Nitrogen efficiency screening of rice cultivars popularized in South China

【Objectives】The objective of this study is to screen and evaluate the nitrogen efficiency of rice cultivars widely grown in South China, and investigate the different phsiology mechanisms between the high and low nitrogen efficient cultivars.【Methods】Thirty four rice cultivars were tested in field trials and pot experiments under both normal and low nitrogen supplies. The high and low nitrogen efficient cultivars were identified by analyzing parameters related to yield and its components, the leaf SPAD value and the aboveground nitrogen accumulation rate of rice. 【Results】 1)Field trials results in 2011 showed that the yields under normal nitrogen treatment were higher than those under low nitrogen treatment for most of the 34 cultivars. Huiliangyou 6, Guangliangyou 35 and Tianyouhuazhan had higher nitrogen efficiencies with 1.11, 1.02 and 1.00, respectively; Xiushui 134, Fudao 2 and Wufengyou 7025 had lower nitrogen efficiencies with 0.55, 0.61 and 0.61, respectively. Similarly, the yield components under the normal nitrogen treatment were higher than those under the low nitrogen treatment for most of the 34 cultivars. The cultivars with higher nitrogen efficiencies have higher panicle number, spikelet number per panicle, as well as thousand grain weight under low nitrogen condition. As a consequence, Huiliangyou 6, Guangliangyou 35 and Tianyouhuazhan were screened as the high nitrogen efficient rice cultivars, and Xiushui 134, Fudao 2 and Wufengyou 7025 as the low nitrogen efficient ones. 2) Field and pot experiments in 2012 showed similar results. Both the actual and theoretical yields and nitrogen efficiencies of Huiliangyou 6 and Guangliangyou 35 are in the high ranking of the tested 34 cultivars, while Xiushui 134, Wufengyou 7025 and Fudao 2 in low ranking. In field trial, both the panicle number and thousand grain weight of high nitrogen efficient cultivar, Guangliangyou 35, were significantly higher than those of low nitrogen efficient ones under normal nitrogen treatment. Under low nitrogen condition, the high N efficient cultivars performed higher or significantly higher SPAD values than the low nitrogen efficient ones. Under normal nitrogen supply in the pot experiment, the SPAD values of the high nitrogen efficient cultivars were slightly higher than the low ones.Under the normal nitrogen condition, there were no significant differences in aboveground N accumulation between the high and low N efficient cultivars at each growth stage, except for Xiushui 134. Under the low N condition, no significant differences at tillering stage, except high N efficient cultivar Huiliangyou 6, in which the N accumulation even lower than N low efficient cultivars. At the heading stage and mature stage, the N accumulation in two high N efficient cultivars were significantly higher than those in low ones. 【Conclusion】Guangliangyou 35 is identified as the highest nitrogen efficient cultivar and Xiushui 134 as the lowest nitrogen efficient cultivar. The high nitrogen efficient cultivar has higher panicle number and spikelet number per panicle under low nitrogen supply condition, which led to the higher yields than the low nitrogen efficient cultivar. Compared with that in the low nitrogen efficient cultivars, the more nitrogen was accumulated in the high nitrogen efficient cultivars at the late growth stage, which enhanced photosynthesis, the carbon and nitrogen assimilation, and then improved yields.