Supplementary Calcium Restores Peanut (Arachis Hypogaea L.) Photosynthetic Capacity Under Short-term Low Night Temperature Stress

Peanut?Arachis hypogaea L.?is widely cultivated globally,and China ranks first in the world on peanut production.Due to the unique conditions of fertile soil and cool climate in northern China,the aflatoxin contents of the soil is very low in this region,the locally grown peanuts are favored by global consumers because of the very low content of aflatoxin and high quality.In early spring,chilling injury occurs frequently and quickly in northern China,which have negative influences on peanut growth and directly leads to the seedling shortage,then severely limits the yield and quality formation.Our research found that exogenous calcium can effectively improve the chilling-tolerance of peanut.However,little is known about the underlying mechanisms of how exogenous calcium application helps peanuts to cope with short-term low night temperature stress.The research took chilling-sensitive and large seed-type peanut cultivar“Fenghua l”as experimental materials.Through the artificial climate chamber to simulate the conditions of short-term low night temperature stress in different scenarios of the early spring stage on the peanut,combined application of Ca²⁺and its inhibitor?EGTA,ethylene glycol tetraacetic acid?on the leaf surface of the peanut to bidirectional verification of Ca²⁺action sites,by measuring biomass,element contents,photosynthetic gas exchange parameters after long-term low night temperature stress?1,6,11d?and chlorophyll fluorescence parameters,electron microscope transmission images after short-term low night temperature stress?10h?.The main findings are as follows:1.The effects of stomatal factors on photosynthesis under short-term low night temperature stress and the alleviating effects of exogenous calcium.In the early stage of the low night temperature stress,stomatal factors are the main reason for the decrease of photosynthesis.However,the influence of stomatal factors is gradually weakened with the increase of stress time,while the influence of non-stomatal factors is gradually strengthened.Calcium can simultaneously alleviate the decrease of Pn caused by stomatal factors and non-stomatal factors under low night temperature stress,and this alleviating effect is effective for a long time.Exogenous calcium can promote the absorption of P,Mg,Fe,and Mn to improve the photosynthetic level of peanut leaves.Exogenous calcium can increase the stomatal movement by increasing the content of K in cells.2.The response of photosynthetic electron transport to exogenous calcium in peanut leaves under short-term low night temperature stress.The short-term low night temperature stress does more damage to photosystem II?PSII?than photosystem I?PSI?.The acidification of thylakoid membrane was aggravated,the membrane structure was destroyed,the oxygen-evolving complex?OEC?is damaged,the number of plastoquinone?PQ?is decreased under low night temperature stress,which prevents the transmission of excitation energy to PSI and protected the PSI.Meanwhile,cyclic electron transport?CEF?enhancement also protects PSI.Exogenous calcium can effectively promote CEF so that it can avoid being damaged by excessive light energy.The activity of the ATP enzyme was decreased in the thylakoid membrane,which resulted in the increase of transmembrane proton potential??p H?.The occurrence of inhibition is directly related to the increase in?p H.The increased acidification of thylakoid stroma limits the reduction of OEC,P680,PQ,and P700,and the electrons cannot be transferred to the downstream,resulting in photoinhibition.Exogenous calcium reduces?p H,resulting in faster electron transfer.The reason for CEF enhancement may be that the low temperature reduced the activity of key enzymes in the Calvin cycle,resulting in the reduction of NADP⁺/NADPH,which started the cyclic electron transfer,slowed down the excitation energy pressure of PSII and PSI,and generated a large amount of ATP.Exogenous calcium can promote the cyclic electron transport,and then alleviate the damage of excessive light energy to the photosystem.3.The response of photochemical activity to exogenous calcium in peanut leaves under two different kinds?light-dark"evening"and dark-dark"night"?of short-term low night temperature stress.It may be affected by the low night temperature stress and the change from light to dark,which a site of action in the photosynthetic electron transport chain at the early stage of the short-term low night temperature stress.CEF could be rapidly initiated under low night temperature stress,and exogenous calcium could rapidly promote CEF.The CEF and NPQ mechanisms would start up rapidly under the influence of a single environmental factor?room-low temperature?,while the CEF and NPQ mechanisms would start up slowly under the cross stress of light-dark and low night temperature.