Dormancy Break of Papaya (Carica papaya L.) Seeds by Heat and Gibberellins Treatments
Date Issued
2009
Date
2009
Author(s)
Lin, Ya-Shuan
Abstract
Papaya (Carica papaya L.), an important economical fruit crop in Taiwan, can be propagated by seeds, grafting, cutting and tissue culture, but mainly from seeds. Therefore, the germinability of seeds are vital to the industry. The characteristics of papaya seed germination are slow and asynchronous. Numerous researchers had proposed different kinds of methods to improve papaya seed germination, but the efficiency is significant different and contradictory depends upon cultivars and environment. The problem of poor and asynchronous germination of papaya seeds still exists. Thus, it is important for papaya nursery industry to look for effective methods to increase germination. Fresh papaya seeds harvested from‘Tainung no. 2’fruits at yellow-ripen stage are dormant after removing the sacrotesta and washing. Scarification and H2SO4 treatment have no effect on improving germination. The germination percentage can be achieved to 65% after removing the testa and tegmen, suggested that testa is the main physical constraint to radicle protrusion, instead of limiting the water and air permeability of seed. Fresh papaya seeds were after-ripened at 20℃ for 4 weeks and the water content of seeds decreased to 5.31~12.24%, but couldn’t germinate as well. Papaya seeds can improve germination by treated GA3 and GA4+7 solutions, and the most effective treatment is GA4+7 100 mg/L, implies that the main bioactive GAs for papaya seed germination may be GA4 and/or GA7. Seeds that were after-ripened for 14 days was soaked in 100 mg/L GA4+7 for 24 hours and incubated in Paclobutrazol and Ancymidol solutions. The inhibitory effects of these two inhibitors enhance as the concentration increases, demonstrates indirectly that the ability of papaya seeds to synthesize GAs continuously is quite important. Papaya seeds can’t germinate without adding GAs, implies that the embryo of papaya seed is weak. In addition to the weak embryo and the mechanical constraint of testa, papaya seeds are belonged to physiological dormancy. Although GAs treatment can improve the germination and synchronicity, it’s improper to be put in use due to the 30% abnormal seedlings occurrence. Papaya seed dormancy can be alleviated by hot water immersion treatment. 34, 36, 38, 40℃ hot water immersion treatment can enhance the germination of after-ripened seeds imbibed for 1 day. The best condition is 36℃ treated for 5 hours, and the germination percentage is 58.67%. The efficiency will be better as the imbibition duration before hot water immersion treatment increases to more than 5 days, the germination can be increased to 74%, but there’s no significant difference when the imbibition duration is more than 5 days. Compare the effects of different heat shock treatment, the positive improvement of hot water immersion (36℃, 5h) is better than hot air treatment (36℃, 5h), and after-ripened seeds are more sensitive to heat response than fresh seeds. Seedlings after heat shock treatment are healthy, have no side effects, unlike GAs treatment which will produce abnormal seedlings, and the method can be regarded as a dormancy breaking technology in papaya seedling propagation. In order to figure out the mechanism of heat shock treatment to improve papaya seed germination, we incubated seeds in Paclobutrazol and Ancymidol solutions after 36℃ hot water-immersion treated for 5 hours. 10 μM Ancymidol completely reverse the promoting effect of hot water immersion treatment. Counteracting effects of 10~250μM Paclobutrazol enhance as the concentration increases, implies that hot water immersion treatment should improve papaya seed germination via GAs de novo synthesis. According to the results of endogenous GA1, GA3, GA4, GA7, GA20 and ABA analysis, the GA4 content of hot water immersion treated seeds increases 1.14 times and enhances continuously as incubation time increases, but the heat treatment has no significant effect on endogenous GA3 content, implies that the bioactive form of papaya seed germination is GA4. The ABA contents of papaya seeds is not detected, suggested that ABA may not be the main factor influencing papaya seed dormancy and germination. This is conformed that 10~100 μM Fluridone treatment has no effect on improving germination. In conclusion, we proposed a hypothesis of mechanism how heat shock treatment can improve papaya seed germination. Seeds can restore physiological metabolism during the imbibition period. After heat stimulated, seeds were directly or indirectly induced increasing the rate of endogenous GAs synthesis or inhibiting the rate of GAs breakdown, and the GA4 content increases. Through GA signal transduction to start the degradation and mobilization of seed reserves and weaken the physical constraint of the cell wall of endosperm cap and testa, and then seeds can germinate successfully.
Subjects
papaya
seed
dormancy break
germination
heat treatment
gibberellins
Type
thesis
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