The Effects Of Exogenous Nitric Oxide On Regulating Photosynthetic Capacity Of Wheat Leaves Under Water Deficit

The growth, water physiology and photosynthetic capacity of wheat seedlings in response to different concentration of SNP were determined after they were treated with SNP (exogenous NO donor) at five different concentrations ranging within 100μmol·L^-1–2000μmol·L^-1. The experiment was conducted under controlled conditions in laboratory. It showed that at different concentrations NO had different effecs on the growths of shoots and roots. The experiment was conducted under drought stress of– 0.5 MPa PEG, while we chose 100μmol·L^-1 sodium nitroprusside (SNP) as NO donor. The effects of SNP on the ability to distribute and utilise the light in PSII of winter wheat (Triticum aestivum L.) under water deficit were studied through chlorophyll a fluorescence kinetic technique in the paper. The results are as followed:1. At different concentrations SNP stimulated or inhibited the growths of shoots and roots of wheat seedlings. At low concentration SNP (100μmol·L^-1) significantly ameliorated the photosynthetic performance, stimulate the dry-matter accumulation of shoots and formation of the osmotic substances. In addition, the gas exchange and chlorophyll a fluorescence parameters were significantly concentration dependent. 100μmol·L^-1 SNP significantly increased osmotic substances soluble sugar (SS) and free amino acids (FAA) of Triticum aestivum L. roots, but it had less effect on the dry-matter accumulation of the roots. After treated with SNP at more than 200μmol·L^-1 concentration, the dry-matter accumulation of roots was less than the control, the osmotic substances began to decrease. Therefore, it showed that SNP significantly promoted the growth and regulated the physiology of wheat seedling at 100μmol·L^-1.2. The content of photosynthetic pigment increased, but the proportion of opened PSII reaction centers (indicated by qP) decreased under drought stress, thus the utilization of absorbed energy in PSII (F_v'/F_m') and the exertion of photosynthetic function (LPFD) were restricted. SNP could help to increase photosynthetic pigment content, and reduce the cell permeability, thus avoiding the high confined value of photosynthetic function (LPFD) by drought. Furthermore, SNP increased the proportion of opened PSII reaction centers (indicated by q_P), leading to the high excitation energy transfer to PSII function center and the less excitation energy dissipating throug antenna pigments (NPQ), which increased the more absorbed light to participate in photochemical reaction (F_v'/F_m'). In addition, SNP alleviated the excitation pressure in PSII reaction centers imposed by drought and lowered the reoxidation of Q_A (1- q_P). Therefore, exogenous nitric oxide donor (SNP) may participate in the synthesis of photosynthetic pigments and the utilization of light-energy in PSII of wheat under drought stress.3. Drought reduced the distribution of absorbed energy in photochemical reactions (P), enhanced the close of PSII reaction centers. NO alleviated water deficit and Chlorophyll decay. NO maintained the photosynthetic electron transport (Ф_PSII, F_m/F₀) and potential activity of PSII (F_v/F₀, F_v/F_m) under drought stress. Furthermore, NO increased the allocation of absorbed light in photochemical reactions (P), while alleviated drought-induced inhibition to transfer efficiency of absorbed energy (F_v'/F_m') and the excess light dissipation (E). However, the effects gradually became weakened with increasing SNP treatment duration. And bovine hemoglobin (Hb), a NO scavenger, partly or wholly eliminated the effects of NO on PSII function. In addition, under PEG stress, the addition of NO metabolite NaNO2 (NO2-) did not get rid of inhibitory effects of PEG. Therefore, these findings provide direct evidence supporting that NO protects PSII function and ameliorates the absorbed light allocation in chloroplast of Triticum aestivum L. suffering drought stress.