Effects Of Chemical Topping On Plant Morphological, Canopy Structure, And Canopy Photosynthetic Production Of Cotton

【Objective】Plant growth regulators compounded as chemical topping agents which could inhibit effectively bud growth, and result in a more compact plant compared with manual topping, this achieves the goal of manual topping. The objectives of this two-year field experiment were(i) to study the effects of two chemical topping agents in the production of cotton, Flumetralin and Mepiquat chloride, on the characteristic of plant morphological, canopy structure, canopy photosynthetic production and yield of cotton;(ii) to discuss how changes in canopy structure affect canopy photosynthetic production and yield,(iii) to determine how topping way affect defoliation ripening effect. The results of this study provide increased understanding that could be used for more effective use of chemical topping agents.【Methods】The study included one cotton cultivar Xinluzao 45 of main variety and one cotton cultivar Zhangmiansuo 50 and one cotton strain 45-21 of main varieties. Conventional manual topping was the control treatment. To Measure systematically different topping treatments(i) the characteristic of plant morphological, yield and fiber quality of cotton;(ii) canopy structure index, the light distribution of different canopy layers;(iii) canopy photosynthetic production; the discipline of dry matter accumulation and distribution; yield components and the characteristic of boll forming;(iii) the rate of shed leaves, the rate of hanging leaves, the rate of boll open.【Results】Plant height, number of main stem section and main stem leaf number were signifiantly greater and after spraying growth was larger in the chemically-topped treatments than in the manually-topped treatment; the length of the branch of the upper layer and plant wide were signifiantly lesser in the chemically-topped treatments than in the manually-topped treatment, and lateral growth was inhibited significantly at after spraing. Cotton chemical topping could reduce cost of production, but cotton fiber quality of the chemically-topped treatments didn’t affect. The LAI and leaf chlorophyll contents were greater in the chemically-topped treatments than in the manually-topped treatment, the difference in LAI and leaf chlorophyll contents were extremely significant at the early boll opening stage; light transmittance in the upper and middle canopy layers were greater in the chemically-topped treatments than in the manually-topped treatment. However, late in the growing season, light transmittance in the lower canopy layers was lesser than in the manually-topped treatment. Leaf chlorophyll contents increased to a maximum and then decreased at the end of the growing season. Chemical topping increased leaf chlorophyll content. Canopy apparent photosynthetic rates were significantly greater in the chemically-topped treatments than in the manually-topped treatment. Furthermore, canopy apparent photosynthetic rates remained high for a longer time in the chemically-topped treatments. At the early boll opening stage, canopy apparent photosynthetic rates in the chemically-topped treatments were 16.04 μmol·m-2·s-1 which was 14.35%-16.40% greater than those in the manually-topped treatment. Canopy respiration rates were significantly greater in the chemically-topped treatments than in the manually-topped treatment during the first part of the season. After reaching a maximum there was no significant difference among the treatments. The ratio of canopy respiration rate to total apparent photosynthetic rate was higher in the chemically-topped treatments than in the manually topped treatment. The total dry matter accumulation was greater averagely in the chemically-topped treatments than in the manually-topped treatment, and the linear growth duration of total dry matter accumulation was long averagely 4.88 days; The ratio of dry matter accumulation of the reproductive organs and vegetative organs to total dry matter accumulation was more harmonious. The total furit branch numbers and boll number per were greater in the chemically-topped treatments than in the manually-topped treatment. Cotton yield in the Flumetraline treatment was greater than that in the manually-topped treatment. The rate of shed leaves and the rate of boll open were greater in the chemically-topped treatments than in the manually-topped treatment, the rate of hanging leaves was lesser in the chemically-topped treatments than in the manually-topped treatment.【Conclusions】Cotton chemical topping agents can inhibit effiectively apical dominance and control lateral growth of branch and shape compact plant type and influence the formation of cotton canopy structure. Compared with manual topping, chemical topping increased the LAI and the leaf chlorophyll content. The LAI and chlorophyll content both remained high for a relatively longer time in the chemically-topped treatment. Chemical topping improved light distribution in the canopy and increased the photosynthetic area. This ensured that the chemically-topped treatments had the highest canopy apparent photosynthetic rate and the longest duration of photosynthetic activity, which provided the safeguard for high yield formation. Meanwhile, the total dry matter accumulation was greater averagely in the chemically-topped treatments than in the manually-topped treatment, and the linear growth duration of total dry matter accumulation was long averagely 4.88 days; The ratio of dry matter accumulation of the reproductive organs and vegetative organs to total dry matter accumulation was more harmonious, which provided security for high-yiled formation. The compact plant type and reasonable canopy light distribution of chemical topping cotton inproved cotton defoliation ripening effect.