Abstract

Media summary

Key words

Short-seasoned cotton (SSC), quantitative traits, antioxidant enzymes

Introduction

The acreage of double or multiple cropping counted for 60% of total acreage of farming land in cotton producing areas in China, the intercropping index reaching 165%. One of the critical measurements for multiple cropping practices is the exploitation and planting of short-seasoned cotton cultivars (SSC). However, under the current double or multiple cropping farming system, especially in the cotton producing areas of the Yellow River Valley, the key obstacle for such practice is that the maturity of some SSC is still not early enough. Cotton plants in these cultivars usually show slower early growth and late maturity with high percentage of post-frost harvest, which decreases lint yield and qualities. Therefore, increasing lint yield and improving fibre quality are the most important purposes of cotton genetic improvement in SSC. In order to attain an early maturity of SSC that fits the requirements of double or multiple cropping practices in various cotton producing areas, an effective means for delaying premature senescence of early maturing SSC is through coordination of vegetative and reproductive growth. This paper examines changes in parameters relating to early maturity without premature senescence and understanding genetic control.

Materials and methods

Two categories of SSC cultivars were used in the experiments: Type A cultivars show premature senescence and include Zhongmiansuo 10 (designated as A₁), Zhong 450407 (A₂), Zhong 652585 (A₃), Zhong 619 (A₄), and Yuzao 28 (A₅); type B cultivars show early maturity without premature senescence and include Liao 4086 (designated as B₁), Zhong 925383 (B₂), Zhong 061723 (B₃), Zhong 961662 (B₄) and Yu 1201 (B₅). Five reciprocal crosses between Type A and B were made in 2000, with their progenies referred to as AnBn,BnAn respectively(N=1,2,3,4,5). All F₁ and F₂ progeny and parental lines were sown in a randomised complete block design with 3 replications at the China Cotton Research Institute, Chinese Academy of Agricultural Science, Anyang, Henan. The 4^th leaves from the topmost leaf of each cotton plant was sampled during flowering and boll-set. Activities of antioxidant enzymes including CAT, POD, SOD, and content of MDA were measured. Data were analysed by SAS and with the genetic model described by Zhu (1993, 1994).

Results and discussion

Population distributions of biochemical inheriting traits in F₂ between Type A and Type B

It is generally believed that the biochemical traits possibly associated with the early maturity of SSC are quantitative, exhibiting a continuous and normal distribution in the F₂ generation. Distributions in F₂ populations from the reciprocal crosses between A₁ and B₁ for the 5 biochemical traits are presented in Figures 1 to 4. The distributions of these traits in all crosses reasonably fitted a normal distribution indicating that activities of CAT, POD and the contents of MDA and chlorophyll are consistent with segregation for a quantitative trait. However, there existed two peaks in the distribution of the activity of SOD indicating non-normality (P < 0.0001).

The inheritance characteristics of biochemical traits

Using an additive-dominance genetic model, significant cytoplasmic maternal effects were observed in the activities of CAT, POD and SOD, MDA, ABA and soluble proteins. The activity of CAT, specific activity of SOD, and chlorophyll content were primarily affected by maternal effects while dominant nuclear genetic effects were also important for specific activity of CAT and POD, SOD activity. Additive genetic effects were important for POD activity and ABA content.

Heritabilities of the biochemical traits in SSC population

POD activity and contents of ABA, IAA and chlorophyll all had relatively higher heritability. POD activity was the highest (53%), and then ABA (52%). CAT specific activity, activities of POD, SOD, and contents of IAA, ABA had comparably higher heritabilities. POD activity was the highest (79%), and then IAA content (78%). Therefore, if early maturing lines without premature senescence are to be bred, early generation selection based on POD activity, contents of ABA, IAA and chlorophyll content could be effective.

Correlations among the biochemical traits

The additive-dominance genetic model was used for correlation analysis. CAT activity had a positive genetic and phenotypic correlation with SOD activity and IAA content, while it had a negative genetic and phenotypic correlation with POD activity, MDA, ABA and chlorophyll contents. The results indicated that CAT together with SOD and IAA could share some common roles in the early developmental stages of cotton. POD activity had positive genetic and phenotypic correlations with SOD activity, MDA and chlorophyll contents, but negative genetic and phenotypic correlations with IAA and ABA contents, indicating that POD and SOD could share some joint roles in the later developmental stages. ABA content had a negative phenotypic correlation with IAA and MDA content. Since SOD activity had positive genetic and phenotypic correlations with the CAT and POD activities, and IAA and MDA content, negative genetic and phenotypic correlations with ABA content, it may render a major role of in promoting early maturity without premature senescence of cotton plants.

Specific expression of biochemical traits in boll-setting stage

The additive-dominance genetic model indicated that from 74 to 105DAP maternal effects on CAT, POD, SOD activity and MDA content reach an extreme significance level. Dominance effects on CAT activity and MDA content increased gradually, reaching the highest expression level (61.7% and 45.8%) at 95 DAP, while dominance effects on POD and SOD activity decreased gradually, after reaching the highest expression level (62.8% and 51.2%) at 81 DAP. The additive effect on CAT, POD, SOD activity was detected until 95 DAP, while the additive effect on MDA content was detectable and decreased gradually. Heritability of CAT, POD, SOD activity and MDA content were prominent at different developmental stages. Heritability of CAT activity was highest (63.4%) at 95 DAP, and then smallest (57.4%) at 81 DAP. Heritability of CAT, POD, SOD activity maintained a low level all the time while Heritability of MDA content maintained a high level at 74 to 88 DAP then tended to decline.

Conclusion

Genetic Basis of the Physiological and Biochemical Traits

The biggest obstacle for developing early maturing, high yielding, improved fibre SSC cultivars is premature senescence of such cultivars. As early maturity and premature senescence are positively correlated, it is difficult to improve lint yield and quality. Therefore, new methods in cotton breeding for high yielding SSC should be taken into consideration. Biochemical assistance breeding could be one of the solutions to such problems. Research indicates that the major enzymes in antioxidant system, such as CAT, POD, SOD and ABA have large, significant maternal and dominance gene effects. The specific activities of CAT and POD, activity of SOD, contents of soluble proteins, IAA, and MDA are largely dominant whereas POD activity and ABA content are largely additive. Accordingly, the genetics of biochemical traits in cotton can be thought to be controlled by both nuclear and cytoplasm gene effects. Traits such as POD activity, and ABA and IAA content have significant additive effect making selection for these effective in development of early maturing cotton. The relationship among the biochemical traits in cotton can be showed that in the early growth stage of cotton, CAT, SOD and IAA have a cooperative effect. In the late growth stage of cotton, POD and SOD have a cooperative effect. SOD plays a major role in promoting early maturity without premature senescence in the total developmental stages.

Inheritance of Biochemical Traits during Boll-setting Period

During boll setting from August 3^rd to September 3^rd (74 to 105DAP), the activities of CAT, POD, and SOD were mainly maternal in nature, and to a lesser extent nuclear dominant. The nuclear controlled additive was much smaller and somewhat undetectable until after September. Therefore, through manipulating the genetic effect of biochemical traits from August 3^rd to September 3^rd in early maturing SSC cultivars with no premature senescence, we can understand the developmental and genetic basis for such biochemical traits. This study has laid a foundation for further exploration of how and when the relevant genes express for these biochemical traits.

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