The Photosynthetic Characteristic Of TaSG1 Wheat Mutant With Stay-green Phenotype And The Physiological Mechanism Responsible For Stay-green

Plant senescence is an important period of plant development, which is a programmed process that is subjected to gene regulation. The structure, function, metabolism and gene expression of the plant cell go through a series of coordinated changes during the plant senescence, two of the distinct changes are the degradation of chlorophyll and the decline of photosynthetic. Wheat (Triticum aestivum L) is a important food crop, and its grain-filling is frequently deteriorated by drought and heat stress combination at the late stage. Premature senescence results in deterioration of the quality of vegetables, poor grain quality and reduced crop yield. Delay leaf senescence is a useful way to increase photosynthetic time, which is beneficial to increase wheat yield. A new wheat species TaSG1 with delayed leaf senescence was obtained in wheat breeding using EMS (ethyl methane sulphonate) as mutagens in our laboratory. In this study, the changes of plant physiology and its stay green mechanism were researched in TaSG1 with wild-type HS2 (WT) as a control. At gain-filling stage, chlorophyll content, net photosynthetic rate (Pn), chlorophyll fluorescence parameters, soluble protein and sugar content, malondialdehyde (MDA) content, superoxide radical (O2-) production rate and H₂O₂ content etc. were determined. The results suggested that the greater antioxidant ablity in TaSG1 than WT may be the important factor that responsible for stay green. Otherwise, to study the mechanism responsible for stay green in TaSG1, dark, light and Methyl viologen (MV) dichloride hydrate were used to induce senescence with seedling leaves, and chlorophyll content and activities of antioxidative enzymes were compared with each other. The main results are as follows:1. Changes of physiological characteristics in TaSG1 and WT during gain-filling stage(1) Compared with WT, TaSG1 shows an obvious stay green phenotype at later stage of nature senescence.(2) The yield and its compositive parameters were higher in TaSG1 than in WT,indicating that the stay green phenotype of TaSG1 is benefits to high yield.(3) The chlorophyll content, the photosynthetic gas exchange parameters (Pn, Gs, Tr, Ci) and the fluorescence parameters (Fo, Fv/Fm, ETR,ΦPSâ…¡, qP, NPQ) were measured at filling stage to evaluate the characteristics of the mutant TaSG1.The results showed that total chlorophyll content in flag leaves of TaSG1 and WT increased from booting stage to flowering and reached the maximal chlorophyll content at about 15 days after anthesis (DAA), then, declined gradually. But TaSG1 showed a marked slowly diminished rate of chlorophyll and carotenoid accumulation compared to the WT. Net photosynthetic rate (Pn) in flag leaves during anthesis and grain filling stages had the same trend as chlorophyll. Compared with WT, the Pn of TaSG1 was higher and the high Pn value was kept for a relatively long period. Changes of stomatal conductance (Gs) and transpiratory rate (Tr) displayed the same trend as Pn. The intercellular CO2 concentration (Ci) decreased from booting stage to about 15 DAA, and then increased. TaSG1 maintained a lower Ci level than WT. The value of Fo, Fv/Fm (the ratio of variable to maximum fluorescence; a parameter of PSâ…¡activity), ETR,φPSâ…¡, qP and NPQ were significantly higher in TaSG1 than WT during senescence, which indicated that TaSG1 might suffer a small excitation energy pressure, less impairment of the light harvesting complex of PSâ…¡, have greater activity of PSII center and an enhanced thermal dissipation in the PSII antennae at the late grain-filling stage.(4) The soluble protein and sugar content in flag leaves of TaSG1 and WT increased before 20 DAA, and then declined gradually, but they were always higher in TaSG1 than in WT during senescence, indicating a delayed senescence and an enhanced resistance to drought and heat stress combination at late stage in TaSG1 than WT.(5) Changes of activities of antioxidative enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX) and peroxidase (POD) in flag leaves were determined, and the results indicting the same trend with Pn. While the activity of CAT decreased from anthesis. These suggested that, during late grain filling stages, activity of antioxidative enzymes in TaSG1 decreased more slowly and maintained a relatively higher level compared with WT. MDA content, superoxide radical (O2-) production rate and H2O2 accumulation in flag leaves increased significantly after anthesis in both TaSG1 and WT. Nevertheless, TaSG1 maintained lower MDA content, superoxide radical (O2-) production rate and H2O2 content in flag leaves than WT. From the results above, we suggest that high antioxidative system competence may be involved in the stay green characteristic of TaSG1.2 The different senescence characteristics between TaSG1 and WT during artificial induced senescence with wheat seedling leaves(1) In dark-induced senescence experiment, no significant difference in senescence was observed between TaSG1 and WT. But in light-induced experiment, different senescence progression between two wheat species was observed, with TaSG1 displaying a significant delayed senescnece.(2) In MV-induced senescence experiment under weak light, compared with WT, TaSG1 showed a obvious stay green trait. Chlorophyll degradation was slower, and activities of antioxidative enzymes were higher in TaSG1 than in WT.From the results above, we suggest that TaSG1 maintain higher chlorophyll content and photosynthetic rate than WT at the gain-filling stage, indicting a obvious delay senescence in TaSG1. A health oxygen-scavenging system that can effectively scaveng reactive oxygen species maybe one of the important mechanisms underlying delay senescence in TaSG1. Otherwise, a higher non-photochemical fluorescence quenching (NPQ), that can dissipation more excess radiant energy and reduce the accumulation of ROS, maybe another factor underlying the stay green in TaSG1.