Abstract
Water plays a pivotal role in agriculture. Irrigation is a common practice, especially in high value crops such as pomefruit where production is extremely sensitive to water stress. Apple and pear are among the most cultivated fruit trees worldwide, spanning a range of different climatic conditions. Climate change is challenging the success of such crops through the increased frequency of heat and drought stress. Therefore, a deeper knowledge on the response of trees to such stresses is required, together with systems to continuously monitor plant status in order to rapidly detect the onset of stress conditions. The irrigation system itself, depending on the volume of soil it is able to wet, can define the resilience of the orchard towards heat and drought stress. In this work, the role of soil water availability under stress conditions was examined, with an additional focus on the internal redistribution of irrigation water under localized irrigation. To this end, a first study aimed at characterizing the physiological response of apple trees to a simulated heatwave and their recovery from such stress under different watering regimes. The results show that soil water availability is crucial in modulating the response of apple trees under heat stress. The most severe physiological impairments were found in trees that were subjected to combined heat and drought stress. Trees facing heat stress with ample soil water availability were able to maintain their physiological functioning but at the expense of a lower water use efficiency. Surprisingly, even the trees subjected to the most severe stress were able to recover their physiological functioning within five days after the removal of the stress conditions. In a second trial, carried out in a pear orchard, dendrometers were utilized to continuously monitor fruit size, allowing to rapidly detect the onset of drought stress conditions. Daily fruit growth was related to tree water status, with growth slowing down in periods when soil water availability was not sufficient. This study offered interesting insights on irrigation management under challenging environmental conditions, showing that proper management of irrigation is necessary to maximize productivity while dealing with limited water resources. Finally, the uptake and redistribution of water in apple trees subjected to localized irrigation was examined using different approaches. The movement of irrigation water within the tree was assessed with isotope and dye tracing techniques, which showed a sectorial transport in apple tree xylem. This implies that water absorbed from one side of the tree’s roots gets preferentially transported to the portion of canopy directly connected to those same roots. This finding opens up new questions about the allocation of nutrients in the canopy, which could show a heterogenous distribution with possible consequences on fruit quality in different parts of the same tree. Future climate scenarios foresee a reduced water availability for irrigation, coupled with an increased risk of stress conditions. The results of this work show that, even if growing pome fruit trees will become more challenging in the coming decades, knowledge and technology can help to overcome many obstacles. A deep understanding of tree physiology, coupled with proper orchard monitoring and precise irrigation management can help farmers to continue achieve outstanding productive results, while limiting the pressure on natural resources. To this end, a continuous knowledge exchange between research centers and practitioners is crucial.