STUDIES ON THE DISTRIBUTION, RE-TRANSLOCATION AND HOMEOSTASIS OF INORGANIC-PHOSPHATE IN BARLEY LEAVES

Changes in inorganic phosphate (P-i) concentrations in barley leaves d uring growth of plants with sufficient or deficient supplies of P-i we re studied, Measurements of the P-i distribution from subcellular leve ls to the leaf tissue level under the same experimental conditions all owed us to analyse the relationship between the P-i homeostasis of var ious compartments and P-i re-translocation in the whole plant. Under P -i deficiency, the finding of growth-dependent changes in the P-i conc entrations of whole leaves established that P-i was re-translocated fr om the older leaves to the young leaves, Translocation of P-32(i) was also confirmed with an 'imaging plate' system, which made it possible to follow P-i movement in the same plantlet. To analyse the mechanism of P-i re-translocation, the P-i distribution amongst various compartm ents of the leaves was measured, Under P-i deficiency, the cytoplasmic P-i concentration of the first leaf remained constant until 16 d afte r sowing, while vacuolar P-i was completely exhausted after 8 to 10 d, Exhaustion of vacuolar P-i in the first leaf coincided with the appea rance of the second leaf, The P-i concentration in the apoplast change d similarly to that of the whole leaf, However, the apoplastic P-i con centration was affected to some extent by the vacuolar P-i concentrati on and the growth of the younger leaf, because the main change in apop lastic P-i concentration coincided with the time of the disappearance of the vacuolar P-i and the appearance of the younger leaf. The P-i co ncentration in the apoplast was about 0.1 to 1 mol m(-3), even in the absence of P-i, which was much higher than that in the usual soil envi ronment (a few mmol m(-3)). This suggests that the P-i absorbed by roo t cells is concentrated in the transport process from the root to the leaf apoplast, The content of P-i in the xylem exudate was constant ir respective of growth culture conditions. The root may be functioning a s the constant P-i supplier to the above tissues.