Study On Physiological Mechanisms Of Phosphorus Efficiency In Brassica Napus

Phosphorus (P) is one of the essential mineral nutrients for plants. As an important oil crop in China, Rapeseed (Brassica napus) needs more P for plant growth, which shows sensitive to low-P stress. Thus, the study on the mechanisms of P efficiency has important significance to the genetic improvement of P nutrition and reasonable fertilization in rapeseed production. Based on the evaluation of P efficiency in a rapeseed recombinant inbred lines (RILs) population, P-efficient genotype 102 and P-inefficient genotype 105 were screened from the population. Used the two genotypes, rhizo-box experiment, pot experiment and solution experiment were conducted to study systematically the genotypic differences in root morphology and physiology in response to low-P stress, and then the physiological mechanisms of P efficiency were discussed. The main results obtained were summarized as follows:1. Screening the genotypes with extreme P efficiency in Brassica napus1.1 Evaluating P efficiency in a rapeseed RILs population with 135 lines and screening the genotypes with extreme P efficiency by two independent solution experiments.1.1.1 There were significant genotypic differences in plant growth among the RILs, shoot dry weight, root dry weight, the ratio of root and shoot dry weight, the max root length, P uptake efficiency and P use efficiency showed normal distribution in the RILs under both P treatments and presented significant transgressive segregation1.1.2 Relative shoot dry weight and relative root dry weight could be used as available evaluation indices for P efficiency but the effect of genetic factors should be considered at the same time1.2 Extreme P-efficient genotype 102 and P-inefficient genotype 105 were obtained by the two screening trials, and they then were confirmed by a pot experiment.2. Physiological mechanisms of P efficiency in Brassica napus based on the study of P-efficient genotype 102 and P-inefficient genotype 105 responding to low-P stress2.1 Root morphology:P-efficient genotype 102 has more dominant root system under low P treatment, its total root length, root surface and root volume were greater than those of P-inefficient genotype 105, which provides an opportunity for acquiring more P from environments and increasing its P uptake efficiency.2.2 P uptake kinetics:P-efficient genotype 102 has greater net P uptake rate; Moreover, at pretreatment low P concentration its Imax value was greater than P-inefficient genotype 105, and Cmin was significantly less than P-inefficient genotype 105, indicating P-efficient genotype 102 has greater P uptake rate and stronger tolerance to low P stress.2.3 Root-exudated proton and organic acids:The amount of proton and organic acids exudated by roots of P-efficient genotype 102 were greater than that of P-inefficient genotype 105 under low P treatment, thus it could mobilize and acquire more sparingly soluble P in soil.2.4 Root-exudated acid phosphatase:The activity of acid phosphatase released by roots of P-efficient genotype 102 were higher than that of P-inefficient genotype 105 under low P treatment, thus it could mineralize more organic P in soil and has stronger ability to utilize organic P.2.5 Acid phosphatase in plant:The activity of acid phosphatase in plant induced by low-P stress increased significantly and was negative correlated to the concentration of Pi. P-inefficient genotype 105 has less inorganic P concentration, therefore it has higher acid phosphatase activity and hydrolyzed more organic P in matured tissue, which increased its P use efficiency.2.6 Photosynthetic characteristics and photosynthetic products:Under low P treatment, the photosynthesis of P-efficient genotype 102 was stronger, and the concentration of sucrose in new leaves was greater than that of P-inefficient genotype 105, but there was not significant difference between genotypes. The higher Pi concentration in old leaves of P-efficient genotype 102 could transport much more sucrose to the young tissues to supply more carbon for plant growth.2.7 The ability of P translocation:Seed yield and seed P accumulation for P-efficient genotype 102 was greater than those of P-inefficient genotype 105, which indicated P-efficient genotype 102 could distribute more P to seeds.2.8 P use efficiency of P-efficient genotype 102 was less than that of P-inefficient genotype 105 under low P treatment, so its higher P efficiency was the result of higher P uptake efficiency.In a word, the mechanism of P efficiency in Brassica napus was mainly contributed to uptake and transport of P. Concretely, P-efficient genotype showed more developer root system, stronger ability of mobilizing insoluble inorganic-and organic-P in soils, and higher activity of absorbing and transporting P under low P stress condition, thus it had stronger photosynthesis. However, P-inefficient genotype showed much more advantage in P reutilization ability under low P stress compared with P-efficient genotype.