INTRAPLANT DIFFERENCES IN LEAF CHLOROPHYLL FLUORESCENCE PARAMETERS INPERENNIAL FRUITING PLANTS

Chlorophyll fluorescence analysis was used to examine intra-plant diff erences in the activity of photosystem II (PS II) associated with sour ce: sink balance, in mid summer, on pear (Pyrus communis L.), plum (Pr unus domestica L.), and apple (Malus domestica L.) trees growing in th e field. For all three species, maximal fluorescence, F(m), and the ra tio 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) a veraging 0.81). Significantly higher non-photochemical quenching (qN) of fluorescence for leaves on extension shoots (averaging 0.41) compar ed to leaves on fruiting spurs (averaging 0.38) was consistent with '' down regulation'' of photosynthesis. For pear, qN was lower than for p lum and apple, indicating inter-specific differences in mechanisms of resistance to photoinhibition. For apple, qN was significantly lower f or leaves in shaded portions of the canopy (0.33), compared to that fo r leaves in exposed portions of the canopy (0.37). The quantum efficie ncy of photosynthetic electron transport, PHI(e), measured in the fiel d at varying quantum flux density (Q), did not differ significantly fo r leaves on extension shoots or spurs, for any species tested. For app les, F(v)/F(m) was significantly lower for leaves on trees that had be en deblossomed early in the season (averaging 0.81), than for leaves o n trees with fruit (averaging 0.82). This difference was mostly the re sult of increased basal fluorescence (F(o)). PHI(e) decreased with inc reasing Q more rapidly for trees without fruit than for trees with fru it.