Floral induction in longan (Dimocarpus longan, Lour.) trees IV. The essentiality of mature leaves for potassium chlorate induced floral induction and associated hormonal changes P. Potchanasin a , K. Sringarm b , D. Naphrom b , K.F. Bangerth a, * a Institute for Special Crop Cultivation and Crop Physiology, University of Hohenheim, 70599 Stuttgart, Germany b Faculty of Agriculture (Central Laboratory), Department of Horticulture, Chiang Mai University, 50200 Chiang Mai, Thailand 1. Introduction Leaves are essential for FI in many temperate and subtropical/ tropical angiosperm fruit tree species (Reece et al., 1949; Huet and Lemoine, 1972; Davenport and Nunez-Elisea, 1997). Their physio- logical/hormonal role in the FI process, however, is largely unknown but may be different from their similarly essential function in annual plants (Bernier and Pe ´rilleux, 2005; Bangerth, 2006). Ying and Davenport (2004) demonstrated that the flowering of litchi trees, a close relative to longan, requires leaves as the source of a putative florigenic promoter and Davenport et al. (2006) could show for mango that this florigenic signal can be produced by less than a whole leaf and can be transported to the shoot apical and lateral buds over considerable distances. This is similar to the earlier proposed and now confirmed property of florigen (Zeevaart, 2006). However, beside this hypothetical function of leaves as a source for florigen production in trees, leaves also serve a different role more related to the quantitative character of FI in trees (Bangerth, 2009, this Vol.), which is also demonstrated in the positive correlation between the number of leaves and the number of flower buds induced (Huet and Lemoine, 1972). Preliminary experiments have shown that potassium chlorate (KClO 3 ) treatment is ineffective in inducing FI in longan trees if a shoot below a terminal bud is girdled and the leaves above that girdle removed (Bangerth, unpublished and Fig. 1). This demon- strates that the ‘‘off season’’ effect of KClO 3 similar to the naturally low temperature induced FI also requires leaves although the mechanism of action may differ between both. Fruit trees in commercial orchards are almost always in their adult stage and, as mentioned before, FI in this case is a quantitative rather than a qualitative process. If, as proposed by Kulkarni (1991) and Davenport et al. (2006), a floral promoting substance with functional similarity to florigen plays a role in perennial fruit trees then one needs to assume two levels of regulation of FI. One level would regulate FI via the molecular genetic pathway known from annual/biennial plants and a second level of regulation would operate mainly via long distance signals, Scientia Horticulturae 122 (2009) 312–317 ARTICLE INFO Article history: Received 4 June 2008 Received in revised form 21 April 2009 Accepted 2 June 2009 Keywords: Auxin Cytokinins ‘‘Off season’’ floral induction Potassium chlorate ABSTRACT This study was conducted to elucidate the function of leaves and their role in the alteration of plant hormone homeostasis during potassium chlorate (KClO 3 ) induced ‘‘off season’’ floral induction (FI) of longan. KClO 3 application had almost no effect on the concentration of gibberellins in receptive apical and lateral shoot meristems, whereas the auxin IAA initially decreased, but later remarkably increased. The most significant changes caused by KClO 3 treatments was an increase in zeatin/zeatin-riboside (Z/ ZR) type cytokinins (CKs) 8 days after treatment in the shoot apical and close by lateral buds as well as in the tissue beneath these buds. The 6–7-fold increase in CK concentrations were greatly reduced in the absence of leaves (leaves removed between the apical bud and a girdle). A significant exception was a strong accumulation of isopentenyl adenine/isopentenyl adenosine (iP/iPA) type CKs in wood and bark beneath the bud of defoliated shoots that were isolated against the rest of functional leaves of the tree by a girdle. This accumulation of iP/iPA, the immediate precursors of Z/ZR, together with the strong increase of Z/ZR in the shoot apical bud (SAB) after KClO 3 application indicate that leaves may be involved in the biosynthesis of CKs. Overall, the results suggest that cytokinins play a significant role in KClO 3 induced ‘‘off season’’ floral induction in longan. This role may not be restricted to the biosynthesis of these hormones but also to their mobilization and accumulation in the SAB of newly flushing shoots. ß 2009 Elsevier B.V. All rights reserved. Abbreviations: CKs, cytokinins; FI, floral induction; GAs, gibberellic acid; IAA, indole acetic acid; iP/iPA, N6(D2-isopentenyl) adenine/N6(D2-isopentenyl) adenosine; SAB, shoot apical buds; Z/ZR, zeatin/zeatin-riboside. * Corresponding author. Tel.: +49 0 7022 59347; fax: +49 711 45923946. E-mail address: fkbang@uni-hohenheim.de (K.F. Bangerth). Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti 0304-4238/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2009.06.007