QUANTITATIVE-ANALYSIS OF THE COMBINED EFFECTS OF TEMPERATURE, EVAPORATIVE DEMAND AND LIGHT ON LEAF ELONGATION RATE IN WELL-WATERED FIELD AND LABORATORY-GROWN MAIZE PLANTS

The respective effects of meristem temperature, vapour pressure defici t (VPD) and photosynthetic photon flux density (PPFD) on leaf elongati on rate (LER) of maize, in the absence of water deficit in the soil ha ve been quantified. This analysis was carried out in a series of field experiments in northern and southern France over several seasons and years, and in growth chamber experiments. LER was measured with 10 min steps, together with meristem temperature, VPD and PPFD at leaf level in three types of experiments: in growth chamber experiments with ste ps in PPFD or VPD at constant meristem temperature, in growth chamber experiments with several combinations of constant, but contrasting, PP FDs, VPDs and meristem temperatures, and in the field with fluctuating conditions. (i) When evaporative demand was low (night or day with lo w air VPD), LER was only linked to meristem temperature, regardless of other climatic conditions. (ii) Light had no effect per se on LER in the range from 0 to 1500 mu mol m(-2) s(-1) for time-scales longer tha n 2 h, provided that its indirect effects on meristem temperature and on evaporative demand were corrected (in the growth chamber) or taken into account (in the field), and provided that cumulated PPFD over a w eekly time-scale was compatible with field conditions. (iii) Evaporati ve demand sensed by growing leaves, as estimated by meristem-to-air va pour pressure difference, markedly affected LER in the range from 1-4 kPa, at all time-scales under study, with a unique relationship in the growth chamber (constant conditions) and in the field (fluctuating co nditions). This effect was only observed when PPFD was high enough for stomata to open. The negative effect of evaporative demand on LER was probably not due to long distance root-to-shoot signalling, since soi l was wet, calculated root water potential remained close to 0 MPa and concentration of ABA in the xylem sap was very low. Therefore, it is proposed to model maize LER with a two-step process, involving the cal culation of the maximum LER at a given meristem temperature and then t he calculation of the reduction in LER due to evaporative demand. Join t analysis of the whole set of data by using the two equations yielded a r(2) of 0.75. This two-step process would be more accurate than the provision of LER from temperature only in cases where air VPD frequen tly exceeds 2 kPa.