Effects of Storage Temperature on Carbohydrate Content and Seedling Quality of Cabbage Plug Seedlings

Transcription

1 Original Paper Environ. Control in Biol., 37(4), , 1999 Effects of Storage Temperature on Carbohydrate Content and Seedling Quality of Cabbage Plug Seedlings Fumio SATO, Hiroshi Y0sx10KA and Takahiro FUJIWARA National Research Institute of Vegetables, Ornamental Plants and Tea, Ano, Mie , Japan (Received May 14, 1999) The effects of storage temperature on plant growth and carbohydrate content during low-temperature storage and on photosynthesis and growth of transplants after the end of storage were studied in cabbage plug seedlings. Plants were stored in the dark for 14 d at 5, 10 and 15 Ž. Fresh weight in shoots increased as storage temperature increased but dry weight decreased slightly, which reduced the dry matter ratio in shoots. Starch concentration in leaves decreased remarkably during the first 4 d in storage, regardless of storage temperature. Sugar concentrations in leaves decreased at 10 and 15 Ž, due mainly to the reduction in dry matter ratio. Concentrations at 5 Ž, however, increased transiently and remained higher than at the other temperatures. The effects of storage temperature on photosynthesis and shoot relative growth rate of transplants after the end of storage were comparatively smaller than the effects on plant growth and carbohydrate content during storage. These results indicate that storage temperature affects succulent growth during storage by etiolation and this is related to reducing accumulated starch. INTRODUCTION Plug-grown transplants are widely used in the vegetable industry in Japan for leafy crops such as cabbage and lettuce. Plug seedlings grow in an extremely small cell volume and at a high plant density. Such conditions often lead to loss of seedling quality, such as poor appearance, delayed flowering (Kaczperski et al., 1996), and retardation of post-transplant development in tomato (Weston and Zandstra, 1986), when seedlings are held in trays too long. Transplanting is often delayed by lack of time, equipment or labor problems and bad weather. This can result in overgrowth of plants in trays and loss of seedling quality. Therefore, quality preservation in plug seedlings is required for efficient seedling production. Low-temperature storage, which is commonly used for harvested vegetables and fruits, is used for preserving the quality of plug seedlings (Risse et al., 1985; Leskovar and Cantliffe, 1991) and cuttings (Pryor and Stewart, 1963; Davis and Potter, 1985; Panton and Schwabe, 1987). In cuttings, depletion of carbohydrate during storage is assumed to be one of the factors that influence quality (Pritchard et al., 1991). Kozai et al. (1996) reported that low light prevented reduction of dry matter and maintained the photosynthetic capacity of eggplant plug seedlings, preventing carbohydrate loss by respiration. These reports suggest that the quality of plug seedlings is also affected by carbohydrate status during storage. However, there is no information about the relationship between carbohydrate content and the quality of plug seedlings during storage. To investigate this relationship, we determined changes in carbohydrate content and plant growth during storage of cabbage plug seedlings at different Vol. 37, No. 4 (1999) (7)249

2 temperatures and investigated the effect of storage temperature on recovery of transplants after the end of storage. MATERIALS AND METHODS Plant materials and growth conditions. Seeds of cabbage (Brassica oleracea L. cv. Matsunami) were sown in a plug tray with 200 cells (13 ml/cell ; Yammer, Osaka, Japan) filled with soil mixture (Yasaiyodo ; Yammer). Plants were grown in a growth chamber at 25 Ž/20 Ž (day/night). A 12-h photoperiod was supplied by a mercury vapor lamp (HRF 1000X ; Matsushita Electric Works, Osaka, Japan) and a metalhalide lamp (D400; Toshiba, Kanagawa, Japan) with a photosynthetic photon flux density of 350pmol m-2 s-1. All plants were supplied daily with complete fertilizer (Napurapower ; Yammer) diluted to 0.02% with tap water. Storage conditions. Twenty-one days after sowing, when 2 leaves had unfolded, plants in the tray were placed on a shelf covered by vinyl film, and kept in the dark at 5, 10, or 15 Ž for 14 d. Throughout storage, plants were not watered, and the relative humidity was maintained at 85%. Growth measurements. Plants were sampled at the start of storage and at 2, 4, 9, and 14 d after that. The plants were cut from their roots for measurement of their fresh weight and shoot length (from the cut end to the tip of the largest leaf). The plants were then oven-dried at 80 Ž over-night and weighed again. Carbohydrate determination. The lower leaves' (the first from the bottom) were sampled at the start of storage and at 2, 4, 9, and 14 d thereafter. Four discs were cut 1 cm in diameter frozen immediately in liquid nitrogen, and stored at -20 Ž until analysis. The frozen, samples were homogenized in a mortar and pestle in 5 ml of 80% (v/v) ethanol. The homogenized samples were held in a water bath for 30 min at 78 Ž and then centrifuged for 10 min at ~ g. The supernatants were separated, and the residues were extracted 2 more times in 3 ml of 80% (v/v) hot ethanol. The supernatants from each extraction were combined and evaporated to dryness in a centrifugal evaporator at 40 Ž and then dissolved in 1 ml of distilled water. Sugar concentration, sucrose, glucose, and fructose, was measured in aliquots of the supernatants by an enzyme-linked method similar to that described by Kerr et al. (1985). For determination of starch concentration, the residues remaining after ethanol extraction were resuspended in 4 ml of 0.2 M KOH and boiled for 30 min. The samples were cooled and adjusted to about ph 5.5 by adding 0.8 ml of 1 M acetic acid. Aliquots of the samples were digested with 4 units of amyloglucosidase in 50 ƒêl of 50 mm acetic buffer (ph 4.5) at 37 Ž for 90 min. The liberated glucose was measured with a glucose test kit (glucose B test ; Wako Pure Chemical Industries, Osaka, Japan). Measurement of recovery of transplants after the end of storage. Recovery of transplants from storage stress was measured from the relative growth rate (RGR) of shoots and photosynthesis of transplants. At the end of storage, plants were transplanted into 350-mL plastic pots filled with compost (Engeibaido ; Kureha, Tokyo, Japan) and grown for 4 d under the same conditions as they were raised in. One and 4 d after transplanting, photosynthesis was measured in the lower leaves with a portable photosynthesis measurement system (LI ; Li-cor, NE, USA) under 400 ppm of CO2 and 350ƒÊmol m-2 s-1 of radiation. RGR in shoots of transplants was estimated for 4 d after the end of storage. The data presented here are the means from 6 individual plants. 250(8) Environ. Control in Biol.

3 RESULTS Shoot growth during storage During the first 4 d of storage, shoot fresh weight increased with increasing storage temperature (Fig, 1A). It then decreased slightly at 15 Ž, partly due to defoliation of cotyledons, but increased at 10 and 5 Ž. Shoot length also increased in a similar pattern (Fig. 1B). Shoot dry weight decreased slightly throughout storage and was lowest at l5 Ž after 14 d of storage (Fig. 1C). As a result of the changes in fresh and dry weights, the dry matter ratio was lowest at 15 Ž (Fig. l D). These results indicate that plants stored at higher temperature grew more succulently by etiolation in response to darkness during storage. Changes in carbohydrate content during storage Preliminary experiments revealed that carbohydrates were accumulated mostly in lower leaves. Therefore, we analyzed carbohydrate in lower leaves. The starch concentration Fig. 1 Effect of storage temperature on fresh weight (A), length (B), dry weight (C) and dry matter ratio (D) in shoots of cabbage plug seedlings during storage. Plants were stored in the dark at 5 ( ), 10 ( œ) or 15 Ž ( ). Vertical bars indicate standard error (n=6). Vol. 37, No. 4 (1999) (9)251

4 Fig. 2 Effect of storage temperature on starch concentration in lower leaves of cabbage plug seedlings during storage. Plants were stored in the dark at 5 ( ), 10 ( œ) or 15 Ž ( ). Vertical bars indicate standard error (n=6). Fig. 3 Effect of storage temperature on sugar (glucose + fructose + sucrose) concentration in lower leaves of cabbage plug seedlings during storage. Plants were stored in the dark at 5 ( ), 10 ( œ) or 15 Ž ( ). Vertical bars indicate standard error (n=6). Fig. 4 Effect of storage temperature on photosynthesis of cabbage transplants at 1 and 4 d after the end of storage. Photosynthesis was measured in lower leaves. Plants were stored for 14 d in the dark at 5, 10 or 15 Ž and then transplanted to plastic pots (350 ml). Vertical bars indicate standard error (n= 6). decreased considerably to 6% of its initial value at all temperatures (Fig. 2). Although the rate of reduction during the first 4 d of storage was slower at 5 Ž than at 10 and 15 Ž, the starch concentration at 5 Ž was lowest after 9 d of storage. In contrast to starch, sugars were affected markedly by storage temperature (Fig. 3). Sugar concentration was highest at 5 Ž throughout storage. Sugar concentration at 5 Ž increased transiently at day 2 in storage, but concentrations at 10 and 15 Ž decreased constantly. After that, they decreased slightly at all temperatures. At 14 d of storage, sugar concentrations at 10 and 15 Ž were nearly half of the initial value, but that at 5 Ž was similar to that. Recovery of transplant after the end of storage Photosynthesis of transplants increased remarkably from 1 d after the end of storage to 4 252(10) Environ. Control in Biol.

5 Table 1 Effect of storage temperature on growth of transplants for 4 d after the end of storage. Z Just after the end of storage. y On 4 d after the end of storage. X Standard error (n=6). d in all treatments (Fig. 4). The differences between treatments at 1 d after the end of storage became less by 4 d. Shoot dry weight at 4 d increased as storage temperature decreased (Table 1). However, shoot RGR was not as different at each storage temperature, because the initial dry weight (just after the end of storage) also increased as storage temperature decreased. DISCUSSION Yoshioka (1996) reported that a reduction in dry matter ratio and shoot elongation reduced the quality of cabbage plug seedlings, which led to plant injury when plants were mechanically transplanted. In our study, both increased with an increase in storage temperature (Fig. 1) and were due mainly to etiolation. Therefore, etiolation appeared to be a main cause of the deterioration of seedling quality. Since the etiolation occurred concomitant with increasing water absorption by plants, other environmental conditions such as soil water content or humidity during storage may also be substantial factors in the deterioration of seedling quality. A considerable reduction in starch concentration was observed during the first 4 d of storage, when the increase in fresh weight occurred (Figs. 1A, 2). Carbohydrate status affects the quality of stored flower cuttings (Pritchard et al., 1991), in which depletion of respiration substrates may contribute to the deterioration of quality. However, the reduction in starch observed here was not caused merely by consumption for respiration, because the rate of the reduction was extremely fast during the first 2 d and occurred even at 5 Ž to a similar extent as at the other storage temperatures. In contrast, it was reported that respiration rate in cuttings decreased with increasing storage temperature (Pritchard et al., 1991). As the reduction in starch coincided with the increase in shoot fresh weight, the reduction in starch might be related to the etiolation during storage. On the other hand, sugar concentration showed a different pattern from that of starch, in which sugar concentration was lower at higher storage temperatures (Fig. 3). The significant reductions in sugar observed at 10 and 15 Ž during the first 4 d in storage were similar to those in dry matter ratio (Fig. 1D). This suggests that the reductions in sugar in this period were due mainly to the reduction in dry matter. However, sugar concentration in this period at 5 Ž increased even though the shoot dry matter ratio decreased. It was reported that sugar content increased in rye (Antikainen and Pihakaski, 1994), cloudberry (Kaulin et al., 1981), spinach (Guy et al., 1992), and wheat (Tognetti et al., 1989) when plants were exposed to low temperature. Indeed, in cabbage, exposure to 5 Ž induced an increase in sugar content that contributed to freezing tolerance (Sasaki et a!., 1996). In this sense, 5 Ž as a storage temperature induces sugar accumulation. For etiolation to occur, a source of carbohydrate must be available (Schwabe, 1963). At 5 Ž, the carbon from degraded starch may be accumulated in Vol. 37, No. 4 (1999) (11)253

6 sugars, whereas that at 10 and 15 Ž it is immediately used for etiolation. Furthermore, assuming that respiration declines with storage temperature, sugar consumption by respiration would be lower at 5 Ž than at 10 and 15 Ž. These processes may be responsible for the lowest starch and the highest sugar concentration at 5 Ž. A plant growth during storage differed to a large extent with storage temperature, but recovery of transplants was not so different as measured by photosynthesis and shoot RGR (Table 1, Fig. 4). These results indicate that it is possible to store cabbage plug seedlings for 14 d even at 15 Ž without detriment to the transplants at least when plants are given enough water and light. However, considerable effect of storage temperature on photosynthesis of transplants was observed at 1 d after the end of storage. This fact indicates that plants lost their photosynthetic efficiency during storage at higher storage temperatures. Moreover, shoot dry weight just after the end of storage was lower at higher storage temperatures, presumably due to respiration and defoliation of cotyledons (Table 1). Shoot growth relies on carbohydrate reserves in the vegetative organs, so that assimilates can be provided when photosynthesis is low (Volenec and Joern, 1996). If photosynthesis of transplants is inhibited by stresses such as low light intensity or water deficit, storage at a higher temperature will reduce transplant growth. Growth reduction immediately after transplanting results in loss of growth uniformity in fields (Fujiwara et al., 1998a,b), which is necessary for mechanical harvesting (Karahashi et al., 1981). Thus, rapid growth after transplanting is an important quality of plug seedlings. The aim of low-temperature storage of plug seedlings is preservation of quality. This study clarified that etiolation occurred during storage, concomitant with the degradation of accumulated starch, which led to deterioration of seedling quality. Starch degradation therefore might be an important factor in etiolation during storage. Starch degradation is catalyzed by amylase and phosphorylase (Robinson et al., 1988; Forrest et al., 1991) ; these enzymes are assumed to regulate starch degradation during storage. Further work must involve enzymatic analysis for a better understanding of the mechanism of the deterioration of seedling quality and the development of countermeasures against it. REFERENCES Antikainen, M., Pihakaski, S Early developments in RNA, protein, and sugar levels during cold stress in winter rye (Secale cereale) leaves. Ann. Bot. 74: Davis, T. D., Potter, J. R Carbohydrates, water potential, and subsequent rooting of stored rhododendron cuttings. HortScience 20: Forrest, S. I., Verma, D. P. S., Dhindsa, R. S Starch content and activities of starch-metabolizing enzymes in effective and ineffective root nodules of soybean. Can. J. Bot. 69: Fujiwara, T., Yoshioka, H., Shikata, H., Sato, F. 1998a. Effects of transplanting depth of plug seedlings on the establishment and the growth of cabbage. (Japanese text with English abstract) J. Jpn. Soc. Hortic. Sci. 67: Fujiwara, T., Yoshioka, H., Shikata, H., Sato, F. 1998b. Effects of rootball moisture content at the time of transplanting on the establishment and uniform growth of cabbage plug seedlings. (Japanese text with English abstract) J. Jpn. Soc. Hortic. Sci. 67: Guy, C. L., Huber, J. L. A., Huber, S. C Sucrose phosphate synthase and sucrose accumulation at low temperature. Plant Physiol. 100: Kaczperski, M. P., Armitage, A. M., Lewis, P. M Performance of plug-grown geranium seedlings preconditioned with nitrogen fertilizer or low-temperature storage. HortScience 31: Karahashi, M., Inoue, K., Sawamura, N Development of a mechanical harvester for head-forming vegetables. (Japanese text with English abstract) J. Cent. Agric. Exp. Stn. 33: (12) Environ. Control in Biol.

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