ELONGATION RATE OF SORGHUM LEAVES HAS A COMMON RESPONSE TO MERISTEM TEMPERATURE IN DIVERSE AFRICAN AND EUROPEAN ENVIRONMENTAL-CONDITIONS

We have tested whether thermal time can account for the effect of meri stem temperature on leaf appearance rate (LAR) and leaf elongation rat e (LER) of sorghum in a wide range of conditions without water or nun- lent deficits. This requires that responses to temperature should be c onsistent in different locations and different seasons, and that respo nses of LER of different leaves of the plant should be similar. Sorghu m plants (Sorghum bicolor (L.) Moench., cv. E-35-1) were sown in the f ield in Bamako (Mali) from October 1995 to August 1996, in Montpellier (France) in summers 1995 and 1997, and in a growth chamber, Irradianc e, air and meristem temperatures (T-m) and air humidity were measured together with LER in all experiments. Unique and tight relationships w ere observed between T-m and LER of leaves located at different positi ons on the stem, for all experimental conditions when evaporative dema nd was low (meristem-to-air vapor pressure deficit, VPDma, lower than 2.5 kPa). Relationships remained linear over the whole studied range o f T-m, i.e. 13-32 degrees C (R-2, from 0.7 to 0.87). With high evapora tive demand, LER was lower than the LER expected at the same temperatu re but with low VPDma (LERreg); the normalized difference between LER measured on a given day (LERa) and LERreg was Linearly related to VPDm a (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 lowe r than 27 degrees C. The x-intercepts of relationships between T-m and LER or LAR did not differ in a covariance analysis, with a common val ue of 10.8 degrees C. The use of thermal time without corrections for high temperature or photoperiod was the most appropriate way to accoun t for the timing of leaf development. It allowed prediction of LER pro vided that the reduction in LER due to evaporative demand was taken in to account, (C) 1998 Elsevier Science B.V. All rights reserved.