Midday stem water potential as a plant water stress indicator for irrigation scheduling in fruit trees

The manuscript reviews in detail a few studies that have been carried out in the past decade on the relevance and applicability of midday stem water potential as an indicator for irrigation scheduling in fruit trees. Midday stem water potential was found to be more sensitive to irrigation level than midday leaf water potential in apple, nectarine, litchi and grapevine. Stem water potential was highly correlated with stomatal conductance in apple (r(2)=0.85), nectarine (r(2)=0.79), winegrapes (r(2)=0.85) and litchi (r(2)=0.84). Lower correlations were apparent between leaf water potential and stomatal conductance in those species. Relative yields of large apple and nectarine fruits were highly correlated with midday stem water potential. The correlation of midday leaf water potential with large-fruit yield was much lower in apple and that of soil water potential was lower in both apple and nectarine. Midday stem water potential of apple and nectarine trees decreased with increasing crop level. This was apparent with stressed apple trees (r(2)=0.51) and with stressed (r(2)=0.47) as well as with non-stressed (r(2)=0.62) nectarine trees. Our data and others suggest that midday stem water potential is a good plant water stress indicator for irrigation scheduling in fruit trees. Nevertheless, there is still a need for easier-to-use water stress indicators. It is suggested that midday stem water potential be used as a reference against which to test the relevance of other water stress indicators, because of its high sensitivity to irrigation regime and its high correlation with fruit size.