SOURCE-SINK ATTENUATION OF PHOTOSYNTHESIS REDUCES CROP NET ASSIMILATION RATE (NAR). TO ADDRESS THIS REDUCTION, OUR GOAL IS TO QUANTIFY THE RELATIONSHIPS BETWEEN ALLOCATION, TRANSLOCATION AND PHOTOSYNTHESIS IN WOODY PERENNIAL CROPS AND TO DEVELOP STRATEGIES TO OPTIMIZE THESE RELATIONSHIPS. OUR MODEL IS CITRUS BECAUSE IT HAS A DIVERSE MULTI-SPECIES GERMPLASM AND A LARGE INDUSTRY. THE PURPOSE OF THIS PROPOSAL IS TO QUANTIFY THE INTERACTION OF SOURCE-SINK ALLOCATION WITH PHLOEM TRANSPORT SPEEDS AND PHOTOSYNTHESIS IN CITRUS, BOTH ON INDIVIDUAL AND POPULATION LEVELS. OUR CENTRAL HYPOTHESIS IS THAT SELECTION FOR GREATER RELATIVE SINK ALLOCATION WILL LEAD TO GREATER NAR UNTIL PHLOEM LOADING AND TRANSPORT BECOME LIMITING.OUR SPECIFIC OBJECTIVES ARE TO:1.QUANTIFY THE RELATIVE CONTROL OF SOURCE EXPORT, SINK IMPORT AND PHLOEM TRANSLOCATION ON PHOTOASSIMILATE FLUX IN CITRUS. WE WILL COMBINE GROWTH ANALYSIS WITH A UNIQUE APPROACH TO MEASURING 14-C TRANSLOCATION RATES IN VIVO TO QUANTIFY SOURCE-SINKAND TRANSLOCATION FLUXES.2.DETERMINE THE EFFECT OF SOURCE EXPORT ON REGULATION OF PHOTOSYNTHESIS. RAPID A/CI RESPONSE METHODS WILL CHARACTERIZE LEAF-LEVEL PHOTOSYNTHETIC ACTIVITY.3.APPLY THE RELATIONSHIPS DESCRIBED IN OBJECTIVES 1 AND 2 ON A POPULATION LEVEL IN BREEDING FAMILIES WITH OBSERVED VARIATIONS IN SOURCE-SINK RELATIONSHIPS. THE SAME METHODS WILL BE EMPLOYED TO ASSESS SOURCE-SINK OPTIMIZATION OF NAR IN A BREEDING POPULATION, INCLUDING ASSESSING SOURCE-SINK TRANSPORT TO FRUITS.THIS PROJECT WILL ADDRESS THE PROGRAM PRIORITY OF "IMPROVING PLANT PERFORMANCE AND PRODUCTIVITY" BY PROVIDING BREEDERS WITH SELECTABLE TRAITS TO ENHANCE NAR OF CULTIVATED CITRUS BY QUANTIFYING THE RELATIONSHIP BETWEEN SOURCE-SINK ALLOCATION, TRANSLOCATION AND PHOTOSYNTHESIS.