In August, low source-sink ratios precipitated leaf senescence, resulting in a drastic ANmax decline, from 25 to 8 μmol m-1 s-1 (70% drop in two weeks). Branches with high-sink showed an up-regulation (increase) in photosynthesis toward the end of July (at 1,500 growing degree days) during the period of rapid kernel growth rate and increased sink strength, with ANmax been about 7 μmol m-1 s-1 higher than in branches with low-sink. In leaves with high source-sink ratios, there was a gradual decrease in maximum net photosynthetic rate (ANmax) over the growing season, while in branches with low source-sink ratios, there was a sharp decline in ANmax in the first two weeks of August. In addition, we determined changes in leaf non-structural carbohydrates (NSC) and nitrogen (N) concentration. We then characterized the leaf photosynthetic light response curves through leaf aging in pistachio tree branches. In this study, we manipulated the branch level source-sink ratio by reduction of photosynthetic activity (partial branch defoliation) or thinning branch fruit load early in the growing season (after fruit set). Tree source-sink ratio has a predominant and complex impact on tree performance and can affect multiple physiological process including vegetative and reproductive growth, water and nutrient use, photosynthesis, and productivity. We investigated the impact of branch source-sink ratio on leaf seasonal photosynthetic performance in pistachio (Pistacia vera) trees. You just subscribed to receive the final version of the article
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