| Crop genetic resources represent both the basis for agricultural development and areservoir of genetic adaptability that acts as a buffer against environment change,and have the potential to improve crops by means of all of our technological toolsinclude the conventional plant breeding and modern genetic engineering. It is nowwidely recognized that physiological and biochemical indices and agronomicperformance, alternative indices to leaf senescence and to photosynthesis rate and totraditional crop yield, are becoming more and more important in breeding programsto improve food supplies. Wheat (Triticum aestivum L.), as the most important cropworldwide, planted widely in China, has evolved into various ecotypes due to naturaland artificial selection, and the number of accessions contained in the gene bank isseveral thousand. Therefore, how to utilize these genetic resources effectively isworthwhile exploring. In this situation, the concept of "Core Collection", was coinedand developed by Frankel and Brown in 1984, represents with a minimum ofrepetitiveness the genetic diversity of a crop species and its wild relatives. Itsobjective is to describe the genetic diversity of crop species and its wild relatives andto identify a subset of accessions, which contain, or is believed to contain most ofthe known or unknown genetic diversity.In the present study, the wheat core collection included 241 genotypes, screenedfrom all wheat resources collected throughout China based on genetic, morphologicand pedigree analysis, were employed. Physiological indices included heading dateand chlorophyll content (SPAD value), and biochemical indices includedmalondialdehyde (MDA) content of flag leaves, and activities in the flag leaves ofboth superoxide dismutase (SOD) and catalase (CAT) were investigated, and theagronomic performance and yield traits included the plant height, length of spike,tillers, 1000-seed weight, spikelets of per spike, neck-panicle length were alsoinvestigated. Then, (â… ), sampling 66 genotypes from the core collections, then takingthe varieties from the same ecological region as a group, average values of CATactivity, SOD activity and MDA content in the flag leaves of every group were estimated. The average values, variance, standard deviation, and the coefficient ofvariation were calculated. The means±standard errors of means of 1000-grainweight of accessions from different ecological regions were also calculated. (â…¡),sampling 50 accessions from core collections, and the varieties were classified as sixgroups according to the different heading date. The average value of the biochemicaltraits included MDA content and activities of SOD and CAT in flag leaves andagronomic values and 1000-grain weight of accessions in different groups werecalculated.The results revealed that there was large variation in heading date among thecore collections with a range (R) of 40 days in Partâ… and of 37 days in Partâ…¡.In Partâ… , 42 accessions which included all the accessions from MY and YHheaded from 27th March to 16th April within the optimum heading time in SWC,which revealed that the genotypes from MY and YH were well-adapted to southwestChina. In contrast, the varieties from QT did not head during this time periodindicating that the wheat accessions from this region could not well -adapted to theclimate of SWC. Eighteen cultivars headed from 19th April to 30th April. Thosecultivars that headed during this period suffered from high temperature duringgrain-filling that resulted in shrunken kernels, which consequently decreased yield.There are 6 varieties headed from 1st May to 6th May. It is seemed that the wheatvarieties headed during the second and the third periods were not adapted toenvironmental conditions in SWC. Among the genotypes employed, Yunmai34(YM34) that originated from SWC had the earliest heading at 158 days after sowing,the results suggested that YM34 maybe carry the gene(s) responsible for earlyheading date, and detailed genetic analyses will be the objective of further study.The MDA content, the activities of SOD and CAT in flag leaves and1000-grain weight exhibited abundant variation within and between the cultivarsfrom different ecological regions. The content of MDA of accessions from YH andSWC were the lowest throughout the whole measurement period indicating that thecultivars from these regions had no severe oxidation occurring during leafsenescence, and which possess the optimum heading dates and high 1000-grain weight. The pattem of CAT activity was reverse to that of MDA content in our study,a possible explanation is that the higher CAT activity is, the stronger the ability toremove H2O2 is. Therefore those genotypes with lower oxidative stress, have lowercontent of MDA during early senescence. However, the pattern of SOD activity inthe flag leaves during senescence was significantly different from those of CATactivity and MDA content. There were obvious differences between groups in thedirection and rate of change in SOD activity. This suggested that SOD activity hadthe most abundant genetic diversity within and between groups, especially when thenumber of sampling is great.In Partâ…¡, as far as CAT activities was concerned, all genotypes from thedifferent heading date had the similar variation tendency, but the rate of change existvariation; The change of MDA content was reverse to that of the CAT activitieswhich similar to the result of Partâ… ; The SOD activity change of flag leaves wasmore complex than those of CAT activity and the MDA content which also similar tothe result of Partâ… ; Change of chlorophyll content of flag leaves, the similartendency with different rate throughout the measurement period was detected,generally, the group 1, 2, 3 sharply decreased during the end plant life cycle, and incontrast, group 4, 5, 6 maintained the level smoothly throughout the period. Also,there were exist abundant variation in agronomic performance and yieldcompositions of core collections. The data revealed that the plant height of mostcultivars over 120cm which was the challenge for utilization to further improvewheat. The length of spikes, tillers, spikelets of per spike revealed large variationamong six groups. The neck-panicle length showed the normal distribution, and the1000-seeds weight showed the low level in core collection, generally.Generally, the higher 1000-grain weight of accessions possesses the higherlevel CAT activity and lower level of MDA content in flag leaves during senescence.Furthermore, results also showed that the accessions from Yellow-Huai Riversregion (YH), middle and lower reaches of Yangtze River (MY) and southwest ofChina (SWC), which possess the optimum heading time and excellent biochemicaltraits examined and high 1000-grain weight, are most well-adapted in the experimental region for wheat grown. And the results also revealed that the1000-seed weight of awned accessions higher than awnless accessions in general,however, this conclusion exist little variation in different heading date time. Someelite wheat accessions, such as Xingyi4 and Yunmai34, from the core collectionsexhibited excellent traits and high yield data and could be used for yieldimprovement.The results from partâ… and partâ…¡showed that the core collections used in thepresent experiment may represent the maximum variations in the traits studied. Andthe many traits revealed similar and abundant variation between both parts; we canestimate the whole core collection by sampling one part of it, which concluded thatscreening mini core collection is feasible and economic approach for evaluating cropgenetic resources (CGRs) to improve crops.
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