Differences In Response To Low Phosphorus Among Indica Rice Varieties And Their Agronomic And Physiological Characteristics

Phosphorus(P)is an essential element for plant growth and development,and rice is a major food crop that requires a large amount of P.However,phosphorus is a non renewable resource.Excessive application of P will aggravate the depletion of P resources and pollute the environment.Therefore,to screen rice varieties with tolerance to low P and to elucidate their agronomic and physiological characteristics would have great significance in saving phosphate fertilizer resources,realizing high yield and high efficiency and protecting environment.In this study,12 representative indica rice varieties that have been widely used in the production of Jiangsu Province in the last 70 years were used and were grown in nutrient solution during the whole growth period.Two treatments,the normal P level(International Rice Research Institute,standard nutrient solution for rice,Control)and the low P level(P concentration was 1/20 of the normal P level),were conducted.Tolerance to low P and agronomic and physiological traits of the test varieties were investigated.The main results are as follows.1.According to the index of tolerance to low P(the grain yield under the low P treatment × the coefficient of tolerance to low P/the average grain yield of all tested varieties under the low P treatment),the order of tolerance to low P of the varieties was?you 084>Liangyoupeijiu>Yangdao 2>Yangliangyou 6>Zhenzhuai>Yintiaoxian>Taizhongxian>Yangdao 6>Nanjing 11>Nanjing 1>Huangguaxian>IR24.The index of tolerance to low P was>0.8 for the varieties of ?you 084,Liangyoupeijiu,and Yangdao 2 which were classified into the varieties with strong tolerance to low P.The index of tolerance to low P was<0.5 for the varieties of Huangguaxian and IR24 which were classified into the varieties with weak tolerance to low P.The index of tolerance to low P was between 0.49 and 0.79 for other varieties which were classified into the varieties with mid tolerance to low P.2.Compared with the normal P level(control),the low P treatment decreased panicle number per unit area and spikelets per panicle,and increased seed-setting rate and 1000-grain weight.The increase of seed-setting rate and 1000-grain weight could not compensate for the decrease of spikelets per panicle and the total number of spikelets,and accordingly,the low P treatment significantly reduced grain yield(except for ? you 084).In comparison with the varieties with weak tolerance to low P,the varieties with strong tolerance to low P showed a higher grain yield under the low P treatment,due mainly to more spikelets per panicle and total spikelet number.There were no significant differences in seed-setting rate and 1000-grain weight between the two types of varieties.3.Versus the normal P level(control),the low P treatment reduced the accumulation of nitrogen(N),P and kalium(K)in the plants of aboveground,increased the ratio of N,P and K accumulations in the root to those in the shoot,and increased the internal use efficiciency of N,P and K(grain yield/total uptake of N,P,or K in plants).Compared with the varieties with weak tolerance to low P,the varieties with strong tolerance to low P had higher N,P and K uptake,higher N,P and K harvest index and higher internal use efficiciency of N,P and K under the low P treatment4.In comparison with the normal P level(control),the low P treatment reduced the number of tillers,leaf area index,chlorophyll content,and dry mater weight at the main growth stages and photosynthetic rate of the flag leaf at the heading stage.Versus the varieties with weak tolerance to low P,the varieties with strong tolerance to low P exhibited more tiller number,higher leaf area index,chlorophyll content and dry mater weight at the main growth stages,higher photosynthetic rate of the flag leaf and more accumulation of non-structural carbohydrate(NSC)in the stems and sheaths at the heading stage,and higher NSC transport rate and contribution rate of NSC to grain yield from heading to maturity.5.Relative to the normal P level(control),the low P treatment reduced the root length,root diameter and root surface area at the early and mid growth stages,increased the dry weight of roots,the ratio of root to shoot,and the activity of acid phosphatase(APase),and reduced the root oxidation activity.Compared with the varieties with weak tolerance to low P,the varieties with strong tolerance to low P had higher root dry weight,total root surface area,root volume and smaller ratio of root to shoot at the early and mid growth stages,and showed higher activities of root oxidation and APase during the whole growth periodIn conclusion,the index of tolerance to low P was an important index to evaluate the tolerance to low P for rice varieties.According to the index of tolerance to low P,the varieties of ?you 084,Liangyoupeijiu,and Yangdao 2 were classified into the varieties with strong tolerance to low P,and the varieties of Huangguaxian and IR24 were classified into the varieties with weak tolerance to low P.Compared with the varieties with weak tolerance to low P and under the low P treatment,the varieties with strong tolerance to low P had more number of spikelets per panicle and total number of spikelets,higher N,P,K uptake,N,P,K harvest index and N,P,K internal use efficiencies.Some important agronomic and physiological traits,such as more tiller number and higher leaf area index at the early growth stage,higher leaf photosynthetic rate and NSC accumulation in the stem and sheath at the heading stage,and greater NSC transfer rate from the stem and sheath to grain during grain filling,a larger root system at the early growth stage,and stronger activities of root oxidation and APase during the whole growth period,contributed to the higher grain yield and higher use efficiencies of N,P,and K for the varieties with strong tolerance to low P under the low P treatment.