lhe relationship between the susceptibility of photosystem II (PSII) to photoinhibition in vivo and the rate of degradation of the D1 protein of the PSll reaction center heterodimer was investigated in leaves from pea plants (Pisum safivum 1. cv Creenfeast) grown under widely contrasting irradiances. lhere was an inverse linear relationship between the extent of photoinhibition and chlorophyll (Chl) a/b ratios, with low-light leaves being more susceptible to high light. In the presence of the chloroplast-encoded protein synthesis inhibitor lincomycin, the differential sensitivity of the various light-acclimated pea leaves to photoinhibition was largely removed, demonstrating the importance of D1 protein turnover as the most crucial mechanism to protect against photoinhibition. In the differently light-acclimated pea leaves, the rate of D1 protein degradation (measured from [35S]methionine pulse-chase experiments) increased with increasing incident light intensities only if the light was not high enough to cause photoinhibition in vivo. Under moderate illumination, the rate constant for D1 protein degradation corresponded to the rate constant for photoinhibition in the presence of lincomycin, demonstrating a balance between photodamage to D1 protein and subsequent recovery, via D1 protein degradation, de novo synthesis of precursor D1 protein, and reassembly of functional PSII. In marked contrast, in light sufficiently high to cause photoinhibition in vivo, the rate of D1 protein degradation no longer increased concomitantly with increasing photoinhibition, suggesting that the rate of D1 protein degradation is playing a regulatory role. lhe extent of thylakoid stacking, indicated by the Chl a/b ratios of the differently lightacclimated pea leaves, was linearly related to the half-life of the D1 protein in strong light. We conclude that photoinhibition in vivo occurs under conditions in which the rate of D1 protein degradation can no longer be enhanced to rapidly remove irreversibly damaged D l protein. We suggest that low-light pea leaves, with more stacked membranes and less stroma-exposed thylakoids, ate more susceptible to photoinhibition in vivo mainly due to their slower rate of D1 protein degradation under sustained high light and their slower repair cycle of the photodamaged PSll centers.Photoinhibition of PSII is a phenomenon that occurs in a11 oxygenic photosynthetic organisms under high irradiance (Powles, 1984;Prasil et al., 1992). Although light is the driving force of PSII, it also inactivates electron transport and destroys structural components of the PSII reaction center. Susceptibility of plants to photoinhibition under any given '

Leaf discs of the shade plant Tradescantia albiflora Kunth grown at 50 μmol · m(-2) · s(-1), and the facultative sun/shade plant Pisum sativum L. grown at 50 or 300 μmol · m(-2), s(-1), were photoinhibited for 4 h in 1700 μmol photons m(-2) · s(-1) at 22° C. The effects of photoinhibition on the following parameters were studied: i) photosystem II (PSII) function; ii) amount of D1 protein in the PSII reaction centre; iii) dependence of photoinhibition and its recovery on chloroplast-encoded protein synthesis; and, iv) the sensitivity of photosynthesis to photoinhibition in the presence or absence of the carotenoid zeaxanthin. We show that: i) despite different sensitivities to photoinhibition, photoinhibition in all three plants occurred at the reaction centre of PSII; ii) there was no correlation between the extent of photoinhibition and the degradation of the D1 protein; iii) the susceptibility to photoinhibition by blockage of chloroplas-tencoded protein synthesis was much less in shade plants than in plants acclimated to higher light; and iv) inhibition of zeaxanthin formation increased the sensitivity to photoinhibition in pea, but not in the shade plant Tradescantia. We suggest that there are mechanistic differences in photoinhibition of sun and shade plants. In sun plants, an active repair cycle of PSII replaces photoinhibited reaction centres with photochemically active ones, thereby conferring partial protection against photoinhibition. However, in shade plants, this repair cycle is less important for protection against photoinhibition; instead, photoinhibited PSII reaction centres may confer, as they accumulate, increased protection of the remaining connected, functional PSII centres by controlled, nonphotochemical dissipation of excess excitation energy.

D1 protein turnover and restoration of the photochemical efficiency of photosystem II (PSII) after photoinhibition of pea leaves (Pisum sativum 1. cv Creenfeast) acclimated to different light intensities were investigated. All peas acclimated to different light intensities were able to recover from photoinhibition, at least partially, at light intensities far above their growth light irradiance. However, the capacity of pea leaves to recover from photoinhibition under increasing high irradiances was strictly dependent on the light acclimation of the leaves; i.e. the higher the irradiance during growth, the better the capacity of pea leaves to recover from photoinhibition at moderate and high light. In our experimental conditions, mainly D1 protein turnover-dependent recovery was monitored, since in the presence of an inhibitor of chloroplastencoded protein synthesis, lincomycin, only negligible recovery took place. In darkness, neither the restoration of PSll photochemical efficiency nor any notable degradation of damaged D1 protein took place. In low light, however, good recovery of PSll occurred in all peas acclimated to different light intensities and was accompanied by fast degradation of the D1 protein. The rate of degradation of the D1 protein was estimated to be 3 to 4 times faster in photoinhibited leaves than in nonphotoinhibited leaves under the recovery conditions of 50 pmol of photons m-'s-'. In moderate light of 400 pmol of photons m-* s-', the photoinhibited low-light peas were not able to increase further the rate of D1 protein degradation above that observed in nonphotoinhibited leaves, nor was the restoration of PSll function possible. On the other hand, photoinhibited high-light leaves were able to increase the rate of D1 protein degradation above that of nonphotoinhibited leaves even in moderate and high light, ensuring at least partia1 restoration of PSll function. We conclude that the capacity of photoinhibited leaves to restore PSll function at different irradiances was directly related to the capacity of the leaves to degrade damaged D1 protein under the recovery conditions.

Global food security in a changing climate depends on both the nutritive value of staple crops as well as their yields. Here, we examined the direct effect of atmospheric CO(2) on cassava (Manihot esculenta Cranz., manioc), a staple for 750 million people worldwide. Cassava is poor in nutrients and contains high levels of cyanogenic glycosides that break down to release toxic hydrogen cyanide when damaged. We grew cassava at three concentrations of CO(2) (C(a): 360, 550 and 710 ppm) supplied together with nutrient solution containing either 1 mM or 12 mM nitrogen. We found that total plant biomass and tuber yield (number and mass) decreased linearly with increasing C(a). In the worst-case scenario, tuber mass was reduced by an order of magnitude in plants grown at 710 ppm compared with 360 ppm CO(2). Photosynthetic parameters were consistent with the whole plant biomass data. It is proposed that since cassava stomata are highly sensitive to other environmental variables, the decrease in assimilation observed here might, in part, be a direct effect of CO(2) on stomata. Total N (used here as a proxy for protein content) and cyanogenic glycoside concentrations of the tubers were not significantly different in the plants grown at elevated CO(2). By contrast, the concentration of cyanogenic glycosides in the edible leaves nearly doubled in the highest C(a). If leaves continue to be used as a protein supplement, they will need to be more thoroughly processed in the future. With increasing population density, declining soil fertility, expansion into marginal farmland, together with the predicted increase in extreme climatic events, reliance on robust crops such as cassava will increase. The responses to CO(2) shown here point to the possibility that there could be severe food shortages in the coming decades unless CO(2) emissions are dramatically reduced, or alternative cultivars or crops are developed.

Summary• The response of biological nitrogen fixation (BNF) to elevated CO 2 was examined in white clover ( Trifolium repens )-dominated swards under both high and low phosphorus availability.• Mixed swards of clover and buffalo grass ( Stenotaphrum secundatum ) were grown for 15 months in 0.2 m 2 sand-filled mesocosms under two CO 2 treatments (ambient and twice ambient) and three nutrient treatments [no N, and either low or high P (5 or 134 kg P ha − 1 ); the third nutrient treatment was supplied with high P and N (240 kg N ha − 1 )].• Under ambient CO 2 , high P increased BNF from 410 to 900 kg ha − 1 . Elevated CO 2 further increased BNF to 1180 kg ha − 1 with high P, but there was no effect of CO 2 on BNF with low P. Allocation of N belowground increased by approx. 50% under elevated CO 2 irrespective of supplied P.• The results suggest that where soil P availability is low, elevated CO 2 will not increase BNF, and pasture quality could decrease because of a reduction in aboveground N.

Thirty-six mesocosms, each containing a two-species community of Trifolium repens (C 3 legume) and Stenotaphrum secundatum (C 4 grass), were grown in sand with three nutrient regimes, zero N low P, zero N high P and supplied N high P, under ambient (aCO 2 ) and twice ambient CO 2 (eCO 2 ) for 15 months in two greenhouses. Aboveground annual production in the P limited mesocosms did not respond to eCO 2 and was reduced by 50% relative to mesocosms with an adequate P supply, where dry-matter production was increased by 12-24% under eCO 2 . The stimulation of production by eCO 2 occurred throughout the year despite a clear seasonality in growth. There was no effect of eCO 2 on leaf area index (LAI), which was larger under high P than low P. Live root mass at the end of the experiment was higher under eCO 2 in all nutrient treatments, but the response of total belowground C (root 1 soil) to eCO 2 depended on P treatment. Under limiting P, belowground C was not significantly changed by eCO 2 (2-2.3 t belowground C ha À1 ). Under high P supply, both root and soil C pools increased under eCO 2 . Under aCO 2 , low P supply increased belowground C by 0.7-1 t C ha À1 above that added by the high P treatment. P is commonly limiting in Australian ecosystems and the majority of ecosystem N input is provided by biological N fixation. Consequently, the response of legumes to eCO 2 is of particular importance. These results demonstrate that at low P availability, there is likely to be only a limited response of biomass production by T. repens to eCO 2 , which in turn may constrain any ecosystem response.

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