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ply was maintained or withdrawn from plants, and in both treatments the sou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.