Response Of Subtending Leaf Source Capability Of Cotton Cultivars To Short-Term High Temperature Stress During Flowering And Boll-forming Stage

The subtending leaves of cotton are the most important source organ for the development of cotton bolls,and the leaf source capability mainly includes the formation and the transformation of photosynthate.Generally,the yield and quality of cotton are seriously affected by short-term high temperature stress during flowering and boll-forming stage in hot summer.The shortage of source capability caused by heat stress is undoubtedly to induce the reduction of yield and quality of cotton.Due to the differences response of high temperature stress of different cotton thermal sensitive cultivars,the source capability of subtending leaves in response to short-term high temperature stress between two different thermal sensitive cotton cultivars were studied,it can provide theoretical basis for the study of heat resistance mechanism of source capability and stress resistance of cotton cultivation.In this study,two cultivars(PHY370WR thermal insensitive and Sumianl5 thermal sensitive)were used as materials to carry out pot experiments in Nanjing Agricultural University from 2015 to 2017.High temperature(34℃(38/30℃)lasted for 5 days)and normal temperature(26℃(30/22℃))during the cotton flowering and boll-forming stage were controlled by the artificial climate chamber.In addition,13C labeling technique was introduced to mark the photosynthate of the subtending leaves.Combined with the study of the leaf source capability in 5 days after the heat stress release,the main research contents are as follows:(1)the differences of leaf photo synthetic capability between the two different thermal sensitive cultivars in response to short-term high temperature stress and after the high temperature stress release,(2)the differences of leaf photosynthate transportation capability between the two cultivars under the short-term high temperature stress and after the release of stress,(3)the differences of leaf photosynthate storage capability between the two difference thermal sensitive cotton cultivars.The main results were as follows:1.The difference of leaf photosynthetic capability of cotton cultivars with different thermal sensitivities in response to short-term high temperature stressThe subtending leaves photosynthetic capability of the two cultivars decreased significantly during the 5 days of the short-term high temperature stress,while recovered evidently there after the heat stress release.There were significant differences of leaf photosynthetic capability of the two different thermal sensitive cotton cultivars in response to short-term high temperature stress.The reduction percentages of leaf net photosynthetic rate(Pn)of PHY370WR always lower than that of Sumian15 both under and after the heat stress.The differences of leaf photosynthetic capability of different thermal sensitive cultivars in response to short term high temperature stress were not only existed under the stress,but after the stress release.The response of leaf source capability to short term high temperature stress was significantly different between the two cultivars under and after the heat stress.Under the short term high temperature stress,the reduction percentage of the gas exchange parameters,chlorophyll fluorescence parameters,and the activity of Rubisco of PHY370WR were significantly lower than those of Sumian15.There was no significant difference of the chlorophyll fluorescence parameters Fv/Fm and Rubisco activity between the two cultivars after the heat stress release.The difference of the Pn between the two cultivars was highly associated with the difference of the cy-FBPase activity,and the thermal sensitivities differences of the photosynthetic system between the two cultivars was no longer the main reason for the significant difference of the leaf photosynthetic capability after the heat stress release.2.Differences of the leaf photosynthate transportation of the different thermal sensitive cotton cultivars in response to the short-term high temperature stressThe leaf photosynthate transportation capability was significantly restricted by the heat stress.Under the heat stress,the reduction percentage of the leaf photosynthate transportation of PHY370WR was 14.7%and 16.6%in 2015 and 2016,respectively,and the reduction percentage adjusted to 7.3%and 8.7%after the stress release.For Sumianl5,the reduction percentage was 21.6%and 22.6%under the stress and adjusted to 18.1%and 18.3%after stress removal in 2015 and 2016,respectively.The leaf starch content of PHY370WR was higher than that of Sumian15 both under and after the heat stress.The transportation capability of leaf photosynthate was significantly correlated with the diurnal variation of the starch content in leaves.The leaf starch content was accumulated in the daytime while decomposed in the night,and the raised of the minimum starch contents was the main reason for the reduction of the diurnal variation amplitude of leaf starch contents under the heat stress.The differences response of leaf starch degradation capability at night to short term high temperature stress could be the main physiological process which mediated the differences of the leaf photosynthate transportation capability of the different thermal sensitive cotton cultivars in response to the heat stress.3.Differences in temporary storage capability of leaf photosynthetic products of cultivars with different thermal sensitivitiesThe reduction percentage of leaf net photosynthetic rate reached about 80%after the phloem blocking,while the chlorophyll fluorescence parameters Fv/Fm and the activity of photosynthetic system within 48 hours were not so much affected by the blocking.The leaf photosynthate transportations were seriously depressed and caused the accumulation of the starch and sucrose in leaves which reached the maximum level after 30 h and 40 h after blocking,respectively.The maximum values of leaf starch and source contents of PHY370WR were about 2.5 and 2 times of the control maximum value under normal conditions,while the starch and source contents of Sumian15 were 2 and 1.5 times of the control,respectively.Those results indicated that the PHY370WR had stronger storage capability for photosynthate thus could store more photosynthate in their leaves than that of Sumian15.4.Differences of leaf photosynthetic recovery capability and photosynthetic time in response to short-term high temperature stress of cotton cultivars with different thermal sensitivities5-day short-term high temperature stress had a significant negative effect on cotton yield.The difference of boll weight reduction percentage of the two cultivars was the main reason for the bigger yield decrease of Sumian15 than that of PHY370WR.The difference of boll weight response to the short-term high temperature stress of different cotton cultivars was not only related to the difference of leaf source capability under the heat stress,but also after stress release.The PHY370WR had a stronger photosynthetic recovery capability than Sumianl5 after the heat stress release,as indicated by higher recovery speed and recovery extent in leaves during the 5 days after the heat release,and more flush leaves of PHY370WR were observed.5-day short-term high temperature stress significantly shortened the leaf photosynthetic time.The reduction photosynthetic time of PHY370WR was shorter than Sumian15 and could maintain a longer photosynthetic function period.Compared with PHY370WR,Sumian15 had a more sensitive response of leaf net photosynthetic rate and MDA content to starch content.Leaf starch accumulation had a positive promotion effects on leaf senescence when meet with the heat stress could be the main reason for the difference of photosynthetic time in response to short term high temperature stress of different thermal sensitive cultivars.To sum up,the leaf source capability of different thermal sensitive cotton cultivars was different in response to heat stress and after the stress release,the difference in sensitivity of leaf photosynthetic system in response to heat stress resulting the differences of photosynthetic capability was the main difference of leaf source capability between cultivars.After the heat stress release,the differences of leaf photosynthate transportation capability in response to heat stress was the main difference of leaf source capability between cultivars.The decrease of leaf photosynthate transportation capability in response to heat stress was the main reason for the decline of leaf source capability with thermal sensitive cultivar.The leaf photosynthate transportation capability was significantly restricted by the heat stress and caused the photosynthate accumulated in the leaves in the form of starch,the starch accumulation not only inhibited the leaf photosynthesis,on the other hand,it accelerated the leaf senescence process and shortened the photosynthetic function period.