Organic anion exudation by lowland rice (Oryza sativa L.) at zinc and phosphorus deficiency Ellis Hoffland 1 , Changzhou Wei 1,2,5 & Matthias Wissuwa 3,4 1 Dept. of Soil Quality, Wageningen University, P.O.Box 8005, NL 6700, Wageningen, EC , The Netherlands. 2 Key Lab. of Oasis Ecology Agriculture of Bingtuan, Shihezi University, Post box 425, 832000, Xinjiang, P.R. China. 3 Crop, Soil, and Water Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, The Philippines. 4 Current address: Japan International Research Center for Agricultural Sciences, 1-1,Ohwashi, 305-8686, Tsukuba, Ibaraki, Japan. 5 Corresponding author* Received 22 June 2005. Accepted in revised form 6 October 2005 Key words: citrate, deficiency, oxalate, phosphorus, rice, root exudation, tolerance, zinc Abstract The objectives of this paper were to determine (1) if lowland rice (Oryza sativa L.) plants respond similarly to low zinc (Zn) and phosphorus (P) availability by increased root exudation of low-molecular weight organic anions (LMWOAs) and (2) if genotypic variation in tolerance to low soil supply of either Zn or P is related to LMWOA exudation rates. Exudation of LMWOAs can increase bioavailability of both Zn and P to the plant, through partly similar chemical mechanisms. We used seven lowland rice genotypes and showed in two experiments that genotypes that grow relatively well on a soil with low Zn availability also grow well on a sparingly soluble Ca-phosphate (r=0.80, P=0.03). We measured exudation rates of LMWOAs on nutrient solution and found that both Zn and P deficiency induced significant increases. Among the LMWOAs detected oxalate was quantitatively the most important, but citrate is considered more effective in mobilizing Zn. Citrate exudation rates correlated with tolerance to low soil levels of Zn (P=0.05) and P (P=0.07). In a low-Zn-field we found an increased biomass production at higher plant density, which is supportive for a concentration-dependent rhizosphere effect on Zn bioavailability such as LMWOA exudation. We, for the first time, showed that tolerance to low Zn availability is related to the capacity of a plant to exude LMWOAs and confirmed that exudation of LMWOAs must be regarded a multiple stress response. Introduction Zinc (Zn) deficiency is the most widespread micro- nutrient disorder in rice (Doberman and Fair- hurst, 2000). It is common in flooded soils because of the reducing conditions that develop following submergence, with Zn becoming immobilised with ferrous iron compounds and other solid phases formed in reducing processes. In lowland rice Zn deficiency causes multiple symptoms that usually appear 2–4 weeks after transplanting of rice seed- lings. Leaves develop brown spots and streaks that may fuse to cover older leaves entirely, plants re- main stunted and in severe cases may die, while those that recover show a substantial delay in maturity and a reduction in yield. Zinc deficiency is generally corrected by apply- ing Zn sulphate to the soil. Zn application, how- ever, is not always adequate to fully recover rice yields in genotypes with low tolerance (Van Bre- emen and Castro, 1980). Also, Zn fertilizers are unaffordable for many farmers. It has therefore been suggested to intensify breeding efforts to im- prove tolerance to low Zn availability (Neue et al., 1998). * FAX No: +86-993-2057992. E-mail: wei-changzhou@163.com Plant and Soil (2006) 283:155–162 Ó Springer 2006 DOI 10.1007/s11104-005-3937-1