Responses of the rate of net CO2 assimilation (A) to the intercellular partial pressure of CO2 (p i ) were measured on intact spinach (Spinacia oleracea L.) leaves at different irradiances. These responses were analysed to find the value of p i at which the rate of photosynthetic CO2 uptake equalled that of photorespiratory CO2 evolution. At this CO2 partial pressure (denoted Г), net rate of CO2 assimilation was negative, indicating that there was non-photorespiratory CO2 evolution in the light. Hence Г was lower than the CO2 compensation point, Γ. Estimates of Г were obtained at leaf temperatures from 15 to 30°C, and the CO2/O2 specificity of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase (E.C. 4.1.1.39) was calculated from these data, taking into account changes in CO2 and O2 solubilities with temperature. The CO2/O2 specificity decreased with increasing temperature. Therefore we concluded that temperature effects on the ratio of photorespiration to photosynthesis were not solely the consequence of differential effects of temperature on the solubilities of CO2 and O2. Our estimates of the CO2/O2 specificity of RuBP carboxylase/oxygenase are compared with in-vitro measurements by other authors. The rate of nonphotorespiratory CO2 evolution in the light (R d ) was obtained from the value of A at Г. At this low CO2 partial pressure, R d was always less than the rate of CO2 evolution in darkness and appeared to decrease with increasing irradiance. The decline was most marked up to about 100 μmol quanta m(-2) s(-1) and less marked at higher irradiances. At one particular irradiance, however, R d as a proportion of the rate of CO2 evolution in darkness was similar in different leaves and this proportion was unaffected by leaf temperature or by [O2] (ambient and greater). After conditions of high [CO2] and high irradiance for several hours, the rate of CO2 evolution in darkness increased and R d also increased.
When spinach plants were transferred to nutrient solutions without phosphorus, the photosynthetic rate per unit leaf area gradually declined. Stomatal conductance also decreased but was not the sole cause of the decreased photosynthetic rate because the partial pressure of CO2 in the intercellular spaces (CI) was unaltered. Measurements of the photosynthetic rate as a function of CI indicated reductions in both ribulosebisphosphate (RuP2) carboxylase activity and RuP2 regeneration capacity. From assays of RuP2 carboxylase activity in vitro and 'percentage activation', it was concluded that low-P leaves had less enzyme per unit area than controls and that the enzyme was also less activated. The photosynthetic quantum yield was reduced by phosphorus deficiency with no effect on leaf absorptance or chlorophyll content. The reduced quantum yield was accompanied by changes in chlorophyll fluorescence of photosystems I and II measured at 77K. However, since phosphorus deficiency did not affect the uncoupled rate of whole-chain electron transport in vitro, some factor(s) other than photoinhibition probably contributed to the reduced quantum yield. The lack of effect on this electron-transport rate also indicates that the maximal RuP2 regeneration rate in low-P leaves was not limited by the amount of electon-transport components. At ambient [CO2], low-P leaves had significantly less RuP2 and 3-phosphoglycerate (PGA) than controls and the response of photosynthesis to low [O2] was similar to control leaves. Therefore photosynthesis did not appear to be limited by triose-phosphate utilization. The low concentrations of RuP2 and PGA (and presumably other Calvin-cycle intermediates) might have reduced the rate of the Calvin cycle. After returning low-P plants to nutrient solutions with PO4, the percentage activation of RuP2 carboxylase, amounts of RuP2 and PGA, quantum yield and maximal RuP2 regeneration rate increased within 24 h. The quantum yield and photosynthetic rate at higher irradiance also increased when leaf discs from low-P plants were floated on 10 mM PO4 solutions for 2 h.
Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.
Previous reports indicate that ribulose 1,5-bisphosphate (RuBP) binds very tightly to inactive ribulose bisphosphate carboxylase (rubisco)
Phosphorus-deficient spinach plants were grown by transferring them to nutrient solutions without PO4. Photosynthetic rates were measured at a range of intercellular CO2 partial pressures from 50-500 μbar and then the leaves were freeze-clamped in situ to measure ribulose bisphosphate carboxylase (Rubisco) activity and metabolite concentrations. Compared with control leaves, deficient leaves had significantly lower photosynthetic rates, percentage activation of Rubisco, and amounts of ribulose bisphosphate and 3-phosphoglycerate at all CO2 partial pressures. After feeding 10 mM PO4 to the petioles of detached deficient leaves, all these measurements increased within 2 hours. At atmospheric CO2 partial pressure the photosynthetic rate was stimulated in 19 mbar O2 compared with 200 mbar. At higher CO2 partial pressures this stimulation was less but the percentage stimulation in deficient leaves was no different from controls in either CO2 partial pressure. It was concluded that phosphorus deficiency affects both Rubisco activity and the capacity for ribulose bisphosphate regeneration, and possible causes are discussed.
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