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Illumination during storage was proven to extend the storability of seedlings, but little attention has been given to the underlying mechanism. To determine how light conditions affect photosynthetic status in watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] plug seedlings, differences of stomatal development and relative photosynthetic characteristics of expanding new leaves when stored for 8 days at 15 °C in light at a photosynthetic photon flux (PPF) of 15 μmol·m⁻²·s⁻¹ or darkness were investigated. The stomatal density (SD) increased with time to a peak and then decreased from the start of storage in light to the subsequent transplantation. Dark storage retarded the stomatal development and delayed the appearance of the peak of SD. Compared with those under dark storage, light-stored seedlings showed significantly higher SD and stomatal index (SI) in leaves accompanied by higher maximal photochemical efficiency of PSII (F_v/F_m) and quantum yield of PSII (ΦPSII). During storage in darkness, F_v/F_m and ΦPSII declined steeply with increasing storage duration but recovered gradually to reach the same level of those in light at the fourth day after transplanting. Seedlings stored in light for 8 days showed higher net photosynthesis rate (Pn), stomatal conductance (g _S), intercellular CO₂ concentration (Ci), and transpiration rate (Tr) than in darkness. The post-storage recovery of Pn, g _S, and Tr were closely related to the SI, which ensured the fast recovery of photosynthesis during the early stage of transplanting. In agreement with the change of SD, no differences in Pn, g _S, Ci, and Tr between light and dark storage were observed after 8 days of transplantation. Seedlings stored in light appeared vigorous and the shoot dry weight was significantly higher than that of dark-stored ones. Although seedlings in dark storage had a poor appearance during storage, they showed inhibited regrowth potentials during the subsequent transplanting stage. This study exhibited that light in short-term storage contributed to maintaining stomatal development as well as photosynthetic efficiency in watermelon, which could also extend to post-storage for ensuring the transplant quality of seedlings after removal from storage.

Low-temperature storage in darkness is usually used for preserving seedlings for a short period. To investigate whether grafted watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] seedlings are superior to non-grafted ones under low-temperature storage in darkness and to study their physiological differences during storage, watermelon (‘Zaojia 84-24’) scions were grafted to pumpkin (Cucurbita moschata Duch. ‘Zhuangshi’) rootstocks. Carbohydrate levels; chlorophyll and malondialdehyde contents; the activities of superoxide dismutase, catalase, and peroxidase; and photochemical efficiency were assayed during 6 days of storage at 15 °C in darkness. After that, seedlings were transplanted into an artificial climate chamber. The net photosynthetic rate and stomatal conductance (g _S) were measured on the first and third days after transplanting. The results showed that the grafted watermelon seedlings had more soluble sugar and chlorophyll contents, higher activities of antioxidant enzymes, and less malondialdehyde content than the non-grafted ones after 6 days of storage. In addition, low-temperature storage in darkness damaged the photosystem II of non-grafted watermelon seedlings more than that of grafted ones. After transplanting, grafted seedlings had a higher net photosynthetic rate. The results suggest that grafted watermelon seedlings were more suitable for the low-temperature storage in darkness than the non-grafted ones.