The respective effects of meristem temperature, vapour pressuredeficit (
VPD) and photosynthetic photon flux density (
PPFD)on leaf elongation rate (
LER) of maize, in the absence of waterdeficit in the soil have been quantified. This analysis wascarried out in a series of field experiments in northern andsouthern France over several seasons and years, and in growthchamber experiments.
LER was measured with 10 min steps, togetherwith meristem temperature,
VPD and
PPFD at leaf level in threetypes of experiments: in growth chamber experiments with stepsin
PPFD or
VPD at constant meristem temperature, in growth chamberexperiments with several combinations of constant, but contrasting,
PPFDs,
VPDs and meristem temperatures, and in the field withfluctuating conditions, (i) When evaporative demand was low(night or day with low air
VPD),
LER was only linked to meristemtemperature, regardless of other climatic conditions, (ii) Lighthad no effect
per se on
LER in the range from 0 to 1500 molm
2 s
1 for time-scales longer than 2 h, providedthat its indirect effects on meristem temperature and on evaporativedemand were corrected (in the growth chamber) or taken intoaccount (in the field), and provided that cumulated
PPFD overa weekly time-scale was compatible with field conditions, (iii)Evaporative demand sensed by growing leaves, as estimated bymeristem-to-air vapour pressure difference, markedly affected
LER in the range from 14 kPa, at all time-scales understudy, with a unique relationship in the growth chamber (constantconditions) and in the field (fluctuating conditions). Thiseffect was only observed when
PPFD was high enough for stomatato open. The negative effect of evaporative demand on
LER wasprobably not due to long distance root-to-shoot signalling,since soil was wet, calculated root water potential remainedclose to 0 MPa and concentration of ABA in the xylem sap wasvery low. Therefore, it is proposed to model maize
LER witha two-step process, involving the calculation of the maximum
LER at a given meristem temperature and then the calculationof the reduction in
LER due to evaporative demand. Joint analysisof the whole set of data by using the two equations yieldeda
r2 of 0.75. This two-step process would be more accurate thanthe provision of
LER from temperature only in cases where air
VPD frequently exceeds 2 kPa. Key words: Leaf growth, light, evaporative demand, temperature, thermal time, water deficit, ABA, Zea mays L.
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