For each measurement, the leaf was positioned on the thermocouple in the chamber to ensure accurate measurement of leaf temperature for calculation of stomatal conductance.
Although the effect of temperature is usually inherent in these models, it is less common for the effect of leaf temperature over the wide range that plants can experience to be explicitly investigated in the models.
We aimed at specifically determining (i) whether the spatial variability of leaf temperature was similar for the whole crown and for the top-viewed crown parts, and (ii) if the spatial variability at these two levels was comparable in a drought stress situation.
In all cases, stomatal conductance, internal CO2 concentration and PFD were all highly significant where the r for the fit to an additive model for each leaf temperature ranged from 0.75 to 0.90 (Table 1).
Leaf position, time of day and day of season were not significant terms in the model, which suggested that differences in photosynthesis along the shoot and during the season were almost entirely accounted for by leaf temperature, stomatal behaviour, internal CO2 concentration and PFD.
Temperature-dependent coefficients derived from the model for each parameter were then applied to independent datasets to assess the goodness of fit and validation of the model over a range of leaf temperatures.
For example, leaf temperatures could be warmer than the air owing to soil-surface influences, particularly in humid regions, which would result in a more rapid yield than that by air temperatures.
It is not clear what factor allows for a reduction in maximum leaf temperature in the open savanna proper, although the combination of greatly reduced leaf dimensions with a high leaf folding ratio could offer an explanation (see below).
In the upper part of the crown, the leaf temperature gradient was smaller than for the whole crown, and the leaf temperature values were more randomly distributed.
The critical parameters for water stress, such as leaf water potential and leaf temperature, are rarely investigated by in-situ measurement.
