A molecular phylogeny of the genus Alloteropsis (Panicoideae, Poaceae) suggests an evolutionary reversion from C4 to C3 photosynthesis.

Background And Aims: The grass Alloteropsis semialata is the only plant species with both C(3) and C(4) subspecies. It therefore offers excellent potential as a model system for investigating the genetics, physiology and ecological significance of the C(4) photosynthetic pathway. Here, a molecular phylogeny of the genus Alloteropsis is constructed to: (a) confirm the close relationship between the C(3) and C(4) subspecies of A.

Seasonal differences in photosynthesis between the C3 and C4 subspecies of Alloteropsis semialata are offset by frost and drought.

The regional abundance of C(4) grasses is strongly controlled by temperature, however, the role of precipitation is less clear. Progress in elucidating the direct effects of photosynthetic pathway on these climate relationships is hindered by the significant genetic divergence between major C(3) and C(4) grass lineages. We addressed this problem by examining seasonal climate responses of photosynthesis in Alloteropsis semialata, a unique grass species with both C(3) and C(4) subspecies.

Low temperature effects on leaf physiology and survivorship in the C3 and C4 subspecies of Alloteropsis semialata.

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.

Drought constraints on C4 photosynthesis: stomatal and metabolic limitations in C3 and C4 subspecies of Alloteropsis semialata.

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.

A mutant of Arabidopsis lacking the triose-phosphate/phosphate translocator reveals metabolic regulation of starch breakdown in the light.

The chloroplast envelope triose-phosphate/phosphate translocator (TPT) is responsible for carbohydrate export during photosynthesis. Using measurements of carbohydrates, partitioning of assimilated 14CO2, 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.