Physiological responses of Prunus cerasus to whole-plant source manipulation. Leaf gas exchange, chlorophyll fluorescence, water relations and carbohydrate concentrations.

The source-sink ratio of 1-year-old, potted sour cherry trees ( Prunus cerasus L.) was altered by whold-plant partial defoliation or continuous illumination to determine if trees were primarily sink limited and to elucidate the means whereby photosynthetic enhancement or inhibition occurs. Leaf xylem water potential was not affected by either treatment. Although stomatal conductance was reduced by 1 to 3 days of continuous illumination, internal CO₂ concentration was not significantly affected indicating that the enhanced physical limitation imposed by the 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 continuious illumination. The increase in A in partially defoliated plants was associated with reduced leaf starch and increased surose and sorbitol concentrations. The decrease in A in continuously illuminated plants was associated with a decrease in variable fluorescence, photochemical efficiency of photosystem II (PSII) and an increase in instantaneous fluorescence, indicating that leaves were photoinhibited and that irreversible damage had occurred to PSII. In addition, leaves of continuously illuminated plants had 80% more starch and significantly less sucrose and sorbitol. These altered leaf carbohydrate concentrations indicate that the existing sink limitation 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 to the same degree to lead to the observed responses remains equivocal.