Effects Of Exogenous Ca～(2+) On Alleviation Of Photoinhibition In Maize Leaves Under Salt Stress

Zea mays cv.Luyu 14 was used as a material in this study. Salt stress wascreated by adding NaCl to culture solution. The possible function ofexogenous Ca2+ in salt stress was investigated by measuring and analyzingphotosynthesis, Chlorophyll fluorescence, water potential, osmotic potential,activity of antioxidants and content of proline and soluble sugars,activeoxygen species and pigment content. The possible mechanisms of Ca2+ on saltstressed maize leaves were discussed. The main results are as follow:1. Different effect of Ca2+ at different salt stress degree(1) Slight salt stress (NaCl≤50mmol·L-1) hardly damaged maize leaves, however addition of Ca2+ to the NaCl solution acted as salt stress.(2) Moderate salt stress(100mmol·L-1,150mmol·L-1,200mmol·L-1 NaCl) had slightly reversible damage effect on maize leaves, exogenous Ca2+ could relieve the damage effect. Different concentration of Ca2+ had different alleviation on the damage. 8mmol·L-1 Ca2+ had the most alleviation 2mmol·L-1 Ca2+ and 16mmol·L-1 Ca2+ had part alleviation on the salt damage. However, when the concentration of Ca2+ more than 32mmol·L-1 ,it acted as salt stress.(3) Severe salt stress(300mmol·L-1,400mmol·L-1 NaCl) had irreversible damage effect on maize leaves, and any concentration of exogous Ca2+ could not alleviate but increased the salt damage.2. Ca2+ had different effects at different salt stress stages Ca2+ had different effects at different salt stress stages. At the early time of stress, Ca2+ had alleviation effect, however it did not at the late time of stress.3. Ca2+ alleviated the decrease in photosynthesis caused by salt stress(1) Under normal growth conditions, exogenous Ca2+ did not have obvious effect on water potential and osmotic potential. But under salt stress conditions, exogenous Ca2+ could markedly increase water potential and osmotic potential of leaves. 3张乃华 外源钙离子对盐胁迫玉米叶片光抑制的缓解效应(2) Addition of Ca2+ suppressed stomatal conductance(Gs) and photosynthetic rate (Pn) under normal growing conditions. But under salt stress conditions, it did not, on the contrary, it significantly increased stomatal conductance and photosynthetic rate.(3) Under normal growing conditions, Mehler reaction dependent electron transport rate(ETR)m increased with the addition of Ca2+, but the increase was not pronounced. While total electron transport rate (ETR)t decreased evidently. Though (ETR)m increased with addition Ca2+, excess light energy still increased because of the decreases of (ETR)t, which indicated that though exogenous Ca2+ accelerated Mehler reaction under normal growing conditions, it did not favor energy dissipation.(4) With the enhancement of Mehler reaction under salt stress,more active oxygen species were produced. The activity of active oxygen scavenging enzyme SOD and APX increased considerably with addition of Ca2+, indicating that active oxygen produced by Mehler reaction under salt stress could be scavenged by the increased activities of SOD and APX.(5) The activity of violaxanthin de-epoxidase(VDE),a key enzyme in the xanthophyll cycle, increased gradually with the time of salt treatment,and it was further enhanced by addition of Ca2+. It was proposed that exogenous Ca2+ helped the build-up of transthylakoid proton gradient,which activited the VDE catalyzing the reaction of violaxanthin (V)to antheraxanthin (A) and zeaxanthin (Z). A and Z cooperating with other factors would facilitate dissipation of excess light energy to protect photosynthetic apparatus from photo- damage.4 The alleviation effect of Ca2+ on the inhibition of growth in maize leaves was attributed to the alleviation of ionic toxity of NaCl(1) Addition of Ca2+ alleviated the damage resulted from salt stress but could not alleviate that resulted from mannitol stress.(2) A...