Evaluation Of Phosphorus Efficiency Of Different Rice Cultivars And Dissection Of The Physiological Mechanism Of Genotypes With High Phosphorus Use Efficient

Phosphorus(P)is one of the essential mineral nutrients for plant growth and development.It serves as a fundamental constituent of biomacromolecules such as phospholipids and nucleic acids as well as substances for energy and signal transfer.The predominant form of P readily accessible for plant root is inorganic orthophosphate(Pi),the bioavailability of which in many natural and agricultural ecosystems is extremely low due to its precipitation and fixation by cations and microorganisms,respectively.Thus,Pi fertilizers need to be utilized to achieve high yield and superior quality in crop production.On the other hand,the use efficiency of Pi fertilizers in current season in China rarely exceeds 20%.Soil has become a potential reservoir of P upon long-term unreasonable application of Pi fertilizers.It represents a major waste of P resources and leads to a series of environmental issues(e.g.eutrophication)following surface runoff and soil erosion.Pi rock is a non-renewable resource which is estimated to be depleted within several hundred years.Thus,there is an urgent need to develop crops with high P-use-efficiency(PUE).Unfortunately,to date,much has yet to be done to comprehensively and practically evaluate crop PUE,especially with regard to the aspect of ’P taken away by crops’.In this study,a total of nineteen rice cultivars(twelve japonica cultivar,five indica cultivar,one japonica hybrid rice,and one aus-type cultivar)were selected for evaluation of PUE by means of pot and field(Lingshui,Hainan and Nanjing,Jiangsu)experiments.We analyzed the P accumulation and plant growth of these cultivars in response to different Pi supplies at three developmental stages.In addition,we mainly focused on one of the various parameters for crop PUE,namely physiological PUE(PPUE),selected two pairs of cultivars with opposing PPUE,and then preliminarily investigated the mechanism underlying the diverged PPUE.The main results are listed as follows:1.The growth and P accumulation of two rice cultivars(a japonica cultivar Xiushui 134 and a japonica hybrid rice Huayou 14)in response to four different Pi supplies(P0:Pi fertilizer free;P1:40 mg/kg Pi;P2:80 mg/kg Pi;P3:160 mg/kg Pi)were analyzed in a pot experiment.At tillering stage,the tiller number and biomass of Huayou 14 were positively responsive to increasing Pi levels when 80 mg/kg or less Pi was supplied,whereas under P3 condition the growth of Huayou 14 was inhibited.By contrast,Xiushui 134 showed a maximum growth under P1 condition,and its growth was suppressed under P3 condition but not P2 condition compared with that under P1 condition.Moreover,under P1 condition,the Pi content in the second and third youngest leaves of Xiushui 134 was significantly lower than that of Huayou 14;in contrast,under P2 and P3 conditions,the Pi content in the youngest leaves of Xiushui 134 was significantly higher than that of Huayou 14.These results suggest that Xiushui 134 is prone to maintain the growth of new leaves at the expense of Pi reserves in old leaves under low Pi(P1)condition,which might explain the lower Pi content in Xiushui 134 under P1 condition but comparable growth performance as compared with that under P2 condition.Furthermore,two Pi starvation-induced marker genes,OsIPS1 and OsPHT1;4,was significantly upregulated in the three youngest leaves under P0 and P1 conditions compared with that under P2 and P3 conditions.In addition,the expression of the two marker genes was higher in the second and third youngest leaves of Xiushui 134 than that in Huayou 14,probably resulting from the decreased Pi content in these tissues of Xiushui 134.2.Fourteen out of nineteen rice cultivars(eight japonica cultivars:Liangeng 11,Nangeng 9108,Wuyungeng 23,Wuyungeng 27,Xudao 3,Yandao 12,Zhonghua 11,Xiushui 134;five indica rice cultivars:93-11,Minghui 63,Wushansimiao,Yuefengsimiao,Zhenshan 97;one aus-type cultivar:Kasalath)were selected for field experiments in Lingshui,Hainan for two years.The P accumulation and PUE of these cultivars were analyzed in the filed context.Three Pi levels were supplied(HP:6 kg P2O5/667 m2;MP:3 kg P2O5/667 m2;LP:1.5 kg P2O5/667 m2).The results from the two years’field experiments were similar.Specifically,the tiller numbers of all the cultivars under MP and LP conditions were remarkably decreased compared with that under HP condition,whereas only a small proportion of the cultivars showed a further suppression in tillering under LP condition than that under MP condition,reinforcing the established notion that rice plants demand more P at early developmental stages,especially seedling stage and tillering stage.Meanwhile,these results also demonstrate that current rice cultivars differ in PUE.Accordingly,we examined the PPUE of selected cultivars at tillering,booting and mature-grain stages.The result showed that some cultivars showed varied PPUE at all three stages,among which Kasalath displayed a dramatically higher PPUE than other cultivars at mature-grain stage.On the other hand,based on the means of yield under LP and HP conditions as well as on that of MP and HP conditions,the selected cultivars were classified into four types:Ⅰ.Double P-efficient;Ⅱ.P-efficient under LP/MP condition;Ⅲ.Double P-inefficient;Ⅳ.P-efficient under HP condition.Wushansimao was found to be type-Ⅰcultivars,and Nangeng 9108 and Zhonghua 11 were type-Ⅲ cultivars.Notably,most japonica rice cultivars were sensitive to the short photoperiod in the winter of lower latitude and underwent the transition from vegetative stage to reproductive stage earlier than expected.This enormously affected the evaluation of the yield performance of japonica rice cultivars irrespective of the Pi supply.3.To avoid the aforementioned effect of short photoperiod on the evaluation of rice yield,we performed another field experiment in Nanjing,Jiangsu.Except for the 14 cultivars used in Hainan,four additional japonica cultivars were included(Liangeng 7,Nangeng 45,Yangyugeng 2 and Yanggeng 4038).Two Pi levels were supplied(+P:6 kg P2O5/667 m2;-P:without Pi fertilizer).As expected,the growth of all the 18 cultivars were significantly suppressed upon-P treatment as evidenced by decreased tiller number and biomass.At tillering stage,the relative tiller number(RTN;the ratio of tiller number under-P condition and that under+P condition)and relative shoot dry weight(RSDW;the ratio of shoot dry weight under-P condition and that under+P condition)of all the cultivars were less than 0.4,except that the RTN of Liangeng 7 was larger than 0.4.At flowering and mature-grain stages,the RTN and RSDW of all cultivars exceeded 0.4,suggesting that the growth of the rice plants were less affected than that at tillering stage.Two possibilities might explain this:i)a certain suboptimal level of Pi supply might delay the tillering process of rice plants;ii)given that the-P field was not subjected to regular ’soil aeration’treatment during tillering stage,we speculate that delaying or even cancelling the ’soil aeration’ treatment might at least partially alleviate the negative effect of Pi deficiency on the tillering and growth of rice plants.4.At mature-grain stage,～65-80%P was usually distributed to rice seeds,thus grain P content is an important determinant of PUE.The determined the P distribution ratio in the rice grains of different cultivars harvested in fields with(+P)or without(-P)P fertilizer.Nine out eighteen cultivars showed higher(5.1-14.3%)P distribution ratio in grains(PDRG)under-P condition than that under+P condition,and eight cultivars showed a comparable level(<5%difference)of PDRG under both conditions.Interestingly,one cultivar,namely Xudao 3,displays significantly lower(9.5%)PDRG under-P condition compared with that under+P condition.The mechanism underlying the differential PDRG in different cultivars remains to be investigated.5.Two pairs of rice cultivars showing opposing PPUE in field trials(Kasalath and Nangeng 9108;Minghui 63 and Xudao 3)were used for further validation of PUE in a hydroponic system.Consistent with that found in the field trials,Kasalath showed higher PPUE compared with Nangeng 9108,while Minghui 63 was superior over Xudao 3 regarding PPUE.In other words,Kasalath and Minghui 63 were capable of producing more biomass with less P accumulation.Moreover,the root-to-shoot ratio of Kasalath was lower than that of Nangeng 9108 under Pi-sufficient condition;by contrast,the opposite trend was observed under Pi-deficient condition.Furthermore,as compared with Nangeng 9108,Kasalath reallocated more P from mature leaves(source organ)to roots(sink organ).These results indicate that Kasalath has a higher redistribution efficiency of mineral nutrients and carbohydrates than Nangeng 9108 in response to Pi starvation stress.To summarize,seedling stage and tillering stage are two major developmental phases when the selected cultivars show massive demand for P,consistent with established knowledge.Moreover,some cultivars show variance in PPUE,yield-based PUE,and P distribution ratio in seeds,which may be utilized to facilitate the characterization of superior traits regarding PUE,identification of the causal alleles,and dissection of the physiological and molecular mechanisms.