Unravelling the in vivo regulation and metabolic role of the alternative oxidase pathway in C3 species under photoinhibitory conditions

The mitochondrial alternative oxidase pathway (AOP) has been suggested to act as a sink for excess reducing power generated in the chloroplast under high-light (HL) stress and thus may reduce photoinhibition. The aim of this study was to compare different species to investigate the in vivo regulation and role of AOP under HL stress. The in vivo activities of AOP (νalt ) and the cytochrome oxidase pathway, chlorophyll fluorescence, metabolite profiles, alternative oxidase (AOX) capacity and protein amount were determined in leaves of five C3 species under growth light and after HL treatment. Differences in respiration and metabolite levels were observed among species under growth light conditions. The HL response of νalt was highly species dependent, correlated with the AOP capacity and independent of AOX protein content. Nevertheless, significant correlations were observed between νalt , levels of key metabolites and photosynthetic parameters. The results show that the species-specific response of νalt is caused by the differential post-translational regulation of AOX. Significant correlations between respiration, metabolites and photosynthetic performance across species suggest that AOP may permit stress-related amino acid synthesis, whilst maintaining photosynthetic activity under HL stress.

Increasing CO2 from subambient to superambient concentrations alters species composition and increases above-ground biomass in a C3 /C4 grassland

•  The glacial-to-present increase in atmospheric CO₂ concentration is likely to have stimulated plant production, but experimental tests in natural ecosystems are lacking. •  We measured above-ground biomass production, plant nitrogen (N) accumulation, and species dynamics in a C₃ /C₄ grassland exposed for 4 yr (1997-2000) to a continuous gradient in CO₂ from 200-560 mol mol^-1 . •  Biomass increased with CO₂ concentration in 1997-99. Biomass increases ranged between 121 and 161 g m^-2 per 100 mol mol^-1 rise in CO₂ and were similar at subambient and superambient concentrations. Biomass responses to CO₂ were determined by different species or functional groups of species during different years. Increasing CO₂ accelerated a successional shift initiated by release from grazing in which C₃ forbs increased at the expense of a C₄ grass. Effects of CO₂ on tissue N concentration varied among species and functional groups, but CO₂ did not alter total N in above-ground tissues. •  Results imply that rising CO₂ has stimulated plant production and accelerated successional change and that grasslands will remain sensitive to rising CO₂ for several decades.

Atmospheric CO₂ Concentration and Other Limiting Factors in the Growth of C₃ and C₄ Plants

It has been widely observed that recent increases in atmospheric CO₂ concentrations have had, so far, a positive effect on the growth of plants. This is not surprising since CO₂ is an important nutrient for plant matter, being directly involved in photosynthesis. However, it is also known that the conditions which have accompanied this increase in atmospheric CO₂ concentration have also had significant effects on other environmental factors. It is possible that these other effects may emerge as limiting factors which could act to prevent plant growth. This may involve complex interactions between prevailing sunlight and water conditions, variable temperatures, the availability of essential nutrients and the type of synthetic pathway for the plant species. The issue of concern to this investigation is if we should be worried about a possible shift in the C₃-C₄ paradigm driven by changes in the atmospheric CO₂ concentration, or if some other factor, such as water scarcity, is much more relevant within a 30-year time frame. If an opinion is needed on what will have the worst effect on the survival of the planet between the scarcity of water or the reduced efficiency of C₃ plants to sequester CO₂, the issue of water is the more incisive.

Drought Sensitivity of the Carbon Isotope Composition of Leaf Dark-Respired CO2 in C3 (Leymus chinensis) and C4 (Chloris virgata and Hemarthria altissima) Grasses in Northeast China

Whether photosynthetic pathway differences exist in the amplitude of nighttime variations in the carbon isotope composition of leaf dark-respired CO2 (δ13Cl) and respiratory apparent isotope fractionation relative to biomass (ΔR,biomass) in response to drought stress is unclear. These differences, if present, would be important for the partitioning of C3-C4 mixed ecosystem C fluxes. We measured δ13Cl, the δ13C of biomass and of potential respiratory substrates and leaf gas exchange in one C3 (Leymus chinensis) and two C4 (Chloris virgata and Hemarthria altissima) grasses during a manipulated drought period. For all studied grasses, δ13Cl decreased from 21:00 to 03:00 h. The magnitude of the nighttime shift in δ13Cl decreased with increasing drought stress. The δ13Cl values were correlated with the δ13C of respiratory substrates, whereas the magnitude of the nighttime shift in δ13Cl strongly depended on the daytime carbon assimilation rate and the range of nighttime variations in the respiratory substrate content. The ΔR,biomass in the C3 and C4 grasses varied in opposite directions with the intensification of the drought stress. The contribution of C4 plant-associated carbon flux is likely to be overestimated if carbon isotope signatures are used for the partitioning of ecosystem carbon exchange and the δ13C of biomass is used as a substitute for leaf dark-respired CO2. The detected drought sensitivities in δ13Cl and differences in respiratory apparent isotope fractionation between C3 and C4 grasses have marked implications for isotope partitioning studies at the ecosystem level.

Photosynthesis and leaf structure of F1 hybrids between Cymbidium ensifolium (C3) and C. bicolor subsp. pubescens (CAM)

BACKGROUND AND AIMS

The introduction of crassulacean acid metabolism (CAM) into C3 crops has been considered as a means of improving water-use efficiency. In this study, we investigated photosynthetic and leaf structural traits in F1 hybrids between Cymbidium ensifolium (female C3 parent) and C. bicolor subsp. pubescens (male CAM parent) of the Orchidaceae.

METHODS

Seven F1 hybrids produced through artificial pollination and in vitro culture were grown in a greenhouse with the parent plants. Structural, biochemical and physiological traits involved in CAM in their leaves were investigated.

KEY RESULTS

Cymbidium ensifolium accumulated very low levels of malate without diel fluctuation, whereas C. bicolor subsp. pubescens showed nocturnal accumulation and diurnal consumption of malate. The F1s also accumulated malate at night, but much less than C. bicolor subsp. pubescens. This feature was consistent with low nocturnal fixation of atmospheric CO2 in the F1s. The δ13C values of the F1s were intermediate between those of the parents. Leaf thickness was thicker in C. bicolor subsp. pubescens than in C. ensifolium, and those of the F1s were more similar to that of C. ensifolium. This was due to the difference in mesophyll cell size. The chloroplast coverage of mesophyll cell perimeter adjacent to intercellular air spaces of C. bicolor subsp. pubescens was lower than that of C. ensifolium, and that of the F1s was intermediate between them. Interestingly, one F1 had structural and physiological traits more similar to those of C. bicolor subsp. pubescens than the other F1s. Nevertheless, all F1s contained intermediate levels of phosphoenolpyruvate carboxylase but as much pyruvate, Pi dikinase as C. bicolor subsp. pubescens.

CONCLUSIONS

CAM traits were intricately inherited in the F1 hybrids, the level of CAM expression varied widely among F1 plants, and the CAM traits examined were not necessarily co-ordinately transmitted to the F1s.