| A plant'senescence is a complex process, which is intrinsic factors controlled, active, programmed, and has close relationship with natural death. Senescence is a common phenomenon happened in all parts of a plant (plant, tissue, organ, and cell) during the entire developing process. Therefore, all parts in a plant, such as root, stem, leaf, flower and fruit, can be senesced. In this experiment, cucumber cultivars D0313 (premature senility) and 649(comparison) were used as materials in routine farming management to study the senescence characteristics of cucumber in order to find some programmed characteristics and mechanism of senescence, which can be the theoretical basis to find the mechanism of senescence and breeding resisting senescence varieties. The results were as follows:1 From the aspect of time that the morphologic characteristics begin to change, we could get the following conclusion: for the same breed, the leaves and root were the first to senesce, and the beginning times for them were almost same. Then, the fruit also began to senesce. The stem was the last organ in a cucumber plant to senesce. For cucumber cultivars D0313 and 649, the time that D0313 began to senesce was about 15 days earlier than the comparison 649.2 As cucumber became senescent, chlorophyII content in leaves and stem increased first, but decreased later. ChlorophyII content in leaves of early maturing variety D0313 started to decrease dramatically when the plant grew 60days, and the comparison one 649 started to decrease 15 days later than D0313. The times that ChlorophyII content in stems of D0313 and in 649 began to decrease were 75days and 90days separately since grown. In conclusion, the times that ChlorophyII content in leaves and stem of cucumber breed D0313 began to decrease were both 15days earlier than the comparison 649. During whole process of development, ChlorophyII content in 649 was always higher than in D0313.3 As cucumber became senescent, soluble protein content in leaves, stem and root all increased first, but decreased later. Soluble protein content in leaves of breed D0313 began to decrease dramatically when the plant grew 60days, while the others changed gradually during senescence. During the whole growth stage, soluble protein content in leaves, stem and root of breed 649 were always higher than the ones in D0313.4 MDA contents in leaves, stem, and root all raise as cucumber became senescent. In the whole process of development, MDA content in leaves, stem, and root of D0313 were much higher than in comparison 649.5 The activities of POD and SOD in leaves, stem, and root of a plant decreased dramatically during the latter period of growth, but the times were different from one anther. The POD activities in leaves, stem, and root of breed D0313 began to decrease dramatically at 90days, 90days, and 75days separately since grown, while the time that POD activities in the same parts of breed 649 began to decreased were 90days, 105days, and 105days for each. That is to say, the time that POD activities in leaves and root of breed 649 began to decrease was 15days and 30days later separately than the one in breed D0313. SOD activity in leaves, stem, and root of D0313 all began to decrease dramatically since the plant grew 75days, while SOD activity in the same parts of breed 649 began to decrease since it grew 90days, 105days, and 90days separately. In other words, the time that SOD activity in leaves and root of breed 649 began to decrease dramatically was 15days later than D0313 and the same index for stem between breed 649 and D0313 was 30days. Since cucumber became senescent, the activity of POD and SOD kept stable or rise slightly in a period, which might be a resistance reaction of POD and SOD to the senescence.6 During senescence process, ChlorophyII content, SOD activity and POD activity in leaves could be used as physiological and biochemical indexes for cucumber leaves; root vigor and SOD activity could be used for root; and ChlorophyII content, SOD activity could be used for stem. Therefore, we could consider ChlorophyII content, SOD activity and root vigor as physiological and biochemical indexes for cucumber senescence, and root vigor's decrease was probably the inducers of the senescence in roots or even in the entire plant. Despite root vigor and ChlorophyII content in leaves decreased at the same time, the decreasing speed of root vigor was much faster than the one of ChlorophyII content. Therefore, the decrease of root vigor was probably the inductive factor for the senescence of cucumber's above-ground organs.7 The sequence that cucumber's organs senesced was: root, leaves, fruit, stem.8 In terms of whole plant, the time that breed D0313 and breed 649 senesced were 60days and 75days separately since the plant grew.
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