Influence of leaf position, fruit and light availability on photosynthesisof two chestnut genotypes

The effects of canopy leaf position, light availability and the presence of fruit on leaf net photosynthesis rate (P-n) were evaluated in a chestnut o rchard located in Central Italy. The study was carried out on Castanea sati va Mill. cv. Marrone di Stroncone', native to the Umbria Region, and on an interspecific hybrid Castanea crenata x Castanea sativa, cv. 'Marigoule'. ' Marrone di Stroncone' had a higher mean P-n than 'Marigoule'. In both genot ypes, P-n changed during the growing season in response to phenological sta ges and, in particular, was higher during the main fruit growth period. Fro m July to October the part of the shoot with higher P-n moved progressively from the base to the tip. During the day, P-n was high in the morning, and decreased progressively in the afternoon. The nearby presence of fruit inc reased P-n of leaves on shoots exposed to full sunlight, especially during the morning, but did not significantly influence the leaf chlorophyll (Chl) or carbohydrate contents. Light saturation for P-n was relatively high and there was a drastic reduction in P-n in leaves which, due to their positio n, intercepted a photosynthetic photon flux density (PPFD) lower than 300-4 00 mu mol photon m(-2) s(-1) during the day. Shaded leaves were ca. 26% thi nner than sunlit ones. Both the epidermal and mesophyll tissues were thicke r in sunlit leaves, and the "density" of cells was higher in sunlit leaves. Morpho-anatomical and physiological adaptations allow the chestnut to opti mise its use of the limited radiant energy available, but shading greatly r educes productivity. From the results it is affirmed that studies which con sider photosynthetic assimilation must take into account the genotype, time of the day, growing season, leaf position and influence of the fruit. In o rder to increase the efficiency of the tree, it is important to ensure opti mal conditions for whole canopy P-n (light interception, water and nutrient availability, leaf integrity. etc.), particularly during those times when demand fur assimilates is high. The chestnut genotypcs studied have a very low P-n compared to other temperate fruit and nut trees. (C) 2000 Elsevier Science B.V. All rights reserved.