全生育期UV-B辐射增强对棉花生长及光合作用的影响

植物光合系统是UV-B辐射最初和最重要的作用靶标。本文在大田条件下进行紫外灯照射处理,研究全生育期UV-B辐射增强(高于环境20%和40%)对棉花形态、干物质积累、光合色素和产量的影响,并通过分析棉花主茎功能叶片的气体交换参数和叶绿素荧光参数,探讨UV-B辐射增强影响棉花光合作用的机制。结果表明,UV-B辐射增强抑制了棉花生长和干物质积累,籽棉产量显著降低,且UV-B辐射越强,抑制作用越明显。随UV-B辐射的增强,棉花主茎功能叶的净光合速率(P_n)在各生育期均显著降低,叶绿素含量呈先升高后降低趋势,气孔导度(Gs)和蒸腾速率(Tr)未发生变化,胞间CO_2浓度(Ci)反而升高,说明P_n下降主要由非气孔限制因素造成。对叶绿素荧光参数的分析表明,PSⅡ的最大光化学量子产率(F_v/F_m)、实际光化学量子效率(ΦPSII)、线性电子传递速率(ETR)和光化学淬灭系数(qP)随着UV-B辐射的增强而降低,非光化学猝灭系数(NPQ)则显著升高,且各叶绿素荧光参数与Pn变化均显著相关;慢速弛豫NPQ(NPQS)及其在NPQ中的比例均随UV-B辐射的增强而显著提高,表明PSⅡ反应中心受损,光化学效率降低。以上结果证明,全生育期UV-B辐射增强降低了棉花的光合叶面积、叶绿素含量和净光合速率,引起棉花生长与物质积累受抑,产量降低。UV-B辐射增强引起的光合速率下降与PSⅡ反应中心遭到破坏密切相关。

Effect of enhanced UV-B radiation on cotton growth and photosynthesis

It has been shown that the thinning of ozone layer continuously enhances ambient ultraviolet-B (UV-B) radiation. En-hanced UV-B radiation influences the growth, development and metabolism of crops, of which photosystem is the initial and most important target. In this study, UV-B radiation was increased by 20% and 40% by using ultraviolet lamp during the whole growth period of cotton under field condition, and its effect on cotton morphology, dry matter accumulation, photosynthetic pigment content and seed cotton yield were analyzed. The influencing mechanism of enhanced UV-B radiation on photosynthesis was also investigated by determining gas exchange parameters and chlorophyll fluorescence parameters in functional leaves. The results showed that the growth of cotton stems, leaves and dry matter accumulation were significantly inhibited by enhanced UV-B ra-diation. The inhibition effects of enhanced UV-B radiation on cotton were more obvious at seedling stage than that at later growth stages. Seed cotton yield also remarkably decreased with increasing UV-B radiation. The contents of chlorophyll a (Chla) and chlorophyll b (Chlb) increased under the treatment of 20% above ambient UV-B radiation and there was no change in Chla/Chlb. When UV-B radiation increased to 40% above ambient UV-B radiation, Chla, Chlb and Chla/Chlb significantly decreased. With increasing UV-B radiation, net photosynthetic rate (Pn) of functional leaves on cotton main stem significantly decreased. Although there were no change in stomatal conductance (Gs) and transpiration rate (Tr) under 40% increase in UV-B radiation, while inter-cellular CO2 concentration (Ci) increased, which indicated that the decline in photosynthesis was mainly caused by non-stomatal limitation factors. The results of chlorophyll fluorescence parameters analysis showed that with increasing UV-B radiation, maximum quantum efficiency (Fv/Fm), operating efficiency (ΦPSⅡ), linear electron transport rate (ETR) and photochemical quenching (qP) of PSIIremarkably decreased, but non-photochemical quenching (NPQ)increased. All the chlorophyll fluores-cence parameters were significantly correlated withPn changes. Slowly relaxing NPQ (NPQS) and its proportion in NPQ signifi-cantly increased under enhanced UV-B radiation, which indicated that the photochemical efficiency of PSⅡ decreased as its reaction center was damaged by elevated UV-B radiation. The results demonstrated that photosynthetic leaf area, chlorophyll content and photosynthetic rate of cotton dropped under enhanced UV-B radiation during the cotton growth period. This inhibited cotton growth, material accumulation and seed cotton yield. Decrease inPn due to enhanced UV-B radiation was closely related with the destruction of PSⅡ reaction center.