Studies on the distribution, re-translocation and homeostasis of inorganic phosphate in barley leaves

Changes in inorganic phosphate (P-i) concentrations in barley leaves during growth of plants with sufficient or deficient supplies of P-i were studied, Measurements of the P-i distribution from subcellular levels to the leaf tissue level under the same experimental conditions allowed us to analyse the relationship between the P-i homeostasis of various compartments and P-i re-translocation in the whole plant. Under P-i deficiency, the finding of growth-dependent changes in the P-i concentrations of whole leaves established that P-i was re-translocated from 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 compartments of the leaves was measured, Under P-i deficiency, the cytoplasmic P-i concentration of the first leaf remained constant until 16 d after 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 appearance of the second leaf, The P-i concentration in the apoplast changed similarly to that of the whole leaf, However, the apoplastic P-i concentration was affected to some extent by the vacuolar P-i concentration and the growth of the younger leaf, because the main change in apoplastic 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 concentration 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 environment (a few mmol m(-3)). This suggests that the P-i absorbed by root 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 irrespective of growth culture conditions. The root may be functioning as the constant P-i supplier to the above tissues.