Root water uptake and water transport to above-ground organs compensate for winter water losses and prevent shoot dehydration in apple trees

Plants need to maintain a minimum moisture content in their tissues to preserve their living functions not only during the growing season but also during periods of dormancy. Whereas water uptake of trees during the growing season has been widely investigated, very little attention has been paid to tree water uptake and losses during winter dormancy. A study was therefore conducted aiming at quantifying root water uptake of apple trees in winter, comparing the water content of two groups of field-grown apple trees for two winter seasons. One group of trees was isolated from its roots by a transversal cut at the base of the trunk and the other was left intact. In an additional experiment, deuterium-enriched water was added to the soil of potted apple trees during winter dormancy, to monitor water uptake and translocation within the plant. The field experiment revealed incrementing moisture losses of annual shoots in late winter, driven by the rising evaporative demand of the atmosphere. Trees with unimpaired water transport systems compensated shoot water losses by root water uptake and internal translocation, whereas trees with severed trunks only partially mitigated losses by internal translocation from trunk and larger branches. Root water uptake during winter dormancy was also confirmed in the potted tree experiment with deuterium-enriched water. This work provides evidence that water flow along the soil-plant-atmosphere continuum in apple trees takes place also in winter, with possible impacts on tree survival during prolonged periods of soil or trunk freezing.