New study reveals ethephon’s role in regulating dormancy and flowering in litchi

The Agricultural Research Council-Tropical and Subtropical Crops campus conducted research on optimising ethephon applications to completely inhibit any new shoot growth as opposed to frequent spot sprays for control of young flush, which are commonly used in the industry.

1AGRICULTURAL RESEARCH COUNCIL-TROPICAL AND SUBTROPICAL CROPS 2UNIVERSITY OF LIMPOPO

Successful flower induction and initiation in litchi are highly dependent on low temperatures (Figure 1). Litchi flower induction takes place in dormant buds of mature shoots and requires minimum temperatures below 15 °C. At flower initiation, the so-called while millet stage, temperature will determine whether a flower panicle will be pure, i.e. without leaves, or leafy (generally less fruitful). Shoot maturity is an essential prerequisite for flower induction in litchi and any new vegetative growth will reduce flowering.

Traditionally, the plant growth regulator, ethephon, has been used in South Africa for spot spray applications to chemically remove such unwanted shoots. However, in the past decade, warmer winters have caused persistent vegetative shoot growth, which made the frequent use of spot spray applications less efficient and economical. In addition, inconsistent inductive temperatures have caused concurrent flower emergence, making growers hesitant to use ethephon out of concern that its application could damage already developing flower panicles. This necessitated a new approach to chemical flush control using ethephon. Therefore, scientists at the Agricultural Research Council-Tropical and Subtropical Crops campus conducted research on optimising ethephon applications to completely inhibit any new shoot growth prior to, and during, the flower induction period as opposed to frequent

Figure 1. Developmental stages in litchi flowering (according to the BBCH scale, Wei et al., 2013). Source: Zhang et al. (2014).

spot sprays for control of young flush, which are commonly used in the industry.

This research has shown that ethephon applied as a full canopy spray when the last desired postharvest flush has hardened off (late March and April), can inhibit new flush growth for up to six weeks depending on the ethephon concentration and prevailing temperatures after application. The full canopy treatment also delayed flower panicle emergence to a time where temperatures were consistently low enough for successful flower induction and increased flowering and yield without delaying harvest. Based on these results, a further study was initiated to investigate the mode-of-action of ethylene, the breakdown product of ethephon, by determining ethylene release and gene expression of various dormancy- and flowering-related genes in leaves and buds.

Measurements of ethylene gas released by leaves and buds before and after ethephon application at 1000 parts per million (ppm) revealed that immediately after application, a burst of ethylene was released in both leaves and buds. However, while ethylene release in the leaves immediately dropped, ethylene release in buds remained high for about seven days and thereafter declined at a slow rate, only reaching the same level as the untreated buds after more than four weeks (Figure 2).

Ethephon treatment thus delayed bud break by about three weeks (as indicated in Figure 2). These results confirmed that ethephon application is directly responsible for inhibiting bud growth. In addition, ethephon application was able to maintain bud dormancy during a period of high temperatures at the end of May (around Day 42 after application) (Figure 2).

In order to investigate the effect of ethephon/ethylene at a molecular level, gene expression analyses were performed using real-time quantitative polymerase chain reaction (RT-qPCR). The genes studied in this context were, among others, the ethylene pathway gene LcEIN3, the flowering suppressor gene LcFLC, and the flower promoter gene LcFT2.

Ethephon treatment did not have any effect on LcEIN3 ex-

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pression in leaves (data not shown), but significantly increased LcEIN3 expression in buds one day after application and during mid dormancy (Figure 3A). This confirmed the trends seen in the ethylene release data and demonstrated that the target sites for ethephon are the buds.

Likewise, ethephon application significantly increased expression of the flowering suppressor gene LcFLC in the buds (Figure 3B). Therefore, ethephon treatment had a direct effect on these genes and directly influenced tree phenology, i.e. by causing extended bud dormancy and preventing vegetative growth prior to flowering. The prolonged dormancy subsequently allowed bud break to occur at a period with lower temperatures, compared with bud break in untreated trees, which provided a stronger stimulus for the significant upregulation of the flower promoter gene LcFT2 at bud break (Figure 3C), leading to successful flowering with significantly reduced leafy panicles compared with the control treatment.

This study is the first of its kind in South Africa revealing the direct involvement of ethephon/ethylene in the physiological and molecular regulation of dormancy and flowering of litchi. Furthermore, it showed that full canopy ethephon applications can be used to mitigate the adverse effects of seasonal climate changes.

The application of gene expression studies is becoming more and more relevant and available for practical agriculture. Traditionally, the appearance of phenotypic traits has been used as an indication of certain growth conditions. However, phenotype lags behind gene expression, as seen in the above study, and does not reflect the real time developmental and metabolic status of the plant.

Gene expression information can therefore be useful for monitoring and diagnosing specific developmental conditions at the gene level to predict unfavourable change before it happens, thereby allowing timely action to be taken. Such information also provides guidelines for more efficient and cost effective orchard management to improve productivity and quality of horticultural products. E-mail: regina@arc.agric.za

Wei, Y.Z., Zhang, H.N., Li, W.C., Wang, J.H., Liu, L.Q. and Shi, S.Y. 2013. Phenological growth stages of lychee (Litchi chinensis Sonn.) using the extended BBCH-scale, Sci. Hort. 161: 273-277. Zhang, H.N., Wei, Y.Z., Shen, J.Y., Lai, B., Huang, X.M., Ding, F., Su, Z.X. and Chen, H.B. 2014. Transcriptomic analysis of floral initiation in litchi (Litchi chinensis Sonn.) based on de novo RNA sequencing. Plant Cell Rep. 33(10): 1723-35.

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