Effect of branch crop load on assimilation rates and nut growth of fully irrigated pistachio

In this study, we manipulated crop load in pistachio branches to study its effect on leaf net photosynthetic performance (A max) and nut growth. In spring we imposed two treatments: high crop load (branches with 3-4 clusters) and low crop load (branches thinned, with only 1 cluster left). The manipulative treatments influenced the seasonal trend of photosynthesis. Highly loaded branches maintained higher rates of A max at the beginning of the season, during shell hardening (Stage II), but sharply dropped when kernel started to grow (Stage III). In low loaded branches, A max decline was constant through the season. High crop load resulted in reduced nut growth both at the beginning of the season, influencing shell and hull expansion (Stage I), and at the end of the season, influencing kernel growth. Competition for resources can have a strong and complex impact on pistachio photosynthesis and nut growth. Crop load effect should be integrated in management practices aiming to achieve optimal productive and quality objectives.