Study On Boron Nutrition Of Different Genotypes Watermelon Cultivars (Citrullus Lanatus)

Watermelon(Citrullus lanatus)cultivars "Yongshi","Heimeiren" and "Jianzi" grown in pots containing sand were used to study for the growth,photosynthetic system and antioxidant systems under boron concentration of 0,0.05,0.20,0.60 mmol·L^-1.The main results were as follows:1.Boron stress decreased agronomy indexes of three watermelon cultivars,such as plant height,stem diameter and biomass.High boron stress had a greater effect on agronomy indexes of three watermelon cultivars than that of boron deficiency.There was different boron efficiency in different genotypes watermelon cultivars."Yongshi" and "Jianzi" were boron efficient cultivars.The boron efficiency of"Heimeiren" was lower.2.Watermelon under boron deficiency stress did not reveal obvious symptom in the early growth stage before 15 days treatment.Boron poisoning watermelon revealed obvious symptom early.Boron deficiency stress mainly acted on aerial part of watermelon,especially for apical growing point.High boron stress firstly acted on under-ground part and bottom leaves,gradually acted on aerial part of watermelon.3.Boron stress severe damaged the oxygen evolving complexes(OEC)of watermelon leaves, hindered oxygen evolution from water by photodecomposition,inhibited the reaction centers of PSâ…¡get electrons,increased proportion of Non-Q_A reaction center.Boron stress severe decreased the content of PSâ…¡acceptor side,inhibited the reaction center of PSâ…¡transfer electron s,increased proportion of Non-Q_B reaction center too.Boron stress inhibited noncyclic electron transport,effected the dark reaction of photosynthesis.Boron stress may speed up pseudocyclic electron transport,precipitated the creation of a lot of O₂·^-.4.Energy pipeline models of reaction centers in PSâ…¡of watermelon leaves under different boron concentration were obtained.It was presumed that the decreasing photosynthetic rate was more likely resulted from the impairment of electron transfer than that of from the decreasing content of chlorophyll.5.Lots of ROC(O₂·^- and H₂O₂)in watermelon leaves were produced under boron stress, which induced the membrane lipid peroxidation and MDA accumulation.ROC and MDA led to the death of a lot of leaves cells,the stimulation of antioxidative system of watermelon. Watermelon leaves increased POD and APX activities,decreased CAT activities under boron stress.But the antioxidative system of watermelon could not completely resisted active oxygen damage under boron stress.The change of SODase active in watermelon leaves was different under boron stress,which proved that ability of antioxidation of three watermelon cultivars was different.Stress resistance of watermelon cultivar "Yongshi" was stronger.6.Watermelon leaves increased APX,MDAR and GR activities,decreased DHAR activities, proportion of ASA/DASA and proportion of GSH/GSSG under boron stress.Boron stress hindered AsA-GSH cycle,led to ROC accumulation.Because of boron stress inhibited noncyclic electron transport,the impetus of AsA-GSH cycle were decreased.And then AsA-GSH cycle in chloroplast was hindered.H₂O₂ in chloroplast accumulation led to the dark reaction enzyme deactivation.,which destroyed photosynthetic capacity of watermelon leaves.