PHYSIOLOGICAL-RESPONSES OF PRUNUS-CERASUS TO WHOLE-PLANT SOURCE MANIPULATION - LEAF GAS-EXCHANGE, CHLOROPHYLL FLUORESCENCE, WATER RELATIONSAND CARBOHYDRATE CONCENTRATIONS

The source-sink ratio of 1-year-old, potted sour cherry trees (Prunus cerasus L.) was altered by whole-plant partial defoliation or continuo us illumination to determine if trees were primarily sink limited and to elucidate the means whereby photosynthetic enhancement or inhibitio n occurs. Leaf xylem water potential was not affected by either treatm ent. Although stomatal conductance was reduced by 1 to 3 days of conti nuous illumination, internal CO2 concentration was not significantly a ffected indicating that the enhanced physical limitation imposed by th e stomata was of no physiological significance. Net CO2 assimilation ( A) was significantly higher 4 days after partial defoliation and lower from 1 to 4 days following continuous illumination. The increase in A in partially defoliated plants was associated with reduced leaf starc h and increased sucrose and sorbitol concentrations. The decrease in A in continuously illuminated plants was associated with a decrease in variable fluorescence, photochemical efficiency of photosystem II (PSI I) and an increase in instantaneous fluorescence, indicating that leav es were photoinhibited and that irreversible damage had occurred to PS II. In addition, leaves of continuously illuminated plants had 80% mor e starch and significantly less sucrose and sorbitol. These altered le af carbohydrate concentrations indicate that the existing sink limitat ion may have been aggravated by continuous illumination leading to an insufficient utilization of sucrose from the leaf. Whether the altered photochemical and biochemical events occurred simultaneously and/or t o the same degree to lead to the observed responses remains equivocal.