x-intercepts of relationships between T m and LER or LAR did not differ in a covariance analysis, with a common value of 10.8°C. The use of thermal time without corrections for high temperature or photoperiod was the most appropriate way to account for the timing of leaf development. It allowed prediction of LER provided that the reduction in LER due to evaporative demand was taken into account.
Published Version
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