Abstract: To determine the photosynthetic mechanism of storage temperature effecting the quality of seedlings, the leaf ultrastructure and photosynthetic characteristics of watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai) plug seedlings stored in darkness at 25℃ or 15℃ were investigated. In this experiment, watermelon (‘Zaojia 8424’) scions were grafted to pumpkin (Cucurbita moschata Duch.‘Zhuangshi’) rootstocks. The results demonstrated that storage in the dark caused swelling, and starch granule disappearance, reduced and disordered lamella in chloroplasts. The PSⅡ maximal photochemical efficiency (F_v/F_m), PSⅡ actual photochemical efficiency (Φ_PSⅡ), the net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of seedlings decreased continually with the increased of dark storage time. Within 4 days of storage in darkness, the decrease of Pn and Gs accompanied by the decline of intercellular CO₂ concentration (Ci), which indicated that the Pn reduced due to stomatal limitation. While the Ci significantly increased with the decline of Pn and Gs on the 6th day of storage, which showed that the non-stomatal limitation became the main factor for Pn decrease. Compared with those stored at 15℃, the seedlings stored at 25℃ for 6 days had more damaged lamella and osmiophilic globule in chloroplasts, and showed lower F_v/F_m,Φ_PSⅡ, Pn, Gs and Tr. After planting, the chloroplast lamella stacked anew and starch granules accumulated again. On the 6 days after transplanting, the chloroplast ultrastructure, F_v/F_m and Φ_PSⅡ of seedlings stored at 25℃ or 15℃ in darkness for 4 days recovered to control levels. Regardless of storage temperature, the chloroplast ultrastructure and Pn did not recover when the storage time increased to 6 days. The F_v/F_m and Φ_PSⅡ of seedlings stored for 6 days at 25℃ did not recover, while that stored at 15℃ could recover to the same level of control at 6 days after transplanting. The decrease of shoot dry weight caused by dark storage did not recover to control level after post-storage for 6 days, but it was significantly higher in seedlings stored at 15℃ than those stored at 25℃. In conclusion, compared with dark storage at 25℃, storage at 15℃ could maintain a better chloroplast lamella structure, higher carbon assimilation level and PSⅡ photochemical activity, and faster photosynthetic performance recovery and material accumulation rate after colonization. According to the experimental results, it was suggested that the low temperature and dark storage of watermelon seedlings should not exceed 6 days, while the dark storage at room temperature should be controlled within 4 days. The results provide a theoretical reference for seedling storage and the study of physiological mechanism of plant response to light and temperature stress.

Key words: watermelon(Citrullus lanatus) seedlings, storage, temperature, chloroplast, photosynthesis

摘要： 为探索贮藏温度影响种苗质量的光合生理机制,以西瓜品种早佳8424嫁接苗为材料(砧木为南瓜品种壮士),研究黑暗条件下25℃和15℃贮藏温度对西瓜幼苗叶片超微结构及其光合特性的影响。结果表明,黑暗贮藏造成叶绿体内片层排列紊乱、片层数量减少,叶绿体内淀粉粒消失;PSⅡ最大光化学效率(F_v/F_m)、PSⅡ实际光化学效率(Φ_PSⅡ)、净光合速率(Pn)、气孔导度(Gs)和蒸腾速率(Tr)均随着黑暗贮藏时间的延长持续降低;贮藏4 d内,Pn、Gs和胞间CO₂浓度(Ci)同步降低,此时是气孔限制造成的Pn降低;贮藏时间延长至6 d,Pn和Gs继续下降,Ci却显著上升,非气孔限制成为Pn降低的主要因素。与15℃黑暗贮藏相比,25℃黑暗贮藏6 d的叶绿体片层结构受损更严重,叶绿体内有较多的嗜锇颗粒,且F_v/F_m、Φ_PSⅡ、Gs和Tr显著低于前者。定植后,从贮藏时间看,黑暗贮藏4 d的幼苗F_v/F_m、Φ_PSⅡ和叶绿体结构在定植后6 d均能恢复至对照(CK)水平,贮藏6 d的幼苗叶绿体结构和Pn则不能完全恢复;从贮藏温度看,25℃黑暗贮藏6 d,F_v/F_m和Φ_PSⅡ在定植后6 d不能恢复;而15℃黑暗贮藏6 d,F_v/F_m和Φ_PSⅡ在定植后6 d均可恢复。两种黑暗贮藏温度导致的幼苗地上部干重降低在定植后6 d均不能恢复到CK水平,但15℃黑暗贮藏显著高于25℃黑暗贮藏。与25℃黑暗贮藏相比,15℃黑暗贮藏的幼苗能够保持较好的叶绿体结构,较高的碳同化水平和PSⅡ光化学活性以及定植后较快的光合性能恢复和物质积累速度。综上,建议西瓜种苗低温黑暗贮藏不宜超过6 d,而常温黑暗贮藏应控制在4 d内。本研究结果为瓜类种苗贮藏及植物对光温胁迫响应的生理机制研究提供了理论参考。

关键词: 西瓜种苗, 贮藏, 温度, 叶绿体, 光合作用