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 relatives of cultivated rice. Unimproved, wild germplasm of a number of species showed light-saturated assimilation rates in atmospheric air at least as great as cultivars and elite breeding material of the cultigen O. satlva. One of these species was O. australiensis, different accessions of which were significantly (P=0.001) superior in assimilation rate to the 0. satlva genotypes tested, including representatives of the Indica, Japonica and Javanica subgroups. Amongst species, assimilation rate was correlated positively with light saturation and with carboxylation 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. Assimilation rates were also higher in diploids than in tetraploids and this was associated with the fact that all sun species are diploids and all tetraploids are shade species. The carbon dioxide compensation concentrations ranged from 28 to 43μmol mol−1 with the two lowest values (28 and 32μmol mol−1) coming from accessions of O. rufipogon. The mean value for the absolute quantum yield of photosynthesis meas ured on attached leaves was 0.060. There was a large range in the activity of phosphoenolpyruvate (PEP) carboxylase with a number of species having rates several times those of C3 species. Some species with the highest assimilation rates were assessed for photorespiratory losses and these were generally around 30% and similar to O. satlva cultivars. However, a range of O. rufipogon accessions had photorespiration rates significantly (P=0.01) lower than the O. sativa genotypes tested. No species in the genus possessed C4 photosynthetic metabolism though some did overlap with compensation concentrations and phosphoenolpyruvate carboxylase activities reported for C3−C4 intermediate species. The potential value of wild relatives to the improvement of cultivated rice is discussed.