End-product Inhibition of Photosynthesis in Prunus cerasus L. in Response to Whole-plant Source-Sink Manipulation

The source-sink ratio of l-year-old, potted 'Montmorency' sour cherry (Prunus cerasus) trees was manipulated by partial defoliation (D) or continuous lighting (CL) to investigate the phenomenon of end-product inhibition of photosynthesis. Within 24 hours of D, net CO2 assimilation rate (A) of the most recently expanded source leaves of D plants was significantly higher than nondefoliated (control) plants throughout the diurnal photoperiod. Between 2 and 7 days after D, A was 30% to 50% higher and stomatal conductance rate (g,) was 50% to 100% higher than in controls. Estimated carboxylation efficiency(k) and ribulose-1,5-bisphosphate (RuBP) regeneration rate increased significantly within 2 days and remained consistently higher for up to 9 days after D. Leaf starch concentration and dark respiration rate decreased but sorbitol and sucrose concentration increased after D. The diurnal decline in A in the afternoon after D may have been due to feedback inhibition from accumulation of soluble carbohydrates (sucrose and sorbitol) in the cytosol. This diurnal decline indicated that trees were sink limited. By 9 days after D, photochemical efficiency was significantly higher than in control plants. In the long term, leaf senescence was delayed as indicated by higher A and g S in combination with higher chlorophyll content up to 32 days after D. CL resulted in a significant reduction of A, g S, k, variable chlorophyll fluorescence (FV), photochemical efficiency, and estimated RuBP regeneration rate of the most recently expanded source leaves within 1 day. During the exposure to CL, A was reduced 2- to 3-fold and k was reduced up to 4-fold. The normal linear relationship between A and gS was uncoupled under CL indicating that A was not primarily limited by g S and since internal CO2 concentration was not significantly affected, the physical limitation to A imposed by the stomata was negligible. The decrease in FV and photochemical efficiency indicated that leaves were photoinhibited within 1 day. The decrease in instantaneous chlorophyll fluorescence after at least 1 day of CL indicated that there was a reversible regulatory mechanism whereby the damage to photosystem II reaction centers was repaired. Leaf chlorophyll content was not altered by 1,2, or 3 days of exposure to CL, indicating that photooxidation of chlorophytl did not occur. The time to full photosynthetic recovery from CL increased as the duration of exposure increased. CL plants that were photoinhibited accumulated significant starch in the chloroplast in a companion study (Layne and Flore, 1993) and it is possible that an orthophosphate limitation in the chloroplast stroma was occurring. D plants that were continuously illuminated were not photosynthetically inhibited. After 7 days of CL, plants that were then partially defoliated yet remained in CL photosyntheti- cally recovered within 5 days to pre-CL values. Under the conditions of this investigation, end-product inhibition of A occurred in young, potted sour cherry trees but the mechanism of action in D plants was different than in CL plants. Plant growth and productivity may be limited by the production or use of photosyntheti c carbon compounds (photoassimilates). Sucrose and starch are the primary end-products of leaf photosyn- thesis inmost higher plants, where sucrose is the main translocated carbohydrate and starch is a temporary storage carbohydrate that accumulates in the chloroplast. Sorbitol, a sugar alcohol, is an additional important translocated form of photoassimilate in Pru- nus species (Loescher, 1987). The capacity of sink tissues to use