The C4 photosynthetic pathway uses water more efficiently than the C3 type, yet biogeographical analyses show a decline in C4 species relative to C3 species with decreasing rainfall. To investigate this paradox, the hypothesis that the C4 advantage over C3 photosynthesis is diminished by drought was tested, and the underlying stomatal and metabolic mechanisms of this response determined. The effects of drought and high evaporative demand on leaf gas exchange and photosynthetic electron sinks in C3 and C4 subspecies of the grass Alloteropsis semialata were examined. Plant responses to climatic variation and soil drought were investigated using a common garden experiment with well-watered and natural rainfall treatments, and underlying mechanisms analysed using controlled drying pot experiments. Photosynthetic rates were significantly higher in the C4 than the C3 subspecies in the garden experiment under well-watered conditions, but this advantage was completely lost during a rainless period when unwatered plants experienced severe drought. Controlled drying experiments showed that this loss was caused by a greater increase in metabolic, rather than stomatal, limitations in C4 than in the C3 leaves. Decreases in CO2 assimilation resulted in lower electron transport rates and decreased photochemical efficiency under drought conditions, rather than increased electron transport to alternative sinks. These findings suggest that the high metabolic sensitivity of photosynthesis to severe drought seen previously in several C4 grass species may be an inherent characteristic of the C4 pathway. The mechanism may explain the paradox of why C4 species decline in arid environments despite high water-use efficiency.

The chloroplast envelope triose-phosphate/phosphate translocator (TPT) is responsible for carbohydrate export during photosynthesis. Using measurements of carbohydrates, partitioning of assimilated 14 CO 2 , photosynthetic gas exchange, and chlorophyll fluorescence, we show that a mutant of Arabidopsis lacking the TPT increases synthesis of starch compared to the wild type, thereby compensating for a deficiency in its ability to export triose-phosphate from the chloroplast. However, during growth under high light, the capacity for starch synthesis becomes limiting so that the chloroplastic phosphate pool is depleted, resulting in a restriction on electron transport, a reduction in the rate of photosynthesis, and slowed plant growth. Under the same conditions but not under low light, we observe release of 14 C label from starch, indicating that its synthesis and degradation occur simultaneously in the light. The induction of starch turnover in the mutant specifically under high light conditions leads us to conclude that it is regulated by one or more metabolic signals, which arise as a result of phosphate limitation of photosynthesis.During photosynthesis, the triose-phosphate/phosphate translocator (TPT) of the chloroplast inner envelope membrane mediates the counterexchange of stromal triose-phosphates (triose-P) derived from CO 2 fixation with cytosolic orthophosphate (P i ), thus providing the cytosol with the precursors for Suc synthesis. Optimum rates of photosynthesis require the regulated exchange of metabolites through TPT. If triose-P is withdrawn from the chloroplast too quickly, the Calvin cycle is depleted of intermediates. Alternatively, if transport through TPT is too slow, phosphorylated intermediates accumulate in the chloroplast, resulting in a depletion of stromal P i , which in turn has the potential to restrict ATP synthesis, again restricting CO 2 fixation (Edwards and Walker, 1983). Under normal circumstances, the potential inhibition of photosynthesis due to P i limitation is ameliorated by activation of ADP-Glc pyrophosphorylase (AGPase; Sowokinos, 1981;Sowokinos and Preiss, 1982), leading to an increase in the rate of starch synthesis and a resulting release of P i .Studies on transgenic potato (Solanum tuberosum) and tobacco (Nicotiana tabacum) plants possessing altered amounts of TPT have established the central role of this translocator in the integration of assimilate partitioning during photosynthesis (Barnes et al., 1994;Heineke et al., 1994; Häusler et al., 1998 Häusler et al., , 2000a Häusler et al., , 2000b. The patterns of carbohydrate synthesis in such plants suggest that they compensate metabolically for the reduced levels of TPT by diverting assimilate into starch, releasing the P i required for continued photosynthesis. Remarkably, despite the changes in carbohydrate metabolism, plants with as little as 20% of wild-type TPT activity show little or no reduction in the maximum rate of CO 2 assimilation under ambient atmospheric conditions, and no change in growth rate (Ries...

Background and AimsThe grass Alloteropsis semialata is the only plant species with both C3 and C4 subspecies. It therefore offers excellent potential as a model system for investigating the genetics, physiology and ecological significance of the C4 photosynthetic pathway. Here, a molecular phylogeny of the genus Alloteropsis is constructed to: (a) confirm the close relationship between the C3 and C4 subspecies of A. semialata; and (b) infer evolutionary relationships between species within the Alloteropsis genus.MethodsThe chloroplast gene ndhF was sequenced from 12 individuals, representing both subspecies of A. semialata and all four of the other species in the genus. ndhF sequences were added to those previously sequenced from the Panicoideae, and used to construct a phylogenetic tree.Key ResultsThe phylogeny confirms that the two subspecies of A. semialata are among the most recently diverging lineages of C3 and C4 taxa currently recognized within the Panicoideae. Furthermore, the position of the C3 subspecies of A. semialata within the Alloteropsis genus is consistent with the hypothesis that its physiology represents a reversion from C4 photosynthesis. The data point to a similar evolutionary event in the Panicum stenodes–P. caricoides–P. mertensii clade. The Alloteropsis genus is monophyletic and occurs in a clade with remarkable diversity of photosynthetic biochemistry and leaf anatomy.ConclusionsThese results confirm the utility of A. semialata as a model system for investigating C3 and C4 physiology, and provide molecular data that are consistent with reversions from C4 to C3 photosynthesis in two separate clades. It is suggested that further phylogenetic and functional investigations of the Alloteropsis genus and closely related taxa are likely to shed new light on the mechanisms and intermediate stages underlying photosynthetic pathway evolution.

The species richness of C(4) grasses is strongly correlated with temperature, with C(4) species dominating subtropical ecosystems and C(3) types predominating in cooler climates. Here, the effects of low temperatures on C(4) and C(3) grasses are compared, controlling for phylogenetic effects by using Alloteropsis semialata, a unique species with C(4) and C(3) subspecies. Controlled environment and common garden experiments tested the hypotheses that: (i) photosynthesis and growth are greater in the C(4) than the C(3) subspecies at high temperatures, but this advantage is reversed below 20 degrees C; and (ii) chilling-induced photoinhibition and light-mediated freezing injury of leaves occur at higher temperature thresholds in the C(4) than the C(3) plants. Measurements of leaf growth and photosynthesis showed the expected advantages of the C(4) pathway over the C(3) type at high temperatures. These declined with temperature, but were not completely lost until 15 degrees C, and there was no evidence of a reversal to give a C(3) advantage. Chronic chilling (5-15 degrees C) or acute freezing events induced a comparable degree of photodamage in illuminated leaves of both subspecies. Similarly, freezing caused high rates of mortality in the unhardened leaves of both subtypes. However, a 2-week chilling treatment prior to these freezing events halved injury in the C(3) but not the C(4) subspecies, suggesting that C(4) leaves lacked the capacity for cold acclimation. These results therefore suggest that C(3) members of this subtropical species may gain an advantage over their C(4) counterparts at low temperatures via protection from freezing injury rather than higher photosynthetic rates.