Effects of mild water shortage on water relations, leaf gas exchanges, fruit growth and vascular flows of two different cherry cultivars

Sweet cherry trees of the cultivars ‘Black Star’ and ‘Vera’ were subjected to two different irrigation regimes: commercial control and 30% reduced irrigation. Stem water potentials, leaf gas exchanges as well as fruit vascular flows were monitored before (pit hardening stage) and after (cell expansion stage) veraison. Also, fruit growth was monitored during the whole season and growth rates calculated weekly. In both cultivars, pre-veraison fruit growth was characterized by high xylem and transpiration flows with the phloem accounting for about the 20% of the fruit total daily inflows. At this stage, all parameters monitored were not affected by reduced water supply, except for Ψstem, which was reduced in ‘Black Star’ but not in ‘Vera’ trees. At veraison, an increase in daily fruit growth rates and in phloem contribution (reaching up to 80% of fruit total inflows) was recorded in trees of both cultivars. However, ‘Vera’ showed 40% higher fruit phloem inflows and daily growth rates compared to ‘Black Star’. Also, at this stage, trees of both cultivars responded differently to reduced irrigation: ‘Black Star’ trees showed decreased Ψstembut no difference in fruit vascular flows and fruit growth, while those of ‘Vera’ maintained similar Ψstemand leaf gas exchanges but fruit decreased phloem inflow and growth rates. These results lead to conclude that: i) cherry sensitivity to mild water shortage increases from pre- to post-veraison; ii) at post-veraison, ‘Vera’ trees can be considered more sensitive to water shortage due to its need to sustain higher fruit growth rates compared to those of ‘Black Star’; iii) different sensitivities to water shortage among cultivars may depend both on the amount of resource required by their growing fruit and on the tree ability to decrease Ψstem.