Quantifying the importance of deep root water uptake for apple trees’ hydrological and physiological performance in drylands

Abstract

In drylands, planted trees generally develop roots through mechanisms that enhance their capacity to forage for resources, particularly water, in deep soils and thus survive droughts. However, it remains unclear how and to what extent, lack of access to deep root water uptake will affect the water balance, growth and production of trees in water-limited ecosystems. To address this gap, we designed a novel experiment to partition soils and roots below 200 cm depth from higher layers in a dryland orchard on China’s Loess Plateau to evaluate the response of apple (Malus pumila Mill.) trees to the lack of deep roots and access to deep soil resource in 2019 and 2020 (when a natural extreme drought occurred). We found that without partitioning apple trees clearly used deep soil water, thereby causing more soil desiccation than partitioned trees, especially during the drought. The unavailability of deep soil water resulted in much greater water stress, which cause a decrease of 36 % in transpiration, 20 % in photosynthesis rate and 32 % in mid-day leaf water potential relative to unpartitioned trees. Consequently, the yield and quality of apple trees also clearly decreased 11 % and 15 %. These findings suggest that the absence of uptake of deep soil water by roots reduced trees’ drought resistance. However, the partitioned trees had clearly higher water use efficiency (WUE) than the unpartitioned trees, although in both cases WUE was strongly reduced by the extreme drought. Our results demonstrate that deep soil resources and roots play key roles in trees’ responses to drought, and are likely to play crucial roles in future ecosystem dynamics.