Study On Physiological Mechanism Enhancing Resistance Of Drought Stress Of The Exogenous Putrescine Of Lettuce (Lactuca Sativa L.) And Ultrastructure Of Lettuce Cell

Agronomic water saving is an important aspect of the water-saving agriculture.Studying drought tolerance mechanism of crops have an important meaning for improve water use efficiency and develop agronomic water saving.Lettuce?Lactuca sativa L.?is one of the important foliage vegetables.Polyamines are ubiquitous lowmolecular aliphatic amines generated in organism metabolism.It plays an important role in plant growth,stress response,morphogenesis and senescence.Putrescine?Put?is one of polyamines.In recent years,there are scores of researching on polyamine substances mediate and modulate tolerance to abiotic stresses have been reported.However,studies about Physiological mechanism of drought stress in leafy vegetables were rarely reported on.In the present study,polyethylene glycol?PEG?6000 simulated drought stress.The effects of spraying exogenous Put on heat sensitive lettuce under drought stress and its alleviation were investigated.An analysis of the screened conditions gave the following results:200 g/L PEG 6000most significantly affected lettuce seed germination.At this concentration,‘America fast-growing lettuce'was drought-resistance variety,‘Victoria crispy'was drought-sensitive variety;relieving effect of exogenous Put was most obvious.Then,the hydroponic drought-sensitive lettuce cultivar cv.‘Victoria crispy'was cultivated under drought stress simulated by 10%PEG.By using exogenous Put?0.1 mM?foliar spray,we respectively determined the morphological indexes,antioxidant enzymes,photosynthetic parameters,chlorophyll content,abscisic acid content,Ca²⁺flowrate and the ultrastructure on day 0,2,4,6,8.The main results are as follow:1.The fresh weight,dry weight,plant height,root length,root-shoot ratio,water content and relative water content of lettuces reduced by drought stress.Exogenous Put treatment could alleviate the impacts of drought stress.Under drought stress by 10%PEG,the content of MDA was increased,but the activities of CAT,APX,SOD,POD were inhibited.Exogenous Put treatment significantly raised the MDA content and increased antioxidant enzymes activity.2.The net photosynthesis rate,stomatal conductance,intercellular CO₂ concentration and transpiration rate of lettuces were remarkably raised but water use efficiency was significantly increased.Exogenous Put treatment could effectively recover above mentioned photosynthetic parameters.A,b and total contents of chlorophyll were significantly decreased under drought stress by 10%PEG.Exogenous Put treatment was helpful to chlorophyll contents of lettuces.3.ABA content of lettuce leaves and roots were first increased and then decreased.Exogenous Put treatment was weaken the change of ABA content.Calcium influx was predominant in Stomatal guard cells under drought stress by 10%PEG;The calcium flowrate was raised in root tips.Exogenous Put treatment promoted calcium efflux and decreased calcium flowrate in root tips.Under drought stress by10%PEG,in lettuce leaves,stomatal density was increased.But opening degree of stoma was decreased as well as the number of opening stoma.The chloroplasts became round,the thylakoid structure was fuzzy,the starch grains were reduced,and the phenomenon of the plasmolysis appeared in the mesophyll cells.By exogenous Put treatment,stomatal density was decreased,opening degree of stoma was increased,the number of opening stoma was increased;The chloroplasts were elongated,the thylakoid structure was clear,the starch grains were raised,and the phenomenon of the plasmolysis disappeared.