Investigating crop ecophysiological response to water stress to improve precision irrigation strategies

Kiwifruit is a widely cultivated and economically important fruit crop. Italy is one of the largest producers of kiwifruit in the world and its industry plays an important role both nationally and internationally. In recent years, however, kiwifruit plantations and total production in several countries around the world, including Italy, have been threatened by a complex vine physiological decline syndrome triggered by soil water excess and unsuitable conditions. The current climate change scenario has serious implications for crop production and is certainly a new challenge for kiwifruit cultivation in many areas, urgently requiring more sustainable agricultural practices and adaptation strategies. Kiwifruit is known to be extremely sensitive to climate and irrigation levels, which affect the viability of kiwifruit growing areas. A low physiological tolerance to drought and waterlogging makes the soil water environment important for healthy growth and optimum production of kiwifruit vines. This thesis then focuses on evaluating the physiological responses of yellow-fleshed kiwifruit to water stress (both deficit and excess), thereby increasing the knowledge useful for implementing more precise irrigation strategies for this crop. Overall, the aims of this thesis were i) to comprehensively describe the physiological responses of yellow-fleshed kiwifruit vines to water stress, which is increasingly occurring due to climate change and improper irrigation management, ii) to implement more precise irrigation for such a sensitive crop in a very challenging environment (also due to the expected impacts of climate change), and iii) to evaluate the tolerance/susceptibility of different kiwifruit rootstocks to water stress in order to identify more tolerant genotypes to abiotic stress in kiwifruit. First, a field experiment was conducted to evaluate the physiological behavior of yellow-fleshed kiwifruit vines under varying soil water availability in a Mediterranean environment. This study showed that kiwifruit can cope with challenging Mediterranean environmental conditions by precisely managing irrigation, distributing the right amount of irrigation water to meet the vines' water needs at the right time and place. Drought stress was imposed to study the physiological responses of the vines, through the analysis of the diurnal trends of stem water potential and stomatal conductance, identifying the onset of stress. Daily dynamics of soil water availability reduction and root uptake highlighted the importance of water availability in the top soil layers to provide the amount of water needed by the vines. Soil moisture thresholds were identified to provide the most suitable soil water environment and guide irrigation decisions. In addition, plant measurements (i.e., sap flow and trunk water potential) were used in combination with a mechanistic plant model to characterize the hydraulic behavior of kiwifruit vines under different soil water availability conditions (i.e., well-watered and drought stress). As plant variables are monitored continuously, the modelling approach provides valuable information on the hydraulic functioning of vines under field conditions and throughout the season, and shows great potential for the development of irrigation decision support tools. In a further step, drought and waterlogging induced responses of different kiwifruit rootstocks and scion/rootstock combinations were investigated. The differential response of the investigated genotypes was evaluated through an integrated physiological and phenotyping approach. In particular, image analysis was performed at the ALSIA 'Metapontum Agrobios' research centre (PhenoLab platform), which is part of the European Plant Phenotyping Network and infrastructures. Physiological and image-based phenotyping assessment provided an effective methodology to conduct a screening for more tolerant kiwifruit genotypes to water stress. The 'Bounty' rootstock was identified as the least susceptible to water stress and 'Hayward' and 'D1' as the most susceptible. The results obtained will hopefully open the way to further studies that will support kiwifruit production in Italy and help to face new challenges in its cultivation.