Nutritional And Physiological Mechanisms Of Rice Plant Adaptation To Low Phosphorus Stress

Phosphorus (P) deficiency is one of the limiting factors to rice production. Screening and breeding of low P tolerant rice genotypes would be one of effective alternatives to alleviate P deficiency and to increase the utilization efficiency of P fertilizer. Understanding the nutritional and physiological mechanisms of rice plant adaptation to low P stress would be very helpful to the above program of screening or breeding, hi the present study, based on the materials screened from the preliminary study, a sand culture experiment and a solution culture experiment were conducted to evaluate genotypic variation of rice in P acquisition from sparingly soluble phosphates and to elucidate mechanisms relevant to P acquisition from sparingly soluble phosphates. Agar culture experiments were conducted to study genotypic variation of rice in P acquisition from phytate and to investigate the factors to influence the utilization of phytate by rice. A rhizo-box experiment was conducted to study the rhizosphere mechanisms involved in the uptake of different P fractions in the soil by rice. Effect of phosphorus deficiency on leaf photosynthesis characteristic, carbohydrate distribution, and acid phosphatase activity was also studied. The results obtained were summarized as follows:1. Genotypic variation of rice in P acquisition from sparingly soluble phosphates and root uptake and mobilization mechanismsGenotypic variation of rice in uptake and utilization of Fe-P by fifty one rice genotypes was studied. Results showed that frequency distribution for both P uptake efficiency and P use efficiency of 51 rice genotypes were normal distribution, suggesting there was considerable difference in uptake and utilization of Fe-P among different rice genotypes and it would be a great potential for screening out rice genotypes with high efficiency in P uptake or utilization of Fe-P. A sand culture experiment and a solution culture experiment were conducted to evaluate genotypic variation of rice in P acquisition from different sparingly soluble phosphates (Al-P and Fe-P) and its relationship with root morphology, root P uptake kinetics, and mobilization characteristic of root exudation byusing eight genotypes of rice, which differed in their P uptake ability from Fe-P in a preliminary experiment with 51 rice genotypes. Results showed that there was genotypic variation of rice in P acquisition from Al-P or Fe-P. The ability of P uptake from sparingly soluble phosphate was closely associated with root morphology, especially with root surface area, root volume, and total number of lateral root. Low P treatment significantly increased the /max, but significantly decreased the Km for all of eight rice genotypes. The P uptake from Al-P or Fe-P by rice was not significantly correlated to /max, ï¿¡m, or Cmin- Genotypic variation in P mobilization from sparingly soluble phosphates by rice root exudation could not account for the genotypic variation in P uptake from sparingly soluble phosphates. The amount of malic acid and oxalic acid secretion in the root exudation was not consisted with P mobilized from sparingly soluble phosphates by root exudation.2. Utilization of P from organic phosphate by two rice genotypes under sterile and non-sterile conditionsAgar culture experiments were conducted to study in P acquisition from phytate by two rice genotypes under sterile conditions and to investigate the effect of inoculation with soil microorganisms on the phytate utilization by rice. Results showed that for both Zhongbu 51 and Azucena grown in agar under sterile conditions, shoot dry weight, P content, and P concentration of plants supplied with P as myo-inositol hexaphosphate (IHP) were significantly (P < 0.05) decreased in comparison to that for plants supplied with inorganic P (P;) at the equal level, indicating that rice had a limited ability to acquire P from IHP under sterile conditions. By comparison, Zhongbu 51 showed higher ability to acquire P from IHP than Azucena, which would probably be related to the fact that Zhongbu 51 showed higher root secreted APA than Azucena when grown with deficient P supply. Plant dry weight and P uptake of rice were significantly increased with the enhancement of IHP level, and phytate utilization by rice was not enhanced by inoculation with soil microorganisms, suggesting that utilization of P from phytate by rice was probably substrate-limited.3. Rhizosphere mechanisms of P uptake from soil by riceA rhizo-box experiment was conducted to study the changes in pH, phosphatase activity and phosphorus fractions in the rhizosphere of two rice genotypes. Results showed that Pembe showed greater P uptake than Zhongbu 51 under minus-P and plus-P treatments. Significant depletions of resin-Pi, NaHCO3-Pi, NaHCO3-Po, and NaOH-P;occurred in the rhizosphere of both genotypes regardless of P treatments. By comparison, Pembe showed higher ability to deplete resin-Pi, NaHCO3-Pi, NaHCO3-Po, NaOH-Pi and NaOH-Po either in depletion zone or in depletion extent which might be the main contribution to the fact that Pembe had greater ability to uptake P than Zhongbu 51. No significant depletion of HCl-Pj was observed despite significant acidification in the rhizosphere of Pembe under minus-P treatment. In addition, we found that AcPME activity was negatively (P < 0.01) correlated to NaHCO3-Po concentration in the rhizosphere of both Zhongbu 51 and Pembe, indicating that AcPME activity was closely associated with the depletion of organic P in the rhizosphere of rice plants.4. Relationship between acid phosphatase activity and either P nutritional status or P efficiency in riceA hydroponic culture experiment supplied with either sufficient P or deficient P was conducted to study the changes in leaf acid phosphatase activity (APA), root APA, leaf inorganic P concentration, and plant total P concentration in rice by using eight rice genotypes different in their response to low P stress. Plants were sampled at 5, 10, 15, and 20 days after treatments (DAT). Results showed that there were significantly (P < 0.05) genotypic variations in P acquisition efficiency (PAE), P use efficiency (PUE), leaf APA, and root secreted APA under either sufficient P treatment or deficient P treatment. The response of leaf APA and root secreted APA to P deficiency varied significantly (P < 0.05) among eight rice genotypes. Relative leaf APA of rice plants was inversely correlated to relative PAE (P < 0.01), and was positively correlated to relative PUE {P < 0.01), suggesting that leaf APA was closely associated with P efficiency for rice plants. However, root secreted APA showed little correlation to P efficiency in rice. There was inverse relationship between leaf APA and P concentration when rice plants were in thestatus of P deficiency moderately (plants sampled at 10 and 15 DAT). However, there was no significant relationship between leaf APA and P concentration when rice plants were in the status of P deficiency slightly (plants sampled at 5 DAT) or severely (plants sampled at 20 DAT), indicating that the extent of P deficiency dictated whether there was a significant correlation between leaf APA and plant P concentration.5. Effect of phosphorus deficiency on leaf photosynthesis and carbohydrates partitioning in two rice genotypes with contrasting low phosphorus susceptibilityA hydroponic culture experiment was conducted to study the effect of phosphorus deficiency on leaf photosynthesis and carbohydrates partitioning of two rice genotypes Zhenongda 454 (low P tolerant genotype) and Sanyangai (low P sensitive genotype). Results showed that the plant growth of Zhenongda 454 was less affected by P deficiency compared with Sanyangai. Under P-deficient conditions, photosynthetic rate of Zhenongda 454 and Sanyangai were decreased by 16% and 35%, respectively. Zhenongda 454 showed higher photosynthetic rate than Sanyangai at deficient P treatment, whereas there was no difference at sufficient P treatment. Phosphorus deficiency decreased the stomatal conductance for both genotypes, but had no significant influence on leaf internal CO2 concentration (Q), suggesting that decrease in leaf photosynthetic rate of rice plants induced by P deficiency was not due to stomatal limitation. P deficiency treatment increased partitioning of soluble carbohydrates and sucrose to roots for both rice genotypes. By comparison, Zhenongda 454 maintained a higher photosynthetic rate, and had a greater ability to allocate carbohydrates to roots than Sanyangai under P-deficient conditions, which might be related to the fact that Zhenongda 454 was more tolerant to low P stress than Sanyangai.