| Summary: | 碩士 === 國立臺灣大學 === 農藝學研究所 === 65 === The purpose of this thesis is to study the factors concerned in soybean leaf disc senescence, which include varieties, temperature, light, leaf age, hormones and ca++. The roles of roots, nodules, apical bud and trifoliolates on the senescence of soybean primary leaf were also investigated.
The main results are outlined as follows;
1. Different varieties show different senescence rates, but the senescence response of different varieties to hormones, cytokinin for example, is almost the same.
2. Senescence proceeds at maximum rate at 35℃, as judged by chlorophyll degradation. Data support the concept that senescence is an active metabolic process.
3. Leaf discs from 10- to 18-day old seedlings show very similar senescence rate. The most consistent senescence rate is found in leaf discs from 14- to 18-day old seedlings.
4. Judging by the change of chlorophyll, under dark condition 3eavesattached to the plant show slower senescence rate than leaf discs; while leaves attached to the plant under light condition sensesce slower than those under dark condition.
5. BA (0.02-2mg/l), Kinetin (0.2-2mg/l) and GA3 (2-20mg/l) retard the senescence of leaf discs, while ethrel (0.2-2mg/l) and ABA (10(-6)-10(-5)M) promote it. Auxins in the optimum concentration such as 0.002-0.02mg/l of IAA and 0.002mg/l of NAA also delay the senescence of leaf discs, but are not so effective as cytokinins and gibberellic acid.
6. CaCl2(10(-2)M) effectively decrease the leakage of organic materials from leaf discs, but does not retard chlorophyll degradation. When CaCl2 and GA3 were applied together, a remarkable synergistic increase in retarding senescence of leaf discs was observed.
7. The sensitivity of cytokinins, GA3 and IAA in retarding senescence of leaf discs varied with the time when leaf discs were sampled, leaf discs sampled from the plants grown under short day condition show more effective retarding response to BA, GA3 & IAA than those grown under long day condition.
8. BA & GA3 applied together delay senescence of leaf discs more effectively than BA & GA3 applied individually, but less than the addition effects of the BA & GA3. ABA decreases the retarding effects of BA, GA3 or BA+GA3.
9. Compared with intact 14-day old seedling, the senescence of primary leaves is promoted by removing roots (especially lateral roots), and is delayed in seedlings without apical bud and trifoliolate but with roots. No difference in primary leaf senescence is found between intact seedlings and seedlings without roots, apical bud, and trifoliolate. The effects of removing roots, apical bud and trifoliolates on the cotyledon senescence are in a manner similar to the primary leaves.
10. Results indicate BA can replace the roots to prevent senescence of primary leaves, while GA3 and ABA accelearate. The effect of IAA to replace roots in preventing senescence depends on the time young seedlings are grown.
11. Seedlings without root, apical bud and trifoliolates treated with GA3 accelerate the senescence of primary leaves, this accelerated effect of GA3 is more than that of seedlings without roots only.
12. IAA applied to seedlings without roots retards senescence of primary leaf in the seasons with short day-length (e.g. Nov., Dec.), but promotes, it in the seasons with long day-length (.e.g. Mar.). However, when roots, apical bud and triofoliolates are all removed from seedlings grown in long day condition, IAA in the range of 0.0002-0.002 mg/l will delay the senescence of primary leaves.
13. Light delays the senescence of leaf discs, and intensities in the range of 250-2000 lux suffice for the maximum effect.
14. BA or GA3 applied with light causes more effective retarding effect of senescence than BA or GA3 applied in darkness.
15. Similar light effect in retarding senescence of leaf discs from 14- to 18-day old seedlings was found. However, light effect is decreased when seedling age is 22- to 30-day old.
Relations between leaf senescence and changes of chlorophy, α-amino nitrogen or tissue permeability, between growing seasons of seedlings and hormones responses, and between light effect and hormones or leafage were discussed in detail in this thesis. Consideration about the roles of roots, apical bud and trifoliolates in regulating primary leaf senescence was also included.
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