Relationship Between Root Chemical Signals And Grain Quality Of Rice And Its Regulation Techniques

This study was designed to elucidate the relationship between root chemical signals (hormones, organic acids, amino acids, polyamines and ions) and the grain quality of rice and its regulation techniques. Various rice genotypes were used and experiments were conducted in the water-culture pools, pots and paddy field. The relationship between root chemical signals and grain quality and its mechanisms were studied. The techniques to regulate root chemical signals and improve grain quality were investigated. The main results are as follows:1.Relationships of cytokinins in roots and grains with grain filling and cooking quality of rice1)Concentrations of zeatin + zeatin riboside (Z+ZR) in roots and grains at the early grain filling stage were significantly or very significantly correlated with the initial grain growth potential, mean grain filing rate, maximum grain filing rate and brown rice weight, whereas significantly and negatively correlated with active grain filling period. Z+ZR concentrations at the mid or late grain filling stage significantly or very significantly correlated with active grain filling period but those at the late stage negatively correlated with grain filling rate.2)Z+ZR concentrations in roots and grains at mid and late grain filling stages were significantly or very significantly correlated with the gel consistency and alkali spreading value of rice, whereas significantly and negatively correlated with amylase content. The results suggest that cytokinins play an important role in the regulation of grain filling and quality of rice, and the effects that are positive or negative may depend on grain filling stages.2.Relationships of abscisic acid (ABA) in roots and grains with grain filling and cooking quality of rice1)Concentration of ABA in roots and grains at mid grain filling stage were significantly or very significantly correlated with the initial grain growth potential, mean grain filing rate, maximum grain filing rate and brown rice weight, whereas significantly and negatively correlated with active grain filling period. The correlations were not significant between ABA concentrations in roots and grains at early and late grain filling stages and grain filling characteristics.2)Concentrations of ABA in roots and grains at mid grain filling stages were significantly and negatively correlated with gel consistency and alkali spreading values, whereas significantly correlated with amylose content.3.Relationships of 1-aminocylopropane-1-carboxylic acid (ACC) in root bleedings with appearance quality of riceACC concentrations in root bleedings were higher at the early grain filling stage, and showed a small difference among the cultivars. It was reduced rapidly and differed markedly among the cultivars with the progression of grain filling. Regression analysis demonstrated that ACC concentrations in roots at the mid grain filling stage were very significantly correlated with chalky kernel percentage and chalkiness, and the concentration at the late grain filling stage was very significantly correlated with chalkiness and chalky size. ACC concentrations in roots at the early grain filling stage were significantly correlated with chalky kernel percentage, chalkiness and chalky size.Observation form scan electron microscope showed that the cultivars with the lower ACC concentration in grains during the grain filling period had close arrangement and small space in/ between starch granules of endosperm, whereas those with higher ACC concentration in the grains exhibited a loose arrangement, larger spaces and disparity in /between starch granules.The starch granules were loosely arranged and chalky kernel percentage, chalky size and chalkiness increased significantly when 10-6 mol L-1 ACC was applied, whereas the results were reversed when 10-6 mol L-1 amino-ethoxyvinylglycine (AVG inhibitor of ACC synthase) was applied to panicles during 10-15 and 25-30 DAA.4.Relationships of organic acid exuded from roots with grain qualityThe more malic acid and succinic acid exuded from roots for a cultivar, the greater the breakdown values and smaller the setback values in the starch profile, and the results were reversed for a cultivar with more tartaric acid and citric acid exuded from roots during the grain filling period. The cultivar with more lactic acid in exudates had smaller gel consistency and alkali spreading values, but had greater amlyose content. Regression analysis demonstrated that malic acid and succinic acid exuded from roots were significantly correlated with breakdown values, and significantly and negatively correlated with setback values in starch profile. Relationships of tartaric acid and citric acid exuded from roots with breakdown values and setback values in starch profile were reversed. Lactic acid in exudates was significantly and negatively correlated with gel consistency and alkali spreading values, whereas significantly correlated with amylose conent.5.Relationships between organic acid exuded from roots and metal absorbed in rice The root and plant absorbed less amount of cadmium (Cd) for a cultivar with more lactic acid in root exudates, whereas absorbed more amount of lead (Pb) for a cultivar with more the malic acid and succinic acid exuded from the roots. Lactic acid in exudates was significantly and negatively correlated with the amount of cadmium (Cd) absorbed by the root and plant. Pb accumulations in roots and plants were significantly and negatively correlated with contents of malic acid and succinic acid exuded from roots, and significantly correlated with contents of tartaric acid and citric acid.6.Relationships of amino acid and ions in root exuded with grain qualityThe concentration of each amino acid in root exudates decreased gradually with the process in grain-filling. The concentrations of amino acids in root exudates were significantly and negatively correlated with the relative content of amino acids in grains, and the concentration of alkali amino acids in root exudates was significantly and negatively correlated with 1000-grain weight. Concentrations of [Ca2+] and [K+] exuded from roots were significantly and negatively correlated with chalkiness and amylose content. The concentration of [NO3-] exuded from root was significantly and negatively correlated with amylose content. Concentrations of [Na+], [NH4+], and [PO43-] exuded from root were significantly correlated with protein composition.7. Relationships between polyamine concentration in roots and protein composition of riceThe more prolamin content in grains for a cultivar, the higher putrescine concentration and smaller spermine concentration in root. The results were reversed for a cultivar with higher globulin content in grains. Concentrations of spermine in rice roots were very significantly and negatively correlated with prolamine content in the grain, whereas significantly and positively correlated with the content of the globulin. The concentrations of putrescine secreted by roots were very significantly and positively correlated with prolamine content in grains,but significantly and negatively correlated with the globulin content. The ratio of spermine to putrescine (spermine/ putrescine) in roots was very significantly and negatively correlated with the content of the prolamine and significantly and positively correlated with globulin.8. Regulation of cytokinins and ABA in roots with activities of key enzymes in starch synthesisActivities of adenosine diphosphate glucose pyrophosphorylase (ADPGase), soluble starch synthase (SSSase) and Q-enzyme at the early grain filling stage, and the mean and maximum activities of each enzyme during the grain filling period were positively and significantly or very significantly correlated with the mean and maximum grain filling rate and starch content (mg grain-1) in grains. Activities of ADPGase at all grain filling stages and those of Q-enzyme at the early and mid filling stages were not significantly correlated with the cooking quality (gel consistency, alkali spreading value, and amylose content). SSSase activities at the early filling stage were significantly and negatively correlated with gel consistency and alkali spreading value, and positively correlated with amylose content. Activities of SSSase at mid and the late grain filling stages and Q-enzyme at the late filling stage were significantly and positively correlated with gel consistency and alkali spreading value, and negatively correlated with amylose content.Application of zeatin (Z) of 10-6 mol L-1 at 7-10 d after anthesis (DAA) showed no significant effect on activities of the three enzymes at early and mid grain-filling stages (12-24 d after anthesis), whereas significantly increased activities of the enzymes at the late grain-filling stage (27-39 d after anthesis). Spraying ABA of 10-6 mol L-1 at 7-10 d after anthesis significantly increased enzyme activities at early and mid grain-filling stages, but significantly reduced enzyme activities at the late grain-filling stage.9. Techniques of regulating the root chemical signals1) Chemical regulation: The positive or negative effects of chemical regulators on root chemical signals and grains quality depend on categories of the regulators. Spraying ABA at 14-20 DAA increased amylose content, reduced alkali values and gel consistency. The results were reversed when exogenous ZR was applied at 24-30 DAA. Application of ACC increased chalky kernel percentage, chalky size and chalkiness, while spraying AVG reduced chalkiness and improve grain appearance quality.2) Dry-wet alternate irrigation during grain-filling: moderate dry-wet alternate irrigation during grain-filling reduced ACC concentration in roots, increased transparency, peak viscosity and breakdown value, significantly reduced chalky kernel percentage, chalkiness, hot viscosity, peak viscosity and setback value, and improved rice appearance and cooking quality. Moreover, moderate dry-wet alternate irrigation during grain-filling increased activities of SuSase,ADPGase,SSSase and Q-enzyme. The activities of SuSase, ADPGase, SSSase and Q-enzyme were positive and significantly correlated with grain-weight, peak viscosity and breakdown value, negatively and significantly correlated with chalkiness and setback value. The results suggest that moderate dry-wet alternate irrigation during grain-filling could improve rice quality. Enhanced activities of SuSase, ADPGase, SSSase, Q-enzyme play an important role in the improvement of grain quality under such a condition.3) Nutrient management: application of nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) during grain-filling regulated the root chemical signals and rice quality. Deficiency of N, K and Ca accelerated root senescence, reduced organic acids, amino acids and ions exuded from roots, worsened appearance and cooking quality of rice. Deficiency of P significantly increased organic acid, amino acid and ion concentrations in root exudation, reduced protein and amino acid contents in grains. The results suggest that an appropriate application of N, P, K and Ca can regulate root chemical signals and improve rice quality.When rape cake was used as organic fertilizer in the basic fertilizer, the concentrations of malic acid and succinic acid in root exudates were increased, while the concentrations of tartaric acid and citric acid were decreased, and no significant changes in lactic acid were observed. Application of rape cake increased the breakdown value, whereas reduced the setback value, in starch-profile. Moreover, with basic fertilizer increased or panicle fertilizer decreased under the same amount of nitrogen application (225 kg N ha-1), the concentrations of malic acid and succinic acid in root exudates, brown rice, milled rice, and head milled rice were increased, and protein content reduced, and rice quality improved. The results suggest that rational nitrogen strategies can regulate the concentration and component of organic acids in root exudates and then influence rice quality. The nitrogen management for high-yielding and good quality would be 6:1:3 for basic fertilizer: tiller fertilizer: panicle fertilizer.