Conceptual models accounting for the influence of source:sink ratio on water relations of trees are theoretically relevant from
a physiological perspective and practically important for irrigation scheduling. Midday stem water potential of horticultural
trees often declines with increasing crop load but the actual response depends on environmental, management and plant
factors. Here we advance a quantitative synthesis of the response of stem water potential to crop load from the perspective
of phenotypic plasticity, defined as ‘the amount by which the expression of individual characteristics of a genotype are
changed by different environments’. Data sets of stem water potential for contrasting crop loads were compiled for apple
(Malus domestica L. Borkh.), olive (Olea europea L.), peach (Prunus persica L.), pear (Pyrus communis L.) and plum (Prunus
domestica
L.). Phenotypic plasticity of stem water potential was calculated as the slope of the linear regression between stem
water potential for each crop load and the environmental mean of stem water potential across crop loads. Regression lines
for trees with different crop load diverged with decreasing environmental mean stem water potential. For the pooled data,
plasticity of stem water potential was a linear function of relative crop load. This represents a significant shift in perspective:
the effect of crop load on the trait per se (stem water potential) is environmentally contingent, but the effect of crop load on
the plasticity of the trait is not. We conclude that research on the effects of crop load on tree water relations would return
more robust results if plant traits are considered from the dual perspective of the trait per se and its plasticity.