Characterization of root exudates at different growth stages of ten rice (Oryza sativa L.) cultivars

Plant root exudates play important roles in the rhizosphere. We tested thre e media (nutrient solution, deionized water and CaSO4 solution) for three p eriods of time (2, 4 and 6 h) for collecting root exudates of soil-grown ri ce plants. Nutrient culture solution created complications in the analyses of exudates for total organic C (TOC) by the wet digestion method and of or ganic acids by HPLC due to the interference by its components. Deionized wa ter excluded such interference in analytical analyses but affected the turg or of root cells: roots of four widely different rice cultivars excreted 20 to 60% more TOC in deionized water than in 0.01 M CaSO4. Furthermore, the proportion of carbohydrates in TOC was also enhanced. Calcium sulfate solut ion maintained the osmotic environment for root cells and did not interfere in analytical procedures. Collection for 2 h avoided under-estimation of T OC and its components exuded by rice roots, which occurred during prolonged exposure. By placing plants in 0.01 M CaSO4 for 2 h, root exudates of soil -grown traditional, tall rice cultivars (Dular, B40 and Intan), high-yieldi ng dwarf cultivars (IR72, IR52, IR64 and PSBRc 20), new plant type cultivar s (IR65598 and IR65600) and a hybrid (Magat) were collected at seedling, pa nicle initiation, flowering and maturity and characterized for TOC and orga nic acids. The exudation rates were, in general, lowest at seedling stage, increased until flowering but decreased at maturity. Among organic acids, m alic acid showed the highest concentration followed by tartaric, succinic, citric and lactic acids. With advancing plant growth, exudation of organic acids substituted exudation of sugars. Root and shoot biomass were positive ly correlated with carbon exudation suggesting that it is driven by plant b iomass. As root exudates provide substrates for methanogenesis in rice fiel ds, large variations in root exudation by cultivars and at different growth stages could greatly influence CH4 emissions. Therefore, the use of high-y ielding cultivars with lowest root excretions, for example IR65598 and IR65 600, would mediate low exudate-induced CH4 production. The screening of exc iting rice cultivars and breeding of new cultivars with low exudation rates could offer an important option for mitigation of CH4 emission from rice a griculture to the atmosphere.