Intra‐plant differences in leaf chlorophyll fluorescence parameters in perennial fruiting plants

Chlorophyll fluorescence analysis was used to examine intra‐plant differences in the activity of photosystem II (PS II) associated with source : sink balance, in mid summer, on pear ( Pyrus communis L.), plum ( Prunus domestica L.), and apple ( Malus domestica L.) trees growing in the field. For all three species, maximal fluorescence, F m , and the ratio of variable to maximal fluorescence for dark adapted leaves, F v /F m , were significantly higher for leaves from fruiting spurs (F v /F m averaging 0.83) than for leaves from extension shoots (F v /F m averaging 0.81). Significantly higher non‐photochemical quenching (qN) of fluorescence for leaves on extension shoots (averaging 0.41) compared to leaves on fruiting spurs (averaging 0.38) was consistent with “down regulation” of photosynthesis. For pear, qN was lower than for plum and apple, indicating inter‐specific differences in mechanisms of resistance to photoinhibition. For apple, qN was significantly lower for leaves in shaded portions of the canopy (0.33), compared to that for leaves in exposed portions of the canopy (0.37). The quantum efficiency of photosynthetic electron transport, F e , measured in the field at varying quantum flux density (Q) , did not differ significantly for leaves on extension shoots or spurs, for any species tested. For apples, F v /F m was significantly lower for leaves on trees that had been deblossomed early in the season (averaging 0.81), than for leaves on trees with fruit (averaging 0.82). This difference was mostly the result of increased basal fluorescence (F o ). F e decreased with increasing Q more rapidly for trees without fruit than for trees with fruit.