Physiological Responses Of Different Genotypic Rice To Low Phosphorus Stress

Phosphorus(P) deficiency is one of the limiting factors to rice production. Screening and breeding of rice genotypes with high phosphorus efficiency would be one of effective alternatives to alleviate P deficiency and to increase the utilization efficiency of P fertilizer. Understanding the nutritional, physiological and molecular mechanisms of rice plant adaptation to low P stress would be very helpful to the above program of screening or breeding. In the present, based on the materials with different P efficiency screened from the preliminary study, cropland culture experiment was conducted to evaluate the photosynthesis characteristic in different rice genotypes. With the materials of rice with high P uptake efficiency genotypes, a hydroponic culture experiment was conducted to evaluate acid phosphatase activity and endogenous hormone, and also two Pi transporter factor from rice were cloned and its expression under both Pi sufficiency and deficiency was analyzed. The results obtained were summarized as follows:1.Under phosphorus stress, the cropland culture experiment was carried out to study the different rice genotypes. The results showed that the relative tiller numbers, relative dry weight of shoot, relative productive panicle, relative grain yield were used as indexes for screening and identifying rice genotypes for tolerance to low phosphorus at whole growing stage. Phosphorus deficiency decreased the photosynthetic rate and stomatal conductance for rice with high efficiency phosphorus, increased the leaf internal CO₂ concentration. It suggested that decreased in leaf photosynthetic rate of rice plants induced by P deficiency was not due to stomatal limitation. The chlorophyll fluorescence kinetic parameters of different rice genotypes were decreased under the condition of phophorus, such as variable fluorescence (Fv), maximal fluorescence(Fm), conversion efficiency of primary light energy of PS II (Fv/Fm), potential activities of PS II (Fv/Fo), effective quantum yield of PS II (Yield), apparent photosynthetic electron transport ratio(ETR), etc., also increased initial fluorescence(Fo), therefore the potential photosynthetic activities, photosynthetic electron transport in photosynthetic chain, thylakoidal proton gradient, assimilation power had been inhibited, which affected the light energy conversion and yield at last. And the photosynthetic structure of rice with high efficiency was less damaged, had better abilities in restore , adaptability, harmony and stress resistance. Therefore the rice with high phosphorus efficiency showed higher photosynthetic characteristic.2.Results of hydroponic culture study showed that low phosphorus increased the leaf Apase, root Apase and root secreted Apase, but the increased extent of different tissue parts of various tolerance genotypes was different, indicated that their adaptation mechanism to low phosphorus stress was different. The changes of endogenous hormones in rice with high phosphorus efficiency were studied under P deficiency stress. The results showed that phosphorus stress induced ABA,GA₃, IAA content increased but the ZR content decreased, the content of endogenous hormones in genotypes were different, indicated that rice regulated content of endogenous hormones to adapt the low phosphorus. The analysis of different hormones ratio showed that rice with high efficiency phosphorus adjusted the balance of hormones to adapt the P deficiency condition.3.Two cDNA clone, OSPTF1 and OSPTF2, encoding rice phosphate transporter gene were isolated by RT-PCR from the total RNA of phosphate-starved root of Oryza sativa(IR71331). Sequence analysis showed that more than 74% nucleotide sequence identified with some other high-affinity Pi transporter of plants. The deduced amino acid sequence of OSPTF2 was predicted to encode the transcription factor of helix loop helix(HLH).OSPTF1 was lack of the conserved domain. Analysis of RT-PCR indicated that OSPTF2 strongly expressed in roots of IR71331 and IR20 whether under Pi supplied conditions or low phosphorus stress, but the expression of IR74 decreased under Pi supplied conditions. OSPTF2 only expressed in IR64 under phosphorus deficiency.