Physiological Mechanism Of Melatonin Alleviated Seed Germination And Seedling Growth Of Cucumber Under Autotoxin Stress

Autotoxicity is one of the important causes of cucumber continuous cropping obstacles, and has a lot of negative effects on the growth and development of cucumber, such as by inhibiting root growth,reducing leaf chlorophyll synthesis and producing ROS and so on. While melatonin (MT) plays an important role in alleviating plant abiotic stresses, mainly by increasing the formation of lateral roots,dedaying leaf senescence and increasing antioxidant capacity. As autotoxin stress is also a kind of abiotic stresses, it is of great practical significance to explore whether melatonin can alleviate the effects of autotoxin on seed germination and seedling growth of cucumber. In this paper, firstly, the effects of autotoxins on seed germination of cucumber were studied and the relief of MT on seed germination of cucumber under autotoxin stress was explored. Secondly, cinnamic acid (CA), as a well-known major autotoxin, we explored the effect of MT and CA interaction on the cucumber seedling growth. Finally,the mechanism of melatonin to alleviate the cucumber seedling growth under CA stress was explored.The main results are as follows:(1) The effects of cinnamic acid (CA), phenylpropionic acid (PA), benzoic acid (BA) and p-hydroxybenzoic acid (HA) on cucumber seed germination were studied. The results showed that the low concentration of autotoxins could promote cucumber seed germination, while the higher concentration had the inhibitory effects on vigor index, main root length, lateral root number, total root length, total root Surface area and RootVolume. The results also showed that CA and PA were more toxic than BA and HA. In addition, 0.4 mM CA and PA, 0.8 mM BA and 3.2 mM HA appeared obvious inhibitory effect on the cucumber seed germination.(2) The relief of melatonin on cucumber seed gennination under autotoxin stress was studied. The results showed that 0.5?M and 0.25?M MT could respectively alleviate the inhibitory effect of 0.4 mM PA and 3.2 mM HA on cucumber seed germination and 1?M MT could alleviate cucumber seed germination under 0.4 mM CA and 0.8 mM BA stress. Melatonin relieved the inhibitory effects of autotoxins on the cucumber seed germination by increasing the vigor index, main root length, lateral root number, total root length, total root Surface area and RootVolume.(3) The interaction effects of MT and CA on cucumber seedling growth were studied. The results showed that 10 ?M MT had the potential to alleviate the cucumber seedling growth under 0.4 mM CA stress. Under the condition of 0.4 mM CA stress, the leaf became yellowing and wilting and the growth rate of leaf area and leaf dry weight decreased. While pretreatment with 10 ?M MT could alleviate the damage of CA stress to cucumber seedlings by maintaining chlorophyll content, leaf area and shoot dry weight to the normal level.(4) The physiological mechanism of MT alleviating the cucumber seedling growth under CA stress was studied. The results showed that pretreatment with MT maintained the leaf area and leaf dry weight to the normal level, and promoted the total root length, total Surface area and RootVolume,especially the lateral root growth, including 1/4 LRS, 1st Order LR no. and LR density/ MR, and promoting the accumulation of whole plant dry weight. In addition, MT promoted root growth, which promoted the ability of roos to absorb nutrients, enhanced the carbon, nitrogen, phosphorus, potassium,calcium and manganese contents in whole plant and made them more distributed in roots and new leaves. Among them, nitrogen plays a key role, which is closely related to most morphological indicators and endogenous hormone changes. Therefore, MT is mainly by regulating of nitrogen to alleviate CA stress, by regulating the expression of CsNRT1.1 and CsIAA14 gene to control the root morphology, including the total root length, surface area, 1st Order LR no. and LR density/MR, leaf area and dry weights of leaves and whole plant, thereby increasing nitrogen absorption and transportion in plant. MT alleviates the cucumber seedling growth under CA stress by reducing the total ABA, GA3,ZR and IAA contents and regulating GA3 and ABA ratio. In addition, cotyledons were most sensitive to CA stress, so MT had a significant effect on hormone contents in cotyledon and reduced the hormone contents to the normal levels. Moreover, MT reduced the content of MDA in each organ, cleared the ROS (O2·-) in the plant, and stimulated the change of the antioxidant enzyme activities. Seedlings pretreatment with MT had higher SOD, APX and POD activities in roots, maintained the SOD and POD activities in cotyledons and the 2nd true leaves, enhanced the CAT activity in the 1st true leaves.