| Tomato(Solanum lycopersicum L.)is one of the main vegetable crops with wide planting range and large protected cultivation area in China.Because of its warm-loving characteristics,tomatoes are often subjected to sub-low temperature stress due to severe cold in winter and in late spring in protected production in China,which seriously affects their normal growth and development and reduces their yield and quality.The results showed that exogenous 5-aminolevulinic acid(ALA)could enhance tomato’s tolerance to sub-low temperature,but its effect on the absorption and utilization of elements at low temperature and its specific mechanism needed to be further studied.Therefore,in this study,the low-temperature sensitive tomato ’Jinpeng No.1’ was used as the material to study the effects of foliar spraying ALA on the stress resistance,post-stress recovery ability,yield and quality of tomato at sub-low temperature {8-18℃/8-12℃(day/night)},and explore the regulation mechanism of exogenous ALA on element utilization of tomato seedlings at sub-low temperature.The main results are as follows:1.Exogenous ALA promoted the reproductive growth by promoting the vegetative growth of plants under sub-low temperature stress at seedling stage and the rapid recovery after stress,thus alleviating the inhibition of sub-low temperature on tomato yield and improving fruit quality to some extent.(1)With the extension of sub-low temperature stress time,the maximal quantum yield of PSII photochemistry(Fv/Fm)and chlorophyll content of tomato leaves gradually decreased,the content of malondialdehyde(MDA)and relative conductivity(REC)gradually increased,and proline(Pro)accumulated.Exogenous ALA significantly alleviated the decrease of Fv/Fm and chlorophyll content and the increase of Pro,MDA and REC in tomato seedling leaves caused by sub-low temperature stress.The Fv/Fm and REC of tomato seedling leaves can be restored to the control level after 7 days of normal temperature growth when the sub-low temperature stress is less than 6 days,but the above indexes of tomato couldn’t be restored when the stress time exceeds 6 days.Exogenous ALA significantly promoted the recovery of Fv/Fm,REC,MDA content and Pro content in seedling leaves after sub-low temperature stress for different time periods,and significantly alleviated the obstacles of plant recovery after sub-low temperature stress.(2)After the tomato plants suffered from sub-low temperature stress at seedling stage and returned to normal temperature,the yield of tomato at fruiting stage decreased significantly,and the longer the stress time was,the more obvious the yield decreased.Exogenous ALA significantly promoted the recovery of tomato yield after short-term and medium-long-term(within 12 days)sub-low temperature stress.Sub-low temperature stress at seedling stage significantly reduced the content of reducing sugar,lycopene,soluble protein and sugar-acid ratio in tomato fruit.Exogenous ALA improved the tomato quality after sub-low temperature stress to some extent.2.The application of exogenous ALA can alleviate the adverse effects of sub-low temperature stress on the accumulation and distribution of mineral elements in tomato seedlings,promote the recovery of plants after stress,and significantly alleviate the damage caused by sub-low temperature.Under sub-low temperature stress,exogenous ALA can promote the absorption of N,P,K,Ca,Mg and Fe in tomato seedlings,and promote the transport of nitrogen from root to leaf by alleviating the accumulation of nitrogen in tomato seedlings roots,thus maintaining the normal accumulation of total nitrogen in leaves and plants.Promote the accumulation of phosphorus and potassium in tomato seedling stems,total phosphorus in lower leaves and plants,and calcium and magnesium in leaves.Promote the transport of iron from the lower leaves to the upper leaves to realize the accumulation of iron in the upper leaves of tomato seedlings,and significantly promote the recovery ability of plant element absorption after sub-low temperature stress,thus alleviating the damage of sub-low temperature to tomato seedlings.3.Exogenous ALA or abscisic acid(ABA)could alleviate the inhibition of sub-low temperature stress on Fe and Mg accumulation and chlorophyll synthesis in tomato seedling leaves.The expression of zinc-iron transporter genes SIZIP1 and SIZIP3 could be up-regulated by applying ALA or ABA after inhibiting ABA synthesis at sub-low temperature.At the same time,the contents of protoporphyrin IX(Proto IX),magnesium protoporphyrin IX(Mg-Proto IX)and protochlorophyll(Pchl),which are precursors of chlorophyll,were increased,and finally the accumulation of iron elements and the synthesis of chlorophyll in leaves were promoted,and the photosynthetic efficiency was improved.However,compared with spraying exogenous ALA,spraying exogenous ABA under sub-low temperature stress did not play a significant positive role in the accumulation of magnesium in leaves.Exogenous ALA can induce the accumulation of endogenous ALA and ABA in tomato seedling leaves under sub-low temperature stress.At the same time,ABA may be involved in the synthesis of chlorophyll by taking part in the synthesis of proto IX from ALA.The crosstalk between ALA and ABA promoted the accumulation of iron and magnesium elements and chlorophyll synthesis under sub-low temperature stress.In summary,exogenous ALA alleviated the nutrient utilization barrier and stress injury of tomato seedlings under sub-low temperature,and regulated the reproductive growth of plants,thereby alleviating the inhibitory effect of sub-low temperature stress on tomato production at the seedling stage.Under sub-low temperature stress,ABA can be induced by exogenous ALA and participate in the process of ALA synthesis of chlorophyll,and the two synergistically promote the accumulation of iron in leaves. |
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