The Mechanism Of Salt-enhanced Senescence In Cotton Under Salinity Stress

Two experiments were conducted to study the underlying mechanism ofsalinity-induced premature leaf senescence and SNP-delayed leaf senescence incotton under salinity stress at both physiological and molecular levels. Two cottonlines （L21and L22）with differing genetic background were used to study the effectsof NaCl stress or exogenous NO on cotton leaf senescence using hydroponics in thegreenhouse. In NaCl stress experiment, two varieties were treated with a solutioncontaining125mmol·L-1NaCl20days after the fifth main-stem leaf emerged, andirrigated with non-saline water served as the NaCl-free control, the content of leafchlorophyll (Chl), leaf photosynthetic (Pn) rate, concentrations of Na+, K+and someendogenous hormones were examined at a7-day interval after NaCl treatment.Exogenous SNP were foliar applied to salinity-stressed plants and the correspondingphysiological and molecular parameters were determined after salt treatment. Themain results and conclusions are as follow：1. Salt stress enhanced cotton leaf senescenceThe leaf chlorophyll (Chl) content and leaf photosynthetic (Pn) rate are importantindicators of leaf senescence in cotton. Chl content and Pn rate in the mainstem leavesof the two varieties in salt-stressed plants greatly decreased after salt stress, comparedto those in NaCl-free controls. At36d after treatment, salt decreased Chl content andPn rate of L21by68.9%and71.4%, Chl content and Pn rate of L22by58.4%and69.1%, respectively.2. Salt stress increased Na+content and reduced K+contentUnder NaCl stress, at36d after treatment, Na+content of L21and L22in leaves ofsalt-stressed plants were increased6.4times and6.2times compared with theirrespective controls. In leaves and roots of L21, K+content were decreased by78.1% and67.4%; and in L22, K+content were decreased by77.9%and62.9%. Furthermore, Na+/K+rate in leaves and roots of L21were increased by33.1times and15.8times; Na+/K+rate were increased by30.5times and12.6times in L22.3. Salt stress up-regulated abscisic acid (ABA) gene and increased ABAconcentrations, and down-regulated zeatin riboside (ZR) gene and decreased ZRconcentrations.After salt treatment, ABA synthesis gene NCED expression rapidly increased andthus enhanced the leaves of ABA in L21. On the8thday after treatment, ABAconcentrations in leaves of L21and L22were improved by23.1%and29.2%,respectively.On the8thday after salt treatment, zeatin riboside (ZR) gene expression leveldecreased by about1/2than the control group in the leaves of L21; on the36thday, ZRconcentrations were decreased by28.9%and44.1%in leaves and roots of L21, anddecreased by16.5%and37.8%in leaves and roots of L22.4. Salt stress induced the expression of cotton leaf senescence associated NAC gene.Salt stress can also enhance the expression of the three senescence NAC genes(EU706340、EU706342、EU372996).5. Exogenous NO delayed the senescence of cotton leaves.Foliar application of SNP under salt stress increased the content of NO in cottonleaves, promoted the content of leaf chlorophyll (Chl) of binding protein expression,increased the synthesis of chlorophyll, and maintained higher photosynthetic rate.6. Exogenous NO reduced Na+accumulation and slowed K+loss in the leaves underNaCl stress.Foliar application of SNP under salt stress promoted Na+/H+antiporter（SOS1and NHX1）expression in the roots，the absorbed Na+were isolated in the vacuoles ofroot, therefore， reduced the Na+content in the leaves, and alleviated the Na+toxicityin the leaves.7. NO can regulate the hormone level in the cotton leavesFoliar application of SNP under salt stress maintained the balance of ABA incotton, enhance the capacity of the cotton resistance to salt stress and reduce the effect of ABA induced senescence; SNP also can promote the synthesis of CTK, delay thesenescence of cotton.In conclusion, NaCl stress significantly inhibited physiological functions of cottonplants, on one hand, NaCl stress increased Na+content and reduced K+content,thereby lowering the Na/K rates, leading to chloroplast ultrastructure damage,chlorophyll loss, reduced the rate of photosynthesis, and caused leaf senescence. Onthe other hand, NaCl stress up-regulated abscisic acid (ABA) gene and increased ABAconcentrations, down-regulated zeatin riboside (ZR) gene and decreased ZRconcentrations, and thus accelerated cotton leaf senescence. Moreover, NaCl stresspromoted the expression of cotton senescence associated NAC transcription factor,speeding the cotton leaf senescence process. Under salt stress, spraying SNP delayedleaf senescence of cotton. On the one hand, exogenous NO improved the expressionof chlorophyll binding protein, and promoted the synthesis of chlorophyll, anddelayed senescence of cotton leaves; on the other hand, exogenous NO promotedNa+/H+antiporter（SOS1and NHX1）expression in the roots, decreased Na+contentflowing to leaf, K+increased, and reduced ion toxicity and nutrition deficiency.Moreover, NO balanced the transgenic cottons hormone levels and lowered the leafsenescence of cotton.