Girdling decreases photosynthetic electron fluxes and induces sustained photoprotection in mango leaves

Girdling has been suggested as a way to improve earliness and intensity of flowering in mango (Mangifer indica L.). However, the accumulation of carbohydrates associated with girdling may result in a decrease in photosynthesis. We assessed the long-term effect of branch girdling during the prefloral period on leaf net photosynthesis (A(n)) of 3-year old mango trees, cv. Cogshall, growing on La Reunion island. Leaf gas exchange and chlorophyll fluorescence parameters were measured monthly from March to August 2004 on recently matured leaves on girdled and non-girdled branches. Within 28 days after girdling, A(n) was reduced by 77% and remained at about 2 mu mol CO2 m(-2) s(-1) until the beginning of flowering. The decrease in photosynthetic electron transport rate (J) and sustained photoprotection (reflected by the decrease in predawn maximal efficiency of photosystem II) effectively protected leaves on girdled branches from photodamage, as shown by the vigorous recovery of A(n) and J observed immediately after the appearance of inflorescences. These increases in A(n) and J were unaccompanied by a decrease in leaf carbohydrate concentration during the first month following the onset of flowering, indicating that there are carbohydrate-dependent and carbohydrate- independent mechanisms of sink regulation of photosynthesis. It is concluded that girdling does not necessarily lead to irreversible damage, even in the presence of a fourfold increase in leaf starch concentration and in the absence of any sink activity. However, the decrease in leaf nitrogen concentration indicates that there may exist long-term negative effects of branch girdling on photosynthetic capacity. A modified version of the biochemical model of A(n) is presented that takes account of the effect of leaf starch concentration on J.