UNDERSTANDING WHOLE TREE NONSTRUCTURAL CARBOHYDRATE STORAGE AND ALLOCATION IN ALMOND TREE CROPS

<p dir="ltr">Perennial tree crops store nonstructural carbohydrates (NSCs) as energy reserves that can be used to persist during both predictable periods of reduced activity like dormancy and more unpredictable periods associated with stress. Understanding how the storage and usage of NSCs responds to changing environments is critical, as tree crops are increasingly challenged by disturbances that impair photosynthate production and require them to rely on previously stored NSC reserves. For one, almond trees grown in managed systems are bred and managed primarily for traits that contribute to productivity and yield. A potential downside to these management practices is the allocation of carbon (C) to yield at the expense of accumulating NSC reserves throughout the tree. If almond trees are operating on lower NSC reserves, then the impacts of extreme climate events may be felt in the future, especially if heat and water stress cannot simply be ameliorated by increasing irrigation due to water scarcity/limitations. Thus far, our understanding of how high-yielding almond cultivars allocate C resources to storage and carry out physiological processes under variable environmental conditions remains limited. To address the major knowledge gaps in our understanding of the storage and use of NSCs in almond trees, the research outlined in this thesis investigates (1) the seasonal fluctuation of NSCs in relation to growth and yield in almond trees grown in a managed orchard and (2) the belowground allocation of C to root exudation under water-stressed conditions in a greenhouse-based study.</p><p dir="ltr">In the first chapter, I provided a brief overview of the history of almond production as well as their C storage and allocation dynamics to provide background and context for my two thesis projects.</p><p dir="ltr">In the second chapter, I performed a detailed assessment of whole-tree NSC concentrations at the start and end of dormancy in four almond cultivars and related the seasonal fluctuation of NSCs surrounding dormancy to physiological processes that occurred after dormancy like stem growth and yield. I also assessed the potential trade-off between stem growth and yield using historical data. My main findings suggest that NSCs stored in the branches by the start of the dormant season are correlated with the next year’s yield, almond trees have prioritized yield at the expense of stem growth over time, and almond trees operate on lower NSC reserves compared to forest trees.</p><p dir="ltr">In the third chapter, I moved belowground to focus on root exudation into the rhizosphere, a process that is estimated to consume a substantial amount of photosynthetically fixed C. I set out to understand shifts in NSC storage and the metabolite profile of root exudates under water-stressed conditions in almond trees. Almond saplings were exposed to drought and once stomatal closure was reached, conditions were maintained for 1, 7, or 10 days prior to collecting root exudates and harvesting organs for NSCs. My main findings suggest that root NSCs were higher in the drought plants compared to well-watered controls and declined over time, the decrease in NSCs was small enough that the amount of C allocated to root exudation may have been negligible, and the metabolite profiles of drought and well-watered plants differed from each other.</p><p dir="ltr">Together the findings outlined in this thesis advance our understanding of C dynamics in tree crops grown in agroecosystems and provide foundational knowledge for future studies that aim to enhance climate resiliency in tree crops and improve orchard management strategies that optimize tree health and yield.</p>