Photosynthetic Characteristics Of Watermelon Seedlings During Storage Under Low Light

Watermelon [(Citrullus lanatus(Thunb.) Matsum. and Nakai)] is an important cucurbit crop. Watermelons are widely produced in industrial seedling nurseries in China, seedling storage is an indispensable process in large-scale production. However, storage(in light or darkness) affects the photosynthetic system obviously in plants. Thus, investigating the photosynthetic response of seedlings to storage conditions(light or darkness) is necessary. This study was conducted on the effects of storage in light(PPFD = 15μmol·m-2·s-1) or darkness at 15℃and subsequent transplanting on the relative photosythetic charateristics including stomatal development, photosynthetic apparatus, chlorophyll metabolism, thylakoid membrane fluorescence spectrum and metabolites of leaves. The main contents and results are as follows: 1. Stomatal development of watermelon seedlings during light storageTo determine how light conditions affect photosynthetic status in watermelon 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 of 15 μmol·m-2·s-1 or darkness were investigated. Microscopic observation of stomatal development showed that the stomata developed well in light-stored seedlings compared to those in dark storage. The stomatal density(SD) and stomatal index(SI) of light-stored seedlings were 510.5 No./mm2 and 18.0 respectovely, which were significant higher than those in dark storage(SD = 412.2 No./mm2, SI=13.8) after storage for 8 days. Compared to those stored in darkness, light-stored seedlings showed significantly higher SD and stomatal index(SI) in leaves, accompanied by higher Fv/Fm, ΦPSII, Pn, Gs, Ci and Tr after storage for 8 days. The post-storage recovery of Pn, Gs, 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, Gs, 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. While seedlings in dark storage had a poor appearance during storage, and therefore, showed inhibited regrowth potentials during the subsequent transplanting stage. This study exhibited that light in the 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. 2. Responses of photosynthetic organs of watermelon seedlings to light storageLight-stored seedlings showed tightly arranged palisade tissue and the chloroplasts in the palisade parenchyma(PP) distributed in the vertical direction of epidermal cells. Compared with those in dark-stored seedlings, light-stored seedlings have higher thickness of PP, spongy parenchyma(SP), and higher leave thickness, with higher ratio of PP/SP and cell tense ratio after 6 days of storage. Thickness of PP, SP and thickness of leaves from light-stored seedling recovered to control level, while those from dark-stored seedling could not recover to control level after 6 days of transplanting. Storage in the dark caused starch grain disappearance, swelling, disordered granal arrangement in the chloroplasts. In contrast, the chloroplasts stored in the light were relatively normal. As a result, light-stored seedlings showed a significantly higher chlorophyll content, Fv/Fm, and Pn than did dark-stored seedlings. After 6 days of transplanting, seedlings that were stored in the light or darkness for 2 or 4 days showed complete recovery of chloroplast ultrastructure, chlorophyll content, Fv/Fm, Gs and Pn. When the storage period increased to 6 days, the dark-stored seedlings had a significantly lower Fv/Fm and Pn than the light-stored and control seedlings 6 days after transplanting, which was mainly ascribed to incomplete recovery of chloroplast ultrastructure. Furthermore, the light-stored seedlings exhibited a significantly higher shoot dry weight during storage and a higher percentage dry weight increase after transplanting than the dark-stored seedlings. These effects were enhanced by prolonged storage(4 to 6 days). This study demonstrated that dim light during storage is beneficial for maintaining leaf anatomical structure and chloroplast ultrastructure as well as photosynthetic efficiency in watermelon seedlings, thus contributing to the rapid recovery of post-storage photosynthetic performance, which ensures the transplant quality of the seedlings after removal from storage. 3. The metabolism of chlorophyll of watermelon seedlings during light storageTo determine the mechanism of the chlorophyll reduction during dark storage, the intermediates of chlorophyll synthesis and the activities of key enzymes of chlorophyll degradation of leaves when stored for 6 days at 15 °C in light or darkness were investigated. From the 2nd day after dark storage, the chlorophyll content, chla/b and chla were significantly lower, while the contents of Pchlide were obviously higher than thoe in light storage. In addition, no differences were observed in the contents of Proto Ⅸ and Mg-Proto Ⅸ between seedlings in light and dark storage. Compared with control, dark-stored seedlings had 129.7%, 124.9%, 129.9% and 89.9%, 77.8%, 64.5% of Pchlide and Chl a contents after storage for 2, 4, 6 days respectively. These results showed that the acumulation of Pchlide did not increase with the extension of storage period in darkness. During storage under darkness, the Chlase, MDCase and Chl-POD showed higher activities than control and light storage. However,significant decrease in the activities of Chlase and Chl-POD, but obvious higher activities of MDCase were observed after 6 days of dark storage. No differences were observed in the contents of chlorophyll, Proto Ⅸ, Mg-Proto Ⅸ, Pchlide and Chl a, with equal or lower level in the activities of Chlase, MDCase and Chl-POD of light-stored seedlings compared to control at the 4th day after transplanting. In contrast, all those pamameters in dark-stored seedling recovered to control level at the 6th day after removal from storage. In conclusion, relative to dark storage, light storage not only accelerated the chlorophyll synthesis but also delayed the chlorophyll degradation. 4. Changes in the spectral characteristics of thylakoid membrane of watermelonseedlings during light storageIn comparison with those in dark storage, light-stored seedlings showed higher fluorescence emission intensity(excited at 435 nm and 475 nm), excitation intensity(emitted at 681 nm and 730 nm), and protein influorescence emission intensity(excited at 278 nm and 295 nm), accompanied by higher Fv/Fm and ETR. After 6 days of transplanting, all chlorophyll fluorescence spectral intensity of thylakoid membrane from dark and light stored seedlings recovered to cotrol level, while dark stored seedlings showed obvious lower Fv/Fm and ETR than control and light-stored ones. These data indicated that light storage maintained higher abilities in thlykoid membrane to capture, transfer and convert light energy, and had more components of proteins, which contributed to the quick recovery of activities of PSⅡreaction center. 5. Changes of metabolites in leaves of watermelon seedlings during light storageThis study was conducted to determine how light conditions affect the primary metabolites of leaves during storage at 15℃ for 6 days using gas chromatography-mass spectrometry(GC-MS). Total 48 metabolites were indentied, consisting of 20 carbonhydrates, 17 amino acids, and 11 organic acids. Storage significantly decreased the contents of carbonhydrates(fructose, talose, glucose, ribose, myo-inositol) and the contents of intermediates of the TCA cycle(fumaric acid, succinic acid and malic acid,), while obviously increased the contents of homoserine, glutamine, glutamate, asparagine, aspartic acid, tryptophan, ornithine, valine and phenylalanine. After 6 days of transplanting, dark-stored seedlings showed significantly higer contents in most carbonhydrates(such as fructose, glucose, xylose, etc.), but lower Pn than control and light-stored seedlings, which might attribute to the photosynthetic feedback inhibition from carbonhydrates. The contents of tryptophan, asparagine, glutamate and glutamine were obviously lower, however, the contents of leucine, isoleucine, and proline were significantly higher in light-stored seedlings compared with those in dark storage after 6 days of transplantation. Those metaboltes that mainly serve as intermediates in nitrogen metabolism, carbon matabolism, amino acid biosynthesis, TCA cycle, and other stress defense pathways could play important roles in responses of watermelon seedlings to light or dark storage.In summary, supplying low light during storage is beneficial in protecting the photosynthetic system of seedlings, and the post-storage photosynthetic performance was associated with the duration of storage. Better stomatal development, nomar photosynthetic apparatus, higher chlorophyll content and ability to capture and transfer light energy, timely photosynthate export and using and nomar energy supply of watermelon seedlings during light storage, together contributed to maintain photosynthetic performance during storage and helped to the quick recovery of photosynthetic capacity after storage, which ensured the growth of plants after transplanting.