| Lotus(Nelumbo nucifera), a Nymphaeaceae perennial aquatic plant, is named one of the top ten traditional flowers of China. China is one of the origin centers of the lotus, and also is a world center of lotus cultivation. Lotus is a thermophilic plant, when the temperature is less than 12℃, lotus will basically end its growing cycle and enter a dormant state. In Nanjing area, the blooming lotus groups mainly concentrate in the hot June, July and August. which can’t satisfy the ornamental demand for people interested in lotus, and improve the economic value of the lotus. It has been shown that many control measures can change the flowering phase of lotus, including fill light, warming, humidifying and other measures. However, the high cost of facility cultivation limits its development and extensive application, therefore to study the cold resistance mechanism of lotus and improve lotus hardiness from the germplasm is one of the most reliable way.In this study, we took 38 common varieties Nelumbo nucifera as research samples by observing their growth conditions in withering period and determining the leaf and petal median frozen lethal temperature (LT50) of 38 and 27 varieties during their growth period, respectively. By screening of selected, we selected deathdate late but lethal freezing temperatures higher species ’Hongju’, and deathdate earlier but lethal freezing temperatures lower species ’Feitian’. We took ’Hongju’ and ’Feitian’ with different temperature sensitivity as lotus sample, and treated by cold temperature 10℃ for 14 h in the day and at 7℃ for 10 h at night 7 days. At the same time, we observed the growth conditions and basic physiological index changes of the aboveground and underground parts of lotus.The main results are as follows:1. Though statistics of the late flowering time and 50% leaves withering time, a few varieties had different withering time in different years, and the difference was about 7-15 days. Individual species, such as ’Zhubi’,had significant difference in two years statistics, about 40 days. The results found that the half frozen lethal temperature (LT50) of lotus leaf ranged from-8.590℃ to -3.839℃, while the lotus petals frozen lethal temperature was in the range of -8.982℃ to -2.430℃. Hence, the 27 lotus varieties were divided into three levels based on their integrated cold resistance, which were high cold resistance varieties (including the ’Feitian’,’Jinling Changxiang’,2 varieties), general hardiness varieties (including ’Youyi Mudan Lian’,’Yuhuaqing’,’Moling Qiuse’ and so on,16 varieties), and poor cold resistance varieties (including ’Aijiangnan’,’Jin Taiyang’,’Hongju’ and so on,9 varieties), respectively. Moreover, the median frozen LT50 of lotus leaf and flower had no significant relationship to the growth period of flowering and green. There was also no relationship among the median frozen LT50 of the leaves and petals in 27 locus varieties.2. Through low temperature stress, we obtained the physiological mechanism of ’Hongju ’and ’Feitian’ responding to low temperature. From the perspective of external morphology, the growth conditions ’Hongju’ was better than the ’Feitian’ under low temperature environment, which is consistent with the natural growth in the natural cooling conditions. For ’Feitian’, the low temperature stress resulted in the leaves yellow spots firstly appearing on the older leaves and functional leaves, without this phenomenon on tender leaves. For the aboveground, cell osmolytes like proline and soluble sugars played an important role during low temperature stress; Different from other plants, the soluble protein content and enzyme activity of lotus leaf decreased after low temperature stress. After chilling stress, underground rhizomes entered dormancy and improved their hardiness mainly by soluble proteins, the Superoxid dismutase(SOD) activity was significantly enhanced after entering hibernation. After the cold treatment rhizomes of ’Hongju’ and ’Feitian’ changed differently, because the former contains tropical lotus genes, which is less sensitive to temperature, while the underground rhizomes of the latter expand, by dormancy to avoid hypothermia. From the changes of starch grains,’Hongju’ has the tendency to enter dormancy after low temperature stress, but need long or enough lower temperature stress.From this study, we found that ’Feitian’ has better cold tolerance than ’Hongju’.3. Hybrid parents have greater effects on the seed setting rate and germination rate of offspring. The study of leaves withering time showed that female parent has great effect on offspring, while male parental have small effect. Therefore, when choosing the hybrid parents, we should pay attention to choose yellow period late varieties with tropical lotus hybrid gene as female parent. |
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