Dal Cin Valeriano, Boschetti Andrea, Dorigoni Alberto, Ramina Angelo
Department of Environmental Agronomy and Crop Science, University of Padova, Viale dell'Università 16, 35020 Legnaro (Padova), Italy.
Ann Bot. 2007 Jun;99(6):1195-202. doi: 10.1093/aob/mcm062. Epub 2007 Apr 30.
It has been previously shown that abscission of apple fruitlets is preceded by an increase in ethylene evolution and in the amount of transcripts for 1-aminocyclopropane-1-carboxylate oxidase (ACO), an enzyme catalysing the final step in ethylene biosynthesis. These events are concomitant with shedding induction and chemical thinning. There are several thinners but their mode of action and efficacy is poorly understood. One of them is benzylaminopurine (BA), a cytokinin believed to act by enhancing vegetative activity and stressing the competition between shoots and fruitlets, thus leading to fruitlet shedding. Nevertheless, the specific mechanism of action of BA and the variable effect depending on apple cultivar (easy or difficult to thin) are poorly understood.
Abscission, the amount of MdACO1 transcripts and other parameters were followed in immature apple fruits during the period of physiological drop. The cultivars studied were 'Golden Delicious' and the 'spur' type 'Red Delicious'. BA was used as a thinning agent and was sprayed 14 d after petal fall (DAPF). Fruitlets were divided into central (C) and lateral (L) fruitlet populations.
Fruitlet size was significantly different between C and L fruitlets but it did not differ much between the populations within the same cultivar. C fruitlets were characterized by basal ethylene evolution while L fruitlets displayed an increase in hormone biosynthesis during abscission induction. Cluster composition evaluated by the L/C ratio differed in the two varieties, being almost unchanged throughout abscission induction in 'Golden Delicious' and progressively decreasing in 'Red Delicious'. Shoot growth activity evaluated at the end of the season indicated a possible connection with both the ongoing abscission and BA application. MDACO1 transcripts were mainly detected in L fruitlets and the accumulation was related to total abscission in 'Golden Delicious', while in 'Red Delicious' expression was observed in both C and L fruitlets.
BA probably exerts its thinning effect through vegetative growth. In the 'spur' type 'Red Delicious' the chemical is ineffective, probably due to a limited action on shoot growth due to genetic characteristics. The amount of MdACO1 transcripts in seeds is a good indicator of abscission.
先前的研究表明，苹果幼果脱落之前，乙烯释放量以及1-氨基环丙烷-1-羧酸氧化酶（ACO）的转录本数量会增加，ACO是一种催化乙烯生物合成最后一步的酶。这些事件与脱落诱导和化学疏果同时发生。有几种疏果剂，但其作用方式和效果尚不清楚。其中之一是苄氨基嘌呤（BA），一种细胞分裂素，据信其作用是通过增强营养生长并加剧新梢与幼果之间的竞争，从而导致幼果脱落。然而，BA的具体作用机制以及因苹果品种而异的效果（疏果容易或困难）尚不清楚。
在生理落果期对未成熟苹果果实的脱落情况、MdACO1转录本数量及其他参数进行跟踪。所研究的品种为“金冠”和“短枝型”“红富士”。BA用作疏果剂，在花瓣脱落14天后（DAPF）进行喷施。幼果分为中心（C）和侧生（L）幼果群体。
C和L幼果的大小存在显著差异，但同一品种内的群体间差异不大。C幼果的特征是基部乙烯释放，而L幼果在脱落诱导期间激素生物合成增加。通过L/C比值评估的果穗组成在两个品种中有所不同，在“金冠”整个脱落诱导过程中几乎不变，而在“红富士”中则逐渐降低。季末评估的新梢生长活性表明其与正在进行的脱落和BA施用都可能存在关联。MDACO1转录本主要在L幼果中检测到，其积累与“金冠”的总脱落有关，而在“红富士”中，C和L幼果中均观察到表达。
BA可能通过营养生长发挥其疏果作用。在“短枝型”“红富士”中，该化学药剂无效，可能是由于遗传特性导致其对新梢生长的作用有限。种子中MdACO1转录本的数量是脱落的良好指标。
