END-PRODUCT INHIBITION OF PHOTOSYNTHESIS IN PRUNUS-CERASUS L IN RESPONSE TO WHOLE-PLANT SOURCE-SINK MANIPULATION

The source-sink ratio of 1-year-old, potted 'Montmorency' sour cherry (Prunus cerasus) trees was manipulated by partial defoliation (D) or c ontinuous lighting (CL) to investigate the phenomenon of end-product i nhibition of photosynthesis. Within 24 hours of D, net CO2 assimilatio n rate (A) of the most recently expanded source leaves of D plants was significantly higher than nondefoliated (control) plants throughout t he diurnal photoperiod. Between 2 and 7 days after D, A was 30% to 50% higher and stomatal conductance rate (g(s)) was 50% to 100% higher th an 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. Lea f starch concentration and dark respiration rate decreased but sorbito l and sucrose concentration increased after D. The diurnal decline in A in the afternoon after D may have been due to feedback inhibition fr om 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 chlorophy ll content up to 32 days after D. CL resulted in a significant reducti on of A, g(s), k, variable chlorophyll fluorescence (F-v), photochemic al efficiency, and estimated RuBP regeneration rate of the most recent ly expanded source leaves within 1 day. During the exposure to CL, A w as reduced 2- to 3-fold and k was reduced up to 4-fold. The normal lin ear relationship between A and g(s) was uncoupled under CL indicating that A was not primarily limited by g(s) and since internal CO2 concen tration was not significantly affected, the physical limitation to A i mposed by the stomata was negligible. The decrease in F-v and photoche mical efficiency indicated that leaves were photoinhibited within 1 da y. The decrease in instantaneous chlorophyll fluorescence after at lea st 1 day of CL indicated that there was a reversible regulatory mechan ism whereby the damage to photosystem II reaction centers was repaired . Leaf chlorophyll content was not altered by 1, 2, or 3 days of expos ure to CL, indicating that photooxidation of chlorophyll did not occur . The time to full photosynthetic recovery from CL increased as the du ration of exposure increased. CL plants that were photoinhibited accum ulated significant starch in the chloroplast in a companion study (Lay ne and Flore, 1993) and it is possible that an orthophosphate limitati on in the chloroplast stroma was occurring. D plants that were continu ously illuminated were not photosynthetically inhibited. After 7 days of CL, plants that were then partially defoliated yet remained in CL p hotosynthetically 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 p lants was different than in CL plants.