Litchi (Litchi chinensis Sonn.) is an evergreen fruit tree widely cultivated in southeast Asia. However, unreliable flowering is a serious problem in the litchi industry. Litchi flowering is induced by low temperatures and enhanced by drought in winter (Chaikiattiyos et al., 1994; Chen and Huang, 2005; Menzel and Simpson, 1988). Although temperature and soil moisture are low enough during winter to induce flowering, maturity of terminal shoots must occur before growth cessation in winter to avoid poor flower set.
Normally litchi trees flower at the apex of terminal shoots. The accumulation of carbohydrates in these shoots is dependent on mature leaves, which have photosynthetic capacity. The carbon-to-nitrogen in plants has been considered as a factor affecting flowering. Evidence has revealed an intricate and integrated nitrogen and carbon sensing and signaling network to control various developmental processes (Coruzzi and Zhou, 2001; Gibson, 2005). Litchi flowering is considered to be related to the patterns of carbohydrate distribution in trees (Chen et al., 2004). Carbohydrates are accumulated in leaves and branches of litchi trees at fall for flowering (Menzel et al., 1995).
In Arabidopsis, CONSTANS (CO) codes a zinc finger transcription factor, which activates the expression of FLOWERING LOCUS T (FT) by binding to FT regulatory regions (An et al., 2004). The FT protein is a component of the mobile flowering signal “florigen” that moves on its expression in the vascular tissue of leaves to the shoot apex (Yang et al., 2007). FLC (FLOWERING LOCUS C) is a floral repressor. Vernalization decreases FLC expression, which allows the induction of FT in leaves and the transition from vegetative to reproductive growth (Wellmer and Riechmann, 2010).
In the present study, we focused on three important commercially cultivated cultivars, Guiwei, Feizixiao, and Huaizhi. We investigated the carbohydrate accumulation at different developmental stages of terminal shoots in relation to flowering. We also determined expression levels of the litchi homolog CO, FT, and FLC in these trees to determine an appropriate developmental stage for litchi flowering.
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