Photosynthetic net O₂ evolution enhancement as a sign of acclimation to phosphorus deficiency in bean (Phaseolus vulgaris L.) leaves

Abstract

Primary leaves of bean (Phaseolus vulgaris L.) seedlings cultivated for 14 days in a growth chamber on complete (control) and phosphate deficient (-P) Knop liquid medium were used for measurements. The -P leaves were smaller and showed an increased specific leaf area (SLA). Their inorganic phosphate (P i ) concentration was considerably lowered. They did not show any significant changes in chlorophyll (Chl) (a + b) concentration and in their net CO 2 assimilation rate when it was estimated under the conditions close to those of the seedlings growth. Light response curves of photosynthetic net O 2 evolution (P N O 2 ) of the leaves for the irradiation range up to 500 µmol(photon) m -2 s -1 were determined, using the leaf-disc Clark oxygen electrode. The measurements were taken under high CO 2 concentration of about 1 % and O 2 concentrations of 21 % or lowered to about 3 % at the beginning of measurement. The results obtained at 21 % O 2 and the irradiations close to or higher than those used during the seedlings growth revealed the phosphorus stress suppressive effect on the leaf net O 2 evolution, however, no such effect was observed at lower irradiations. Other estimated parameters of P N O 2 such as: apparent quantum requirement (QR A ) and light compensation point (LCP) for the control and -P leaves were similar. However, with a high irradiation and lowered O 2 concentration the rate of P N O 2 for the -P leaves was markedly higher than that for the control, in relation to both the leaf area and leaf fresh mass. This difference also disappeared at low irradiations, but the estimated reduced QR A values indicate, under those conditions, the increased yield of photosynthetic light reaction, especially in the -P leaves. The presented results confirm the suggestion that during the initial phase of insufficient phosphate feeding the acclimations in the light phase of photosynthesis, both structural and functional appear. They correspond, probably, to the increased energy costs of carbon assimilation under phosphorus stress, e.g. connected with raised difficulties in phosphate uptake and turnover and enhanced photorespiration. Under the experimental conditions especially advantageous for the dark phase of photosynthesis (saturating CO 2 and PAR, low O 2 concentration), those acclimations may be manifested as an enhancement of photosynthetic net O 2 evolution.