To finish this special issue, some friends, colleagues and students of Prof. Chow (Emeritus Professor, the Research School of Biology, the Australian National University) have written small tributes to acknowledge not only his eminent career but to describe his wonderful personality.

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 CO2 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 CO2 (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 CO2. 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 CO2 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 CO2. 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 CO2 shown here point to the possibility that there could be severe food shortages in the coming decades unless CO2 emissions are dramatically reduced, or alternative cultivars or crops are developed.

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 CO2 (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+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(-1)). 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(-1) 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.

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 m2 sand-filled mesocosms under two CO2 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 CO2, high P increased BNF from 410 to 900 kg ha(-1). Elevated CO2 further increased BNF to 1180 kg ha(-1) with high P, but there was no effect of CO2 on BNF with low P. Allocation of N belowground increased by approx. 50% under elevated CO2 irrespective of supplied P. The results suggest that where soil P availability is low, elevated CO2 will not increase BNF, and pasture quality could decrease because of a reduction in aboveground N.

D1 protein turnover and restoration of the photochemical efficiency of photosystem II (PSII) after photoinhibition of pea leaves (Pisum sativum L. cv Greenfeast) 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 chloroplast-encoded protein synthesis, lincomycin, only negligible recovery took place. In darkness, neither the restoration of PSII photochemical efficiency nor any notable degradation of damaged D1 protein took place. In low light, however, good recovery of PSII 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 [mu]mol of photons m-2 s-1. In moderate light of 400 [mu]mol of photons m-2 s-1, 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 PSII 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 partial restoration of PSII function. We conclude that the capacity of photoinhibited leaves to restore PSII function at different irradiances was directly related to the capacity of the leaves to degrade damaged D1 protein under the recovery conditions.

The relationship between the susceptibility of photosystem II (PSII) to photoinhibition in vivo and the rate of degradation of the D1 protein of the PSII reaction center heterodimer was investigated in leaves from pea plants (Pisum sativum L. cv Greenfeast) grown under widely contrasting irradiances. There 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. The extent of thylakoid stacking, indicated by the Chl a/b ratios of the differently light-acclimated 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 D1 protein. We suggest that low-light pea leaves, with more stacked membranes and less stroma-exposed thylakoids, are 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 PSII centers.

The turnover in vivo of the Photosystem II (PS II) reaction center D1 protein was investigated by [35S] methionine labeling of leaf discs of Brassica napus and subsequent analysis after thylakoid SDS-gel electrophoresis. The rate of D1 protein degradation was found to have a t1/2 of approximately 2 h, at an irradiance corresponding to the growth irradiance. The rate of D1 protein degradation was not increased further by prior photoinhibitory treatment which inactivated 40% of the PS II centers, but the amount of [35S]methionine label incorporated into the D1 protein during 1 h was increased 3-fold. Whether photoinhibited or not, the incorporation of label into the D1 protein increased with irradiance only up to approximately the growth irradiance and above that it decreased with increasing incident light. These data indicate that a high rate of D1 protein degradation occurs not only after photoinhibition but also under conditions where no net decrease in the number of functional PS II centers occurs and that the number of PS II centers that undergo D1 protein turnover varies depending on prior photoinhibitory damage and on incident irradiance. A down-regulation of PS II in leaves at high irradiances is discussed.

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.