Increment of chilling tolerance and its physiological basis in chilling-sensitive corn sprouts and tomato seedlings after cold-hardening at optimum temperatures

中国人. 研究了冷敏感植物甜玉米和番茄在生长临界温度锻炼下抗冷性的提高以及某些生理指标的适应性变化。结果表明，番茄幼苗经12℃锻炼7d，甜玉米种苗经11℃锻炼8d或15d后，番茄幼苗和甜玉米种苗对2℃低温胁迫的抗性明显提高。经过低温锻炼的番茄幼苗和甜玉米种苗脯氨酸含量较常温下（26℃／21℃）生长的显著增加。电镜细胞化学观察结果表明，甜玉米种苗经11℃8d锻炼后，其质膜Ca. ＋-ATPase在2℃冷胁迫后仍保持高活性，甚至有某些提高。而未经锻炼的种苗在同样低温处理后，质膜Ca. ＋-ATPase活性丧失，遭受冷害。本研究结果揭示和证实，喜温植物，无论是单子叶植物玉米，还是双子叶番茄，用其生长临界温度进行低温锻炼是适宜的，能有效地提高冷敏感植物的抗冷能力。

英语. Many important cultivated plants, such as tomato, tobacco and maize, have a limited capacity to survive temperatures below freezing, these crops are chilling-sensitive plants which can be killed at temperatures below 10 - 12℃.In contrast, leaves of plants such as cabbage, lettuce, and spinach can develop tolerance to below-freezing temperatures in response to low but above freezing temperatures （2 - 4℃ ）, these plants are chilling-insensitive or chilling-resistant plants.It was considered that chilling-sensitive plants can not increase in freezing or chilling tolerance upon exposure to low nonfreezing temperature, a phenomenon known as cold acclimation. This is because researchers acclimate the chillingsensitive plants at temperatures of 2 - 4℃ which is lethal to these plants. To conform chilling-sensitive plants can also acclimate to low but above freezing temperatures, 10 - 12℃ was selected as the optimum temperatures for cold-hardening in this research. Physiological basis for the requirement and increment of chilling tolerance in chilling-sensitive tomato seedlings and sweet corn sprouts after cold acclimation was elucidated. Results showed that chilling tolerance was increased obviously after cold-hardening at 12℃ for 7 days in tomato seedlings . Sweet corn sprouts acclimated at 11℃ for 8 or 15 days could survive 2℃ 3 d chilling stress（ Fig. 1 and Table 1）. Moreover, spectrophotometric measurement revealed that proline content increased by 48% in tomato seedlings leaves after cold acclimation for 7 days and by 117% in sweet corn sprouts after cold-hardening for 8 days（ Table 2）. Changes of plasma membrane Ca. ＋-ATPase activity, indicated by the status of cerium phosphate precipitated grains, were observed by electronic microscopy in sweet corn seedlings during the course of 2℃ chilling stress. The results showed that many reaction products were distributed on the plasma membrane （PM） ,showing the high activity of Ca. ＋-ATPase, in cells of 11℃, 8 d-hardened sweet corn sprouts（Plate Ⅰ -A）. PM-Ca. ＋-ATPase still maintained high activity in cells from cold -hardened sprouts chilled for one day or two days at 2℃ （ Plate Ⅰ -B， C）. However, no reaction products were observed on the PM in cells from un-hardened seedlings chilled for one or two days at 2℃, indicating the Ca. ＋-ATPase activity has been inactivated（ Plate Ⅰ-F, G, H）. The results mentioned above approve that thermophilous plants can really improve their chilling tolerance after cold-hardening at the survival threshold low temperatures. The dynamic difference of PM Ca. ＋-ATPase between cold-hardened and un-hardened sweet corn sprouts suggests that the ability of plants to sustain PM Ca. ＋-ATPase activity during chilling stress is important for plants to survive cold adversity.