Low sink demand limits photosynthesis under P-i deficiency

The role of the demand for carbon assimilates (the 'sink') in regulating ph otosynthetic carbon assimilation (P-n: the 'source') in response to phospha te (P-i) deficiency was examined in tobacco (Nicotiana tabacum L.). P-i sup ply was maintained or withdrawn from plants, and in both treatments the sou rce/sink ratio was decreased in some plants by darkening all but two source leaves (partially darkened plants), The remaining plants were kept fully i lluminated, P-i-sufficient plants showed little variation in rate of P-n, a mounts of P-i or phosphorylated intermediates. Withdrawal of P-i decreased P-n by 75% under the growing conditions and at both low and high internal C O2 concentration, Concomitantly, P-i, phosphorylated intermediates and ATP contents decreased and starch increased, RuBP and activity of phosphoribulo kinase closely matched the changes in P-n, but Rubisco activity remained hi gh. Partial darkening P-i-deficient plants delayed the loss of photosynthet ic activity; Rubisco and phosphoribulokinase activities and amounts of sucr ose and metabolites, particularly RuBP and G6P, were higher than in fully i lluminated Pi-deficient plants, Rates of sucrose export from leaves were mo re than 2-fold greater than in fully illuminated P-i-deficient plants. Grea ter sucrose synthesis, facilitated by increased G6P content, an activator o f SPS, would recycle P-i from the cytosol back to the chloroplast, maintain ing ATP, RuBP and hence P-n, It is concluded that low sink strength imposes the primary limitation on photosynthesis in P-i-deficient plants which res tricts sucrose export and sucrose synthesis imposing an end-product synthes is limitation of photosynthesis.