Adaptation Of Photosynthesis In Leaves Of Mulberry Plants To Low Temperature And Salt-alkali Stresses

In this study, response and adaptations of photosynthesis in leaves of mulberry plants werestudied, using gas exchange and chlorophyll fluorescence technology, to investigateacclimation mechanisms under low temperature, salt and light conditioning stress. Resultsshowed as follows:1. Apparatus of light absorption and light energy transfer in leaves of mulberry wasestablished firstly during growth stage, and then stomata structure and functions and CO₂assimilation were improved gradually. Photochemical induction of sun-light leaves was fasterto approach steady state than that of week-light leaves, and induction in non-photochemicalquenching (NPQ) appeared a high level in the first stage and then was decline. Photochemicalinduction of weak-light leaves took on a biphasic induction, NPQ increased rapidly, thenmaintained high value for a long time. It was inferred from this observation that light activationenhanced significantly light induction in weak-light leaves. Under the high light condition,state-transition quenching (qT) in leaves grown in natural light condition made up 18% of totalnon-photochemical quenching (qN), but qT of the weak-light leaves amounted only to 7% ofqN. It meant that qT in sun-light leaves played more important role in light energy distributionbetween photosystemâ… (PSâ… ) and photosystemâ…¡(PSâ…¡) than that of weak-light leaves. But highenergy quench (qE), which made up over 65% of the total qN, was the main mechanism ofnon-photochemical quenching in leaves grown in sun-light and weak light environments.2. The activity of PSâ…¡reaction center, efficiency of light energy transfer and electrontransport of PSâ…¡declined when PSâ…¡complex and outer antenna proteins (LHCâ…¡) weredamaged by low temperature. Moreover, it was found that increase of fluorescence radiationin mulberry leaves played a very important role in relieving excitation pressure and protectingphotosynthetic apparatus under low temperature. Comparison among three mulberry varieties,Mongolia mulberry was more resistant to low temperature than Qiuyu and Qinglong mulberry.3. Effect of salt alkali stress on mulberry photosynthesis had the time effect andconcentration effect. Photosynthetic capability in leaves of Qiuyu and Mongolia mulberry wasdecline with increase in salt concentration and stress time. Under the same salt ironconcentration condition, effect of salt alkali stress on mulberry photosynthesis was moresignificant than that of neutral salt stress. Moreover, response of photosynthesis to neutral saltand salt alkali stress were differences between Qiuyu and Mongolia leaves. Mongolia mulberrywas more resistant to neutral salt than that of Qiuyu mulberry.4. The mechanism of mulberry leaves adaptive to salt alkali stress: (1) Stomatal conductanceinduction to low CO₂ concentration/high irradiance showed that stomatal conductance in leavestreated for 8 days was nearly equal with leaves of no treatment. It was indicated that thedecrease in stomatal conductance was an efficient adaptation to salt alkali stress, and stomataclose could reduce water loss andincrease water use efficiency; (2) Photosynthetic systemâ…¡ (PSâ…¡) in mulberry leaves was slightly affected by salt alkali stress, and light intensity wassignificance to the level of non-photochemical quenching. Under salt alkali stress and highirradiance conditions, non-photochemical quenching increased because of high energyquenching (qE); (3) Excessive electrons could be used by photorespiration under salt alkalistress. Therefore, the damage of photosynthetic apparatus caused by active oxygen could bealleviated by photorespiration; (4) Activities of SOD, CAT and POD increased within short-time salt alkali stress. The results indicated that adaptations of photosynthesis to salt alkalistress included stomatal behaviour, non-photochemical quenching, photorespiration andprotective enzymes.