Temporal-Spatial Distribution Characteristics Of Cottonseed Important Quality Traits And The Physiological Mechanism Of Their Formation

Cottonseed is not only the by-product of fiber with a high economic additional value,but also the most basic material for cotton production.Because of the increasing costs of cotton planting and the limitation of conventional breeding methods,the research on cottonseed quality has gained increasing attention over recent years.In addition,the time course of cottonseed dormancy release is not yet clear,which won’t help shorten the breeding cycle of cotton and evaluate the cottonseed quality by seed companies quickly and accurately.In the context of climate change,the rainfall during the harvest season of cotton is increasing,especially in the Yangtze River Valley,which lead to the cottonseeds of opened bolls prone to sprout before harvest.The purpose of this study were(1)to summarize the spatial and temporal distribution of cotton yield and quality by harvesting the mature cotton bolls in batches and measuring the 100-seed weight,100-kernel weight,vigor index,oil and protein content of cottonseed;(2)to explore the effects of different planting dates on cottonseed important quality traits by observing and contrasting the development process of different cottonseeds;(3)to clarify the time course and physiological mechanism of cottonseed dormancy release by analyzing parameters of germination,hormones and reactive oxygen metabolism during cottonseed after-ripening.The main results are as follows:1.The spatial and temporal distribution of cottonseed yield and qualityCottonseed planting qualities(100-seed weight,100-kernel weight and seed vigor index)first increased and then decreased with the delay of harvesting time,as well as the kernel oil content.These qualities of cottonseed harvested between 3rd to 6th harvesting times were better,which could roughly correspond to 5-12 fruiting branches.The protein content of cottonseed kernel was higher in the early harvesting time,but lower in the later harvesting time,and then slightly increased after that.Changes of cottonseed qualities were consistent among different cultivars.Quadratic function was suitable to express the relationship between cottonseed quality and its spatial and temporal distribution or the meteorological factors during its ball maturation period.Cottonseed planting qualities and kernel oil and protein content were closely related to the harvesting time,fruiting branch,and mean daily temperature.And they also had higher degree of fitting with GDD(growing degree-days)and TEP(accumulated product of thermal effectiveness and PAR).There was a positive correlation between seed weight,kernel weight seed vigor index and kernel oil content,however,no significant correlation was observed between oil content and protein content.Cottonseed yield,kernel protein and oil yield were decreased with the delay of planting date,while it was different for fruiting branches responding to different planting date.In summary,FB5-12 was the superior fruiting positions of cottonseed qualities and the main contribution position for cottonseed yield,kernel protein and oil production.2.Effects of planting dates on cottonseed qualities(1)Kernel oil and protein contentKernel oil and protein content of the mature cottonseed were negatively correlated in normal planting date,however,they decreased simultaneously at FB10-11 under late planting condition.The significant decrease in Vmax(the maximum velocity of kernel weight accumulation)was the main reason for the decrease in kernel oil content under late planting condition.At the late stage of cottonseed development,the sustained decline in alkaline Inv activity may lead to a short supply of hexose;the decreased activities in glutamic acid synthase and glutamine synthase GS,GOGAT and PEPC could cause a lack of substrate for protein synthesis;the decreased PAPase and G6PDH activities lower the efficiency of oil synthesis and its energy supply;and the above results could be the physiological reasons for the decline in kernel oil and protein caused by late planting.Under the late planting condition,there was a decrease in kernel saturated fatty acid content and an increase in polyunsaturated fatty acid content.The activities of Alkaline Inv,PAPase and G6PDH in XG39K5 were more susceptible to late planting condition and appeared more obvious at FB10-11.Lumian 28 has more unsaturated fatty acids(mainly linoleic acid)and less saturated fatty acids(palmitic and stearic acid)than XG39K5.(2)Planting qualitiesSieed weight,kernel weight and seed vigor of mature cottonseed decreased with the rise of fruiting branches and the delay of planting date.Kernel weight had a more positive effect on seed vigor than seed weight,but the kernel of one cottonseed was not the larger the better.An optimum kernel percentage(kernel weight/seed weight)might be beneficial for cottonseed to reach a maximum vigor.Path analysis showed that the mean daily temperature during boll maturation period was the main meteorological factor influencing these seed properties.Moreover,the final level of kernel malondialdehyde(MDA)content and the electrical conductivity of mature cottonseed leachate were also increased with the delay of planting date and the rise of fruiting branches.Cool temperature caused by late planting prolonged the duration of oxidative stress on membranes by limiting the activities of H2O2-scavenging enzymes at the late stage of seed development to reduce the seed vigor.Among the four H2O2-scavenging enzymes,peroxidase(POD)activity remained relatively stable after reaching maximum and had the highest correlation with seed vigor.While the activities of ascorbate peroxidase(APX)and glutathione reductase(GR)decreased sharply after their peak,which presented more sensitive to the change of water content during dehydration.In addition,seed vigor has stronger correlation with the changes in methionine(Met)content during 4 days of germination than its initial value and final value.3.The physiological mechanism of cottonseed dormancy release by after-ripeningUpland cotton seed is known dormant when freshly harvested,but the time-course and physiological mechanism of its dormancy release remain unclear.In the present study,freshly harvested cottonseeds were subjected to dry after-ripening for 180 days.Cottonseeds at different days of after-ripening(DAR)were sampled for germination.The intact seeds and isolated embryos were germinated contrastively for assessing the effects of seed coat and embryo on cottonseed dormancy.Contents of H2O2 and phytohormones and the activities of antioxidases and glucose-6-phosphate dehydrogenase in embryo were measured during after-ripening and germination.Results showed that the effects of nitrogen application level on the germination and dormancy of cottonseed was more significant than that of after-ripening time.Only 7%of the freshly harvested intact seeds could germinate,while more than 80%of the isolated embryos germinated at 0 DAR.The release of embryo dormancy required 21 DAR,which was 9 days shorter than the time coat-imposed dormancy required,according to the results of T50(the time when 50%of the seeds germinate)and germination index.During after-ripening,drying and coat weakening caused the accumulation of H2O2 in embryos which further promoted the decline of ABA content to release cottonseed dormancy.A noticeable increase was observed in GAs content during germination when ABA content decreased to a lower level.Coat removal treatment made the embryo absorb water faster,more sensitive to H2O2 and meanwhile altered the changes of peroxidase.Cottonseed dormancy release resulted from the interaction of embryo physiological metabolism and seed coat weakening during after-ripening and germination.H2O2 acted as a core link between the response to environmental changes and induction of other physiological changes for breaking cottonseed dormancy.