Intra-crown spatial variability of leaf temperature and stomatal conductance enhanced by drought in apple trees as assessed by the RATP model

The influence of drought intensity on spatial variability of leaf temperature was explored by the RATP model. 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. During the pre-drought period, the temperature gradient along the leaf crown evolved diurnally under the major influence of incoming radiation for the whole crown, with the warmer leaves generally distributed in the top part of the crowns, leading to a significant spatial autocorrelation. 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. As drought developed, the model reproduced realistic differences between leaf temperature and air temperature dynamics. Drought increased leaf spatial variability (in terms of stomatal conductance and temperature), regardless of the tree position considered. Moreover, for the whole crown, drought did not modify spatial autocorrelation with respect to the pre-drought period, whereas for the upper crown, the spatial autocorrelation felt under the significance level for the lowest relative soil water content levels (i.e., leaf temperatures were randomly distributed). When direct radiation reached low values, both the difference between leaf and air temperature and temperature gradient dramatically decreased compared to higher direct radiation levels, regardless of the air temperature and drought level. The geometrical and functional assumptions of the model are discussed. This study provided an example of the use of functional-structural plant models for assessing multiple interactions between climate, tree architecture and plant physiology, as well as the influence of drought on the within-crown microclimate