PHOTOSYNTHESIS AND PHOTORESPIRATION IN THE GENUS ORYZA

Photosynthetic gas exchange has been surveyed in 22 of the 23 species currently placed in the genus Oryza and constituting the wild relative s of cultivated rice. Unimproved, wild germplasm of a number of specie s showed light-saturated assimilation rates in atmospheric air at leas t as great as cultivars and elite breeding material of the cultigen O. sativa. One of these species was O. australiensis, different accessio ns of which were significantly (P = 0.001) superior in assimilation ra te to the O. sativa genotypes tested, including representatives of the Indica, Japonica and Javanica subgroups. Amongst species, assimilatio n rate was correlated positively with light saturation and with carbox ylation efficiency. The wild species fell into two distinct groupings according to whether they originated from sun or shade habitats, with the higher assimilation rates being associated with the sun species. A ssimilation rates were also higher in diploids than in tetraploids and this was associated with the fact that all sun species are diploids a nd all tetraploids are shade species. The carbon dioxide compensation concentrations ranged from 28 to 43 mu mol mol(-1) with the two lowest values (28 and 32 mu mol mol(-1)) coming from accessions of O. rufipo gon. The mean value for the absolute quantum yield of photosynthesis m easured on attached leaves was 0.060. There was a large range in the a ctivity of phosphoenolpyruvate (PEP) carboxylase with a number of spec ies having rates several times those of C-3 species. Some species with the highest assimilation rates were assessed for photorespiratory los ses and these were generally around 30% and similar to O. sativa culti vars. However, a range of O. rufipogon accessions had photorespiration rates significantly (P = 0.01) lower than the O. sativa genotypes tes ted. No species in the genus possessed C-4 photosynthetic metabolism t hough some did overlap with compensation concentrations and phosphoeno lpyruvate carboxylase activities reported for C-3-C-4 intermediate spe cies. The potential value of wild relatives to the improvement of cult ivated rice is discussed.