PHOTOSYNTHESIS IN THE GREEN STEMS OF WOODY PLANTS HAS THE POTENTIAL TO INCREASE BOTH PLANT CARBON GAIN AND WATER-USE EFFICIENCY, THE RATION OF CARBON GAINED TO WATER LOSS. MOREOVER, RECENT RESEARCH HAS SUGGESTED THAT STEM PHOTOSYNTHESIS MAY ALSO PLAY AN UNEXPECTEDLY IMPORTANT ROLE IN THE MAINTENANCE OF STEM WATER FLOW (I.E. HYDRAULIC FUNCTIONING). GIVEN THE THREAT OF REDUCED WATER AVAILABILITY FOR IRRIGATED AGRICULTURE, IT IS IMPORTANT TO UNDERSTAND THE POSSIBLE LINKS BETWEEN STEM PHOTOSYNTHESIS AND DROUGHT TOLERANCE IN AGRICULTURALLY-IMPORTANT PLANTS. BOTH THE SAPLINGS AND BRANCHES OF MATURE TREES OF AVOCADO, A CRITICALLY IMPORTANT CROP IN CALIFORNIA, HAVE PHOTOSYNTHETIC STEMS. HENCE, THE PRIMARY GOAL OF THIS PROJECT IS TO UNDERSTAND THE ROLE OF STEM PHOTOSYNTHESIS IN IMPROVING WATER-USE EFFICIENCY AND MAINTAINING HYDRAULIC FUNCTIONING OF AVOCADO PLANTS DURING DROUGHT.IN THIS GRANT PROPOSAL, WE WILL STUDY THE POSSIBLE ROLE OF STEM PHOTOSYNTHESIS IN PLANT DROUGHT RESPONSES. MORE SPECIFICALLY, WE WILL 1) DETERMINE VARIABILITY IN STEM CARBON GAIN, WATER-USE EFFICIENCY, AND WATER FLOWAMONG 24 AVOCADO CULTIVARS GROWING IN A COMMON GARDEN, AND 2) DETERMINE THE ROLE OF STEM PHOTOSYNTHESIS IN STEM WATER FLOW IN TWO COMMERCIALLY GROWN AVOCADO CULTIVARS IN A GREENHOUSE EXPERIMENT. IMPROVED WATER-USE EFFICIENCY AND MAINTENANCE OF HYDRAULIC FUNCTION ARE TWO KEY TRAITS THAT CAN ALLOW AVOCADO CULTIVARS TO RESIST THE THREATS POSED BY REDUCED WATER AVAILABILITY, WITH IMPORTANT IMPLICATIONS FOR PLANT GROWTH AND FRUIT PRODUCTION.