Leaf photosynthesis (A) generally decreased over
time without significant effects of treatment. The variation
of A was partially explained by the stomatal conductance (gs) especially up to 550 mmolm−2 s−1
(r = 0.91**) (Fig. 4). Over this threshold, gs appeared
clearly less correlated to A and it explained about 25%
of the total variation of A. Stomatal conductance was poorly influenced by treatment, soil moisture content
and crop aging which explained only 18% of the total
variability of gs. Conversely, leaf temperature (Tleaf)
highly influenced gs as well as A (Fig. 5). Furthermore,
Tleaf seemed to influence A more than gs over 33 ◦C.
Specifically, over this threshold gs was no longer affected
by Tleaf (r =−0.22 n.s.) while A still varied significantly
(r =−0.53**). Stomatal conductance was not
significantly affected by relative water content (RWC)
and specific leaf area (SLA) which showed an unexpected
positive correlation with A (r = 0.69**). The
chlorophyll index (SPAD) and the leaf nitrogen content also showed a significant positive correlation with
A (Fig. 6).