Effect of foliar application of auxin, absisic acid and cytokinin hormones on grain yield and yield components of three rice (Oryza sativa L.) genotypes under low temperature stress conditions

To study the effects of auxin (IAA)، abscisic acid (ABA) and cytokinin (CK) hormones on grain yield and its components of rice (Oryza sativa L.) genotypes under low temperature stress conditions، an experiment was conducted during 2008 in phytotron in Agricultural Biotechnology Research Institute of Iran، Karaj، Iran. Three rice genotypes (33: IR72944-1-2-2، 34: IR73688-57-2 as exotic cold-tolerant and Hoveizeh as Iranian coldsensitive genotypes) were foliar-applied with IAA، ABA and CK hormones and treated with two temperatures conditions including; 22/29 oC (night/day) and 15/13 oC (night/day) as control and stress temperatures، respectively. Grain weight. hill-1 (g)، spikelet. spike-1، fertility (%)، maximum tiller number، fertile tiller number were examined in a factorial experiment using completely randomized design (CRD) with four replications. Results showed that there were significant differences among the genotypes. Grain weight. hill-1 of Hoveizeh cultivar was significantly higher in control (26. 2 g) than stress (1. 4 g) conditions. This trait showed a significant reduction in non-applied treatment as compared with hormonal treatments. IAA، ABA، and CK hormones، under stress conditions، increased grain weight. hill-1 from 1. 2 g to 24. 1 g، 20. 8 g and 30. 4 g، respectively. Grain weight. hill-1 under control temperature increased from 26. 2 g in non-applied to 40. 1 g in CK-applied treatment. There were significant interactions between genotypes and hormonal treatments. Application of hormones prevented the reduction of grain weight. hill-1 under stress temperatures. Grain weight. hill-1 of Hoveizeh cultivar with application of IAA، ABA and CK under stress conditions، showed 94. 1%، 93. 3% and 95. 4% increases، respectively. IAA، ABA and CK applications under stress conditions also increased fertile tillers (%) to 85. 5%، 85. 5% and 82. 3 %، respectively، in comparison to non-applied treatment. CK caused a significant increase in grain weight. hill-1 under stress conditions. It increased the number of spike in Hoveizeh up to 14. 4 % under low temperature stress conditions. It is concluded that the amount and balance of hormones are the most important and effective factors in rice plants to tolerate low temperature stress conditions.