Study On The Physiological Mechanisms Of Rice With High K Efficiency

Potassium(K)plays a role in a wide range of functions in plants:photosynthesis,enzyme activation,protein synthesis,osmotic potential,and as a counterion to inorganic ions and organic biopolymers.Potassium deficiency in soils is widespread in China,especially in the paddy soils, where low K has become the major limiting factor for obtaining high yield.China is short of K fertilizer resources,so it is importance to understand the mechanisms of plant adaptation to low K. There existed large genotypic differences in K efficiency in lowland rice(Oryza sativa L.).The major objectives of this study were to investigate the characteristics of root morphology,K absorption kinetics,and root cell membrane tolerance to low K stress,to find out the plant trait for screening and identifying K-efficient rice genotypes,and understand the characteristics of rhizosphere processes of K in the paddy soils.The major results obtained were summarized as follows:1.A field trial was carried out to study the effect of different K supply levels on the grain yield, K uptake and utilization of eight rice cultivars.Results showed that there were significantly genotypic variations in rice grain yield,K utilization efficiency and shoot K accumulation at different growth stage under two different K supply.Compared with normal K supply,rice grain yield and shoot K accumulation were decreased,and K utilization efficiency was increased under low K supply.Correlation analysis showed that under low K supply rice grain yield was positively correlated with K utilization efficiency,shoot K accumulation at both early growth and mid-growth stages(p＜0.05),and under normal K supply rice grain yield was positively correlated with K utilization efficiency and shoot K accumulation at early growth stage(p＜0.05).Therefore,screening and breeding rice genotypes with higher K utilization efficiency and higher ability to accumulate K at grow th early stage would probably be one of alternatives to alleviate the K deficiency of paddy soil in south of China.2.Differential effects of K deficiency on chloroplast ultrastructure and chlorophyll fluorescence parameters were observed between the K-efficient(HA)and the K-inefficient(TLHZ) rice(Oryza sativa L.)genotypes.The net photosynthetic rate(Pn)and stomatal conductance(Gs) decreased at low K,but to a greater extent in TLHZ than in HA.Chlorophyll fluorescence measurements showed that PSâ…¡-available photochemical efficiency(F_v/F_m),electron transport rate of PSâ…¡(ETR),and photochemical quenching(qP)declined more significantly,whereas the non-photochemical quenching(NPQ)increased more in TLHZ than in HA-881043 under low K, indicating that the K-efficient genotype has greater ability to keep PS reaction center less damaged and maintain stronger photosynthetic ability with high resistance to photoinhibition under K deficiency.In addition,better preserved chloroplast ultrastructure was noted in HA-881043 than in TLHZ under K deficiency.The high efficient photochemical capacity in K -efficient genotypes may be a key factor for high K efficiency in rice.3.Disparity in the root morphology of six rice(Oryza sativa L.)genotypes varying in potassium(K)efficiency was studied with three K levels:5 mg L^-1(low),10 mg L^-1(moderate) and 40 mg L^-1(adequate)in hydroponic culture.Morphological parameters included root length, surface-area,volume,and count of lateral roots,as well as fine(＜0.2 mm)and thick(＞0.2 mm) roots.The results indicated that root growth of all genotypes was reduced under low K,but moderate K deficiency increased root length of the efficient genotypes.At deficient and moderate K levels,all the efficient rice genotypes developed more fine roots(＜0.2 mm)than the inefficient ones.Both fine root and root surface area were found to be the best parameters to portray K stress in rice.In accordance with root morphology,higher relative K concentration was noted in shoots of the efficient genotypes when grown at moderate and deficient K levels,indicating that root morphology parameters are involved in root uptake for K and translocation it up to shoots. Potassium deficiency affected not only the root morphology,but also the root ultra-structure.The K_m,C_min,andβvalues of K uptake by root of the efficient genotypes were both lower than those of the inefficient ones,whereas minimal differences were observed for the I_maxvalues among different rice genotypes.The results indicated the K efficiency was relative to the root affinity for K⁺,the minimal K⁺ concentration in the culture medium(C_min),but not to the maximum K⁺ uptake rate.The roots of high-efficient genotypes had stronger tolerance to K deficient stress for root membrane damage,and could maintain developed root architecture to adapt the low K in growth medium.And the results of field experiment were similar with hydroponic study.4.Differential changes of K fractions in the rhizosphere in field experiment and root exudates constitutes were investigated between the K-efficient and K-inefficient rice cultivars under low and adequate soil K supply.The results showed that considerable depletion of readily-available K and exchangeable K in rice rhizosphere occurred at both low and adequate K levels.At low K,the efficient cultivar(HA-881043)reduced more dramatically the rhizospheric concentrations of both readily-available K and exchangeable K than the inefficient cultivar(TLHZ),but the opposite trends were noted at adequate K supply.Both cultivars could deplete readily-released K in the rhizosphere.And similar difference was noted for pH of root exudates.Moreover,the root exudates of the K-efficient cultivars grown at low K contained higher organic acids and showed more effective in extracting K from philogopite.These results indicated the K efficiency in lowland rice resulted from more effective use of slowly-released K and mineral K in soil by means of induced root exudation of acids and/or organic acids at low K.5.We chose 200 isolated rice(Oryza sativa L.subsp.Japonica,cv.Ribenqing)mutants as material from the mutant system derived from gene transformation mediated via Agrobacterium tumefaciens.By different K treatment,we obtained the K-sensitive/tolerant mutant rice(STOW). Our results indicated that K accumulation of the K-tolerant mutant were higher than that of the wild type,so the capacity of transporting K to shoots might be greater in the mutant.Then the high/Iow -accumulation K-tolerant mutant was added a generation to analyze the amplification patterns of Homomycin gene in T3 plants of K-tolerant mutant.The results indicated that there was not the Hyg in T2 and T3 lines,so we concluded that different inheritable character of accumulating K in the mutant was not caused by T-DNA inserting.Our studies suggested that tolerant mutants had stronger root morphology to adapt to low K,but sensitive mutants had no these characters.