| Litchi(Litchi Chinensis Sonn.) is responsive to low temperature to induce flowering in a very short period around bud break. Control of flush growth rhythm so that latest autumn flush fully matures, without the occurrence of winter flush prior to the commence of winter chill, is a key practice to ensure litchi flowering and yield. Ethephon is commonly used for flush suppression and flower thinning in litchi production. However, the effects of flush killing and suppressing are influenced by flush developmental stages and ethephon dose. Improper use of ethephon will cause panicle withering and tree decline. Therefore,we used flushes at different developmental stages(various leaf ages) of litchi(cv. ’Feizixiao’ and ’Nuomici’) as materials for quantifying the sensitivity of flush of litchi to ethylene by exposing flushes at different stages to different concentrations of ethephon. On the top of observing the biological effects of flush responses to ethylene treatment, mechanisms of differential responses of flushes at different stages were analyzed by examining the expression of ethylene biosynthesis and signal transduction genes. Moreover, we examined the flower and bud thinning effects of different concentrations of ethylene and paraquat treatments during litchi flowering season(cv. ’Feizixiao’). The study aims to provide scientific references for rational use of ethephon for flushing control and for flower thinning in litchi production. The followings are the major results obtained from this study:1.Tender leaves at mid-expanding stage and the old leaves beneath the latest mature flush were very sensitive to ethylene as ethephon treatment induced severe leaf drop. Newly mature leaves were much less sensitive to ethylene as ethephon below 2000mg/L did not induced leaf drop. However, ethephon higher than 4000mg/L caused senescence and abscission of newly matured leaves. The leaf abscission rate was positive to the concentration of ethephon applied at different developmental stages. The semilethal concentration of ethephon, a parameter showing sensitivity of leaves to ethylene, at mid-expanding stage, end-expanding stage, newly mature stage and over-mature stage(old leaves) was 305.3mg/L, 573.7mg/L, 7810mg/L, 680.1mg/L, respectively, revealing the sensitivity to ethylene of the leaves different developmental stages was, in a descending order, mid-expanding stage> end-expanding stage > over-mature stage > newly mature stage. Therefore, the sensitivity of leaves to ethylene decreased as new leaves matured, and increased with leaf aging. Ethephon treatment not only induced old leaf abscission but also promoted break of lateral buds. Silver thiosulfate(STS) at less than 10mg/L could inhibit leaf abscission at mid-expanding stage, induced by 1000mg/L ethephon treatment. The relation of semilethal concentration of ethephon and leaf SPAD displayed a normal distribution pattern. Ethephon semilethal concentration increased first and then decreased with SPAD increased in a range from 12 to 59. Moreover, the ethephon semi-lethal concentration followed a exponential trend with Pn, revealing that Pn of leaf is negatively correlated with sensitivity to ethylene.2. The Pn of newly matured leaves of litchi was the highest, Pn of the old leaves was slightly higher than that of the leaves at end-expanding stage. Ethephon treatments caused significant decrease in Pn at all stages, and the range of decrease increased with the increase of ethephon concentration. The Fv/Fm of newly matured leaves was highest under normal circumstances. Ethephon treatments decreased Fv/Fm at all stages, the decrease being greater in higher ethephon concentrations. The leaf SPAD increased as new leaves matured. The SPAD of leaves at mid-expanding and over-mature stages decreased at the early time of ethephon treatment,while no significant changes in SPAD was observed in newly matured leaves after ethephon treatments.3. Shoot pruning exposing the old leaves at inner layer of the shoots, reduced the sensitivity of the old leaves of both cvs. ’Feizixiao’ and ’Nuomici’ to ethylene. The treatment increased Pn of the old leaves and could alleviate the inhibition of leaf photosynthesis by ethephon treatment. The longer exposure time after pruning before ethephon treatment, the smaller Pn decrease caused by ethephon treatment, and the faster Pn recovery after ethephon treatment. Meanwhile, shoot pruning caused photoinhibition in the old leaves at inner layer reflected by decreased Fv /Fm, but the photoinhibition was reduced with exposure time. Ethephon(1000mg/L) treatment increased daily photoinhibition rate in the old leaves in shoots with or without pruning, indicating that ethephon treatment damages the photosystem II.4. The IAA content in newly matured leaves in cv.’Feizixiao’ was the highest, followed by leaves at end-expanding stage, while it was lowest in mid-expanding leaves and over-mature leaves. The IAA content in the leaves at different developmental stages displayed a negative correlation with the sensitivity to ethylene. The ABA content increased with leaf development, but there was no significant difference among leaves at different developmental stages, suggesting that ABA was not the key factor determing the sensitivity of leaves to ethylene, while IAA content of leaves might be a key factor.5. The expression levels of LcACS6, LcACO1, Lc CTR1-3, Lc EIN2 and Lc EIN3-1 genes in the new matured leaves and old leaves were higher than those in the leaves at midand end-expanding stages, but the expression level of Lc ETR1 gene was lower, indicating that LcACS6, LcACO1, Lc CTR1-3, Lc EIN2, Lc EIN3-1 and Lc ETR1 genes may be associated with the evolution of sensitivity to ethylene with leaf development. Lc CTR1-3 expression was most negatively correlated with sensitivity to ethylene. The expression of Lc EIN3-3 gene was the lowest in newly matured leaves with the lowest sensitivity to ethylene, but it increased in old leaves. The expression level of Lc CTR1-1 was highest in newly matured leaves, but decreased in old leaves. These results suggested that the increased sensitivity of old leaves to ethylene increased, might be related to up-regulation of Lc EIN3-3 gene and down-regulation of Lc CTR1-1 gene.6. Response of the expression of ethylene biosynthesis gene to ethephon treatment differed at different stages. In leaves of litchi, LcACS3 gene may involved in autocatalytic ethylene synthesis system in the leaves at mid-expanding and end-expanding stages, when leaves are highly sensitive to ethylene and its expression was suppressed by ethephon treatment but become inhibited by ethylene(autoinhibitory) in newly matured leaves and old leaves. The result suggested that LcACS3 gene plays an important role in regulating responsiveness of leaves to ethylene. LcACS6 expression was promoted by ethephon treatment and might belong to a member of ethylene autocatalytic synthesis system in the leaves at end-expanding stage, but became suppressed by ethephon at mid-expanding stage and newly mature stage. LcACO1 gene might belong to a member of ethylene autoinhibitory synthesis system in leaves at all developmental stages examined as its expression was suppressed by ethephon treatment. LcACO3 gene was suppressed by ethephon treatment and might be a member of ethylene autoinhibitory synthesis system in leaves at mid-expanding and over-mature stages.7. Three days after shoot pruning, the LcACS3, LcACO3, Lc CTR1-2 and Lc CTR1-3 genes in the old leaves were up-regulated, and Lc ETR1, Lc ETR2, Lc EIN3-3, Lc ERF1-1 genes were down-regulated.8. Ethephon treatment increased the flowerlet and flower bud abscission rates in ’Feizixiao’ panicles and the increase was faster with higher concentration of ethephon applied. High concentrations of ethylene(> 100 mg / L) caused excessive flower shedding, and 50mg/L concentration of ethylene was relatively safe for flower thinning. The sensitivity of opened flowers to ethephon was higher than unopened flower buds. The effect of ethephon was closely related to temperature, and the responsiveness to ethephon was increased by paraquat. |
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