Studies on leafy vegetable cultivation in the snowed cold region in winter with special emphasis on freezing tolerance, sugar and ascorbic acid contents of spinach (Spinacia oleracea L.) and komatsuna (Brassica rapa L.)

1 Freezing tolerance of spinach (Spinacia oleracea L.) and komatsuna (Brassica rapa L.) 1) Changes in osmotic concentration, sugar content betaine content proline content and Telso of spinach and komatsuna exposed to low temperature treatments. Telso decreased from approximately -6.5 to -17 deg C in spinach and from approximately 5.6 to -18 deg C in komatsuna when exposed to low temperatures for 10 weeks (at 5 deg C in the first 5 weeks and at 21 in the subsequent 5 weeks) with corresponding increase in osmotic concentrations. There was a high correlation between the Telm and osmotic concentration over the range of 375 and 750 mmol/kg in spinach and 338 and 800 mmol/kg in komatsuna. In addition there was a high correlation between the Telm and sugar content over the range of 0.3 and 3 g/100gFW and 0.3 and 3.5g/100gFW in spinach and komatsuna, respectively. On the other hand, when the sugar content increased above the ranges, indicated the Telso did not change anymore in both crops. In spinach, glycinebetaine content increased from 70 to 200mg/100gFW during the first 5 -week low temperature treatment and there was a high correlation between the T(EL50) and glycinebetaine content over the range of 70 and 150mg/100gFW. Glycinebetaine content increased in upper leaves but not in lower leaves. On the other hand, glycinebetaine was not detected at any stage of low temperature treatment in komatsuna Free amino acid content increased during the first 2-week low temperature treatment both in spinach and komatsuna, but then decreased gradually during the last 3 to 10 weeks. Specifically in proline, the content increased during the first 2-week low temperature treatment both In spinach and komatsuna from 9 to 100 and 8 to 270mg/100gFW, respectively, and did not change thereafter. These results demonstrate that there are apparent correlations among freezing tolerance and osmotic concentration, sugar content glycinebetaine content and proline content in spinach and komatsuna 2) Freezing tolerance of spinach and komatsuna cultivated in the greenhouse. Freezing tolerance of spinach and komatsuna increased gradually from October to the coldest season (i.e., late January or early February). T(EL15) decreased from -12 to -16 deg C and -11 to -14 deg C in spinach and komatsuna, respectively. T(EL15) reversely increased in both crops while temperature increased There was a high correlation between the freezing tolerance and the mean minimum air temperature for 7 days before the determination of freezing tolerance was conducted Therefore, it is possible to estimate the freezing tolerance at the time by taking the mean minimum air temperature for 7 days before the measurement of freezing tolerance. This estimated value of freezing tolerance would be applicable to the management of air temperature in the greenhouse in order to avoid the freezing injury of the crops. 2 Sugar and vitamin C contents of spinach and komatsuna in the greenhouse. When komatsuna was grown to about 20cm in height in the greenhouse at the mean air temperature of 13 to 15 deg C and then transferred to the greenhouse at 2 to 3 deg C, the sugar and vitamin C contents increased after the transfer. On the other hand, the sugar and vitamin C contents in komatsuna did not change when grown continuously in the greenhouse at 13 to 15 deg C. These results indicate that cold treatment could improve the nutritional qualities of komatsuna even under limited solar radiations in winter. The relationship between the contents of sugar and vitamin C in spinach and komatsuna and the maximum, minimum and mean air temperatures during cultivation period was analyzed under conditions of limited solar radiations in winter. The results indicated that the mean minimum and mean air temperatures in 10 days before the harvest greatly affected the contents of sugar and vitamin C in both crops. Therefore, it is possible to estimate the sugar and vitamin C contents with the observation of the mean minimum air temperatures for 10 days before the harvest If the temperature in a greenhouse is managed at -5 deg C, the high Qualities of spinach and komatsuna which contained high sugar and vitamin C contents could be obtained 3 Cultivation techniques of spinach and komatsuna with the high sugar and vitamin C contents without freezing injury. 1) Avoiding freezing injury in spinach and komatsuna. There are no or little possibilities of freezing injury at the coldest season because spinach and komatsuna plants were highly acclimated To the contrary, there are quite high possibilities of freezing injury in early winter or early spring when the plants were not acclimated enough or were deacclimated To avoid the freezing injury in early winter, it is effective to increase the freezing tolerance by exposure of spinach and komatsuna to cold environments after the suitable size for shipment (greater than 25cm in height) was reached. When plants were smaller than 25cm, insulation should be used only if there is a possibility that plants suffer freezing injury due to a sudden hit of cold wave. Freezing tolerance of plants that had grown with heat insulation was lower (by approximately 2 to 4 deg C than that of plants that had grown without heat insulation. Therefore, the usage of heat insulation greatly increases the possibility of freezing injury in plants. Introduction of cold air into the greenhouse by opening doors/windows during the period of mid-winter (from end of December to early March) at which plants reached the harvesting stage increased freezing tolerance of both spinach and komatsuna plants by 2 to 5 deg C compared to plants grown in the closed greenhouse. In addition, opening greenhouse resulted in a decrease in the possibility of deacclimation of plants occurring when air temperature considerably increased during daytime in unclouded days. These results indicate that the exposure of plants to cold air may reduce the possibility of freezing injury. 2) Production technique of spinach and komatsuna containing the high sugar and vitamin C contents. In order to ship spinach and komatsuna in January, it is necessary to determine the planting date suitable for each area Furthermore, the management manual of plant growth must be created So that plant growth must be controlled to obtain suitable plant size before the air temperature in the greenhouse drops to 5 deg C or below. Since the greenhouses and equipments are usually occupied with cultivation of summer crops (tomato and others), spinach or komatsuna plants cannot be planted at the best time in the greenhouse directly. Thus, it is good to perform transplant cultivation. Cultivation of komatsuna plants with heat insulation resulted in several undesirable characteristics compared to unheated cultivation. Those include that plants grow too soft leaf blades become less greenish, and resultant plants have less commercial values. In addition, freezing tolerance of plants that had grown with heat insulation was lower than that of plants that had grown without heat insulation. Furthermore, the sugar and vitamin C contents in komatsuna plants were lower when growing with heat insulation than without heat insulation. Therefore, it was concluded that insulation of plants continuously from planting to harvest time was not a good choice for agricultural production. The insulation should be used only if there is a possibility that plants may suffer freezing injury due to a sudden hit of cold wave. Introduction of cold air into a greenhouse by opening doors/windows during the period of mid-winter at which plants reached the harvesting stage increased the contents of sugars and vitamin C compared to plants growing in a closed greenhouse. In order to build a confidential relationship with a consumer, it is important for crop producers to ship spinach or komatsuna in the high sugar and vitamin C contents reliably. For that purpose, it is good to use the Brix value of spinach or komatsuna with 7 % or greater when deciding whether to ship or not Taken together with these results in the present study, exposure of plants to cold environments at the harvesting stage results in additional values to the producers commercially, such as increasing in nutritional values, matching demands of market and mass marketers as well as increasing income of producers during winter.