Effect of girdling and defoliation of longan shoots on hormonal changes during flower induction by KClO<inf>3</inf>

Abstract

The use of KClO3as a flower inducing agent in longan (Dimocarpus longan, Lour.) offers scientists an ideal model to study the physiology of flower induction (FI) in fruit trees within 3-4 weeks of application. Defoliation but also insufficient maturity or shading of leaves is inhibiting to flowering success. To study the involvement of leaves in hormonal changes within buds and to further elucidate the sequence, origin and transport of the hormonal signals accounted to be relevant for FI, shoots of longan trees were girdled and defoliated in the apical region to prevent "signal trafficking" from leaves to buds. Half of the trees were treated with KClO3as a soil drench. Up to 20 days after treatment, buds and stem material dissected into bark and wood, from above and below the girdle were collected for plant hormone analysis. Around 28 days after treatment first flowers appeared below the girdle while above no flowering occurred. Significantly elevated cytokinin (CK) concentrations, especially zeatin/zeatinriboside (Z/ZR), in buds at simultaneously reduced indolylacetic acid (IAA) and gibberellin (GAs) levels, as observed in KClO3treated trees below the girdle, seem to be essential for FI, while a rise in IAA and GAs in consequence of girdling and defoliation above the girdle seems to inhibit FI. Together with reduced Z/ZR concentrations this might create a low CK/auxin ratio in buds, which might be unfavourable for FI. Considerably increased CKs as measured in buds, wood and bark following KClO3application might originate from splitting of CK-conjugates, for example in wood, bark and roots, being transported as precursors isopentenyladenine/ isopentenyladenenosine (iAdo/iAde) via xylem to the leaves, where they are converted into active CKs (Z/ZR) and then translocated to the buds. Defoliation might inhibit the transport and/or conversion of CKs, notable in an accumulation of iAdo/iAde below the girdle and reduced Z/ZR concentration in the defoliated shoot above the girdle. Under normal conditions the basipetal flow of auxin out of leaves might suppress auxin transport out of buds by autoinhibition and, therefore, down regulate IAA biosynthesis in buds. Therefore, defoliation could activate IAA biosynthesis in buds, while girdling in combination with defoliation causes a backup of IAA in the shoot and inhibits IAA biosynthesis in buds, resulting in an up and down of IAA concentration as observed in buds above the girdle. The involvement of nitratereductase in the FI process, as an enzyme converting KClO3in leaves and linked to CK metabolism, still needs to be investigated.