[Objective] Melatonin is a small molecules widely exist in higher plants which is regarded as a new plant growth regulator and biological stimulant.Melatonin plays an important role in improving plant resistance,but research about melatonin involved in regulation of plant photosynthesis under adversity stress is rarely reported.The objective of this study is to explore the influence of exogenous melatonin on photosynthesis of tomato leaves under drought stress.[Method] Tomato cultivar ‘LiaoYuanDuoLi’ was used as the experimental materials,concentration screening tests were firstly carried out:CK:Leaf spray with water and root application with 50 mL water.R5,R50,R100,R150,R250:leaf spray with water and root application with 50 mL 5,50,100,150,250 μmol·L-1 melatonin.L5,L50,L100,L150,L250:root application with 50 mL water and leaf spray with 5,50,100,150,250 μmol·L-1 melatonin.This process was repeated in the morning and afternoon for three consecutive days and the drought stress treatment was conducted for subsequent three days (CK0:normal water after leaf spray with water and root application with 50 mL water,CK1:drought treatment after leaf spray with water and root application with 50 mL water).The optimal melatonin concentration was screened comparing the maximum photochemical quantum yield of PSⅡ (Fv/Fm) and a parameter representing the quantity of efficient PSⅠ complex (Pm).Then the influence of root application and leaf spray with exogenous melatonin on gas exchange parameters,light energy distribution and electron transfer rate of PSⅠ and PSⅡ and the integrity of the thylakoid membrane and ATP enzyme activity of tomato leaves under drought stress were analyzed by using photosynthetic fluorescence synchronous measurement technology.[Result] Root application and leaf spray with different concentrations of melatonin both increased the Fv/Fm and Pm of tomato leaves under drought stress,and all showed a trend of increase at first and then decreased with the increase of concentration,the values of Fv/Fm and Pm were the highest under L100 and R100,both significantly higher than that of control,thus it was determined that L 100 and R100 were the optimal concentration treatments for leaf spray and root application,respectively.L100 and R100 significantly alleviated the inhibition of drought stress on the gas exchange parameters,and the leaf net photosynthetic rate (Pn) was 2.04 μmol·m-2·s-1 and 1.71 μmol·m-2·s-1,respectively,which both significantly higher than the control (CK1) (0.52 μmol·m2·s-1);transpiration rate (E) was 0.66 mmol·m2·s-1 and 0.54 mmol·m-2·s-1,respectively,and both significantly higher than that of CK1 (0.25 mmol·m2·s1).L100 and R100 treatments significantly increased the stomatal conductance (GH2O) and the maximum water use efficiency (WUE) and significantly reduced stomatal limitation (Ls) of tomato leaves under drought stress,it was also found that L100 treatment was superior to R100.The results of rapid light response curve showed that L100 and R100 enhanced Fv'/Fm'and qP of tomato leaves,indicated melatonin is beneficial to improve photochemical reaction efficiency of PSⅡ of tomatoes under drought stress.Cyclic electron flow of tomato seedlings under drought stress was significantly enhanced,while melatonin treatment reduced on cyclic electron flow,but strengthened ETRⅠ and ETRⅡ,and they were both higher under L100 compared with R100.Y (Ⅰ) and Y (Ⅱ) both improved under L100 and R100 compared with CK1,indicate melatonin treatment strengthened light energy distribution to the direction of photochemical reaction of PSⅠ and PSⅡ under drought stress.P515 induction curves of L100 and R100 were higher than that of CK1 after dark adaptation,and after illumination,P515 signal of CK0 fell fast,followed by L100 and R100,CK1 treatment decreased slowest,showed that exogenous melatonin protected the thylakoid membrane from damage caused by drought stress and strengthened the ATP-synthase activity.[Conclusion] Root application and leaf spraying with exogenous melatonin can relieve the inhibition of drought stress on photosynthetic performance of tomato leaves,strengthen the photosynthetic efficiency,leaf spraying is a more simple and efficient measure compared with root application.Melatonin can enhance crop photosynthesis adaptability to the environment stresses and has an regulatory role in crop growth and development.%[目的]褪黑素是一种广泛存在于高等植物体内的小分子物质,被认为是一种新的植物生长调节剂和生物刺激剂,对于提高植物抗逆性具有重要作用.探索外源褪黑素对干旱胁迫下番茄叶片光合作用的影响,为揭示褪黑素调节植物抗逆性的机制打下基础.[方法]以番茄‘辽园多丽’为试材,首先采用叶片喷施和根施不同浓度褪黑素进行预处理:CK:叶片喷施清水、根施50 mL清水;R5、R50、R100、R150、R250:叶片喷清水,分别根施50mL 5、50、100、150和250 μmol·L-1褪黑素;L5、L50、L100、L150、L250:根施50 mL清水,叶片分别喷施5、50、100、150和250 μmol·L-1褪黑素;连续处理3d后将植株移至温室中,以不浇水作为干旱处理(其中CK0:叶片喷施清水、根施50 mL清水预处理后正常浇水,CK1:叶片喷施清水、根施50 mL清水预处理后干旱处理).干旱胁迫5d后,通过比较暗适应下PSⅡ最大光化学量子产量Fv/Fm和PSⅠ最大氧化状态Pm,确定根施和叶片喷施的最佳浓度处理.然后利用光合荧光同步测量系统分析根施和叶片喷施褪黑素对干旱胁迫下番茄叶片气体交换参数,PSⅡ和PSⅠ的光能分配和电子传递速率,类囊体膜的完整性和ATP酶活性的调节.[结果]根施和叶片喷施不同浓度褪黑素均提高了干旱胁迫下番茄叶片的Fv/Fm和Pm,并且随浓度增加表现出先升高后降低的趋势,L100和R100处理下的Fv/Fm和Pm最大,显著高于对照.L100和R100显著缓解了干旱胁迫对气体交换参数的抑制,其中叶片净光合速率(Pn)分别为2.04和1.71 μmol·m-2s·-1,显著高于对照(CK1)(0.52 μmol·m-2·s-1);蒸腾速率(E)分别为0.66和0.54mmol·m-2·s-1,显著高于CK1(0.25 mmol ·m-2·s-1),并且显著提高了番茄叶片气孔导度(GH2O)和最大水分利用效率(WUE),降低了气孔限制值(Ls),而L100优于R100处理.快速光响应曲线结果表明L100和R100处理提高了干旱胁迫下番茄幼苗PSⅡ的光化学反应效率Fv'/Fm'及PSⅡ光化学淬灭系数qP,表明褪黑素处理更利于干旱胁迫下番茄叶片PSⅡ光化学反应的高效进行;干旱胁迫下番茄幼苗环式电子传递速率得到显著加强,而L100和R100处理降低了环式电子传递速率,但加强了线性电子传递速率,且L100处理下番茄叶片ETRⅠ和ETRⅡ均高于R100处理;L100、R100处理提高了干旱胁迫下番茄叶片的Y(Ⅰ)、Y(Ⅱ),表明褪黑素处理有利于干旱胁迫下番茄叶片吸收光能向光化学反应的方向分配;暗适应后,L100和R100处理番茄叶片P515诱导曲线均高于CK1,照光后,CK0处理番茄幼苗P515信号快速下降,其次是L100和R100处理,而CK1处理降低较慢,表明褪黑素具有保护叶绿体类囊体膜和ATP合成酶免受干旱胁迫伤害的作用.[结论]根施和叶片喷施外源褪黑素能缓解干旱胁迫对番茄幼苗光合性能的抑制,加强光合运转效率,而叶片喷施是一种更简单高效的处理方式;褪黑素能加强作物光合作用对环境胁迫的适应性,对于农作物的生长发育具有调节作用.
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