486 AJCS 8(4):486-494 (2014) ISSN:1835-2707 Near-lethal heat treatment induced metabolic changes associated with endodormancy release of Superior Seedless grapevine cv. (Vitis vinifera L.) buds Hatem Ben Mohamed 1* , Ahlem Zrig 2 , Jan M.C. Geuns 3 , Habib Khemira 2,4 1 Laboratory of Horticulture, National Agricultural Research Institute of Tunisia (INRAT), Rue Hédi Karray 2049 Ariana, Tunisia 2 Research Unit Biodiversity and Valorization of Bioresources in Arid Zones, Faculty of Sciences of Gabès, University of Gabès, Erriadh City, Zrig 6072 Gabès, Tunisia 3 Laboratory of Functional Biology, KU leuven, Kasteelpark Arenberg 31, B-3001 Heverlee, Belgium 4 Centre for Environmental Research & Studies, Jazan University, Jazan, Saudi Arabia *Corresponding author: benmohamed.hatem@yahoo.fr Abstract To assess the efficacy of near lethal heat stress for endodormancy release of Superior Seedless grapevine buds, single-node cuttings were soaked for 1 h in hot water (50°C) and then forced for one month. The effects of hot water treatment (HWT) on budburst, metabolic changes of carbohydrates, proline, free polyamines PAs (putrescine (Put), spermidine (Spd) and spermine (Spm)) and antioxidant enzymes’ activity were investigated in bud under forcing conditions. Near-lethal heat stress caused a transient increase in starch hydrolysis, leading to an abrupt accumulation of soluble sugars, especially sucrose concentration during the first 3 days after treatment. This accumulation of soluble sugars coincided with a brief stimulation of the antioxidant system represented by ascorbate peroxidase (APX; EC 1.11.1.11), peroxidase (POD; EC 1.11.1.7) as well as an increase in the concentration of proline and free polyamines, especially putrescine (Put) and spermidine (Spd). These changes, which occurred immediately upon treatment, appear to be related with a process leading to endodormancy release. These results support the argument that a temporary and acute oxidative stress is involved in the mechanism leading to dormancy release and budbreak. Furthermore, it is possible that the stimulation of both peroxidases’ activity and proline biosynthesis activated the pentose phosphate pathway (PPP) which helped the bud to overcome endodormancy. After this initial period and towards budbreak initiation, there was a rapid decline in the concentration of soluble sugars, proline and Put, while, Spm and Spd became abundant. Such post stress changes appear to be associated with the reactivation of growth leading to an earlier and more vigorous budbreak. The metabolic response to HWT was compared to those observed after bud chilling or the application of restbreaking agents such as hydrogen cyanamide (HC). The similarity in the response to these various stimuli suggests the presence of common regulatory pathways involved in bud dormancy release and subsequent sprouting. Keywords: Grape bud dormancy; Hot water treatment; Carbohydrates; Antioxidant enzymes; Polyamines. Abbreviations: APX_Ascorbate peroxidase, BSA_Bovine serum albumin, CAT_Catalase, Ctrl_Control, EC_Electrical conductivity, DMAB_3-dimethylamino benzoic acid, HC_Hydrogen cyanamide, HWT_Hot water treatment, MBTH_3-methyl-2- benzothiazolinone hydrozone hydrochloride monohydrate, OM_Organic matter, Pas_Polyamines, POD_Peroxidase, Put_Putrescine, Spd_Spermidine, Spm_Spermine. Introduction Perennial plants have the ability to suspend and resume growth recurrently in response to seasonal fluctuations in environmental conditions (Rohde and Bhalerao, 2007). This cessation of active growth and bud dormancy seen in most deciduous fruit trees and vines is an adaptative mechanism to adverse conditions. In the temperate zone, prolonged exposure to short days and/or low temperatures have been known to govern growth cessation and the onset of dormancy (Dokoozlian et al., 1995), while the release from endodormancy requires the accumulation of chilling during the winter period (Westwood, 1993). Hence, to resume growth, dormant buds must receive an amount of chilling called chilling requirement which is genetically controlled (Balandier et al., 1993). As for most temperate fruit trees, dormancy limits grapevines production in mild-winter climates where natural chilling requirement for budbreak is often not fulfilled. Such conditions may prolong dormancy and trigger uneven and abnormal patterns of budbreak and development leading to low commercial production (Fuchigami and Nee, 1987; Dokoozlian et al., 1995). To overcome this problem, horticulturists attempted to compensate for the lack of natural chilling needed to stimulate budbreak of deciduous fruit trees and vines by using artificial restbreaking agents such as potassium nitrate, paraffin mineral oil, thiourea and HC (Westwood, 1993). The latter is considered the most efficient on grapevines to achieve satisfactory budbreak and yield (Dokoozlian et al., 1995; Mohamed et al., 2010a). Near-lethal heat stress brought about by soaking in hot water was found effective in releasing buds of woody plants from dormancy (Shirazi and Fuchigami, 1995; Wisniewski et al., 1997; Halaly et al., 2011). However, the mechanism by which a short exposure to such stress overrides intrinsic locks on growth and development is still poorly understood. Bud growth regulation and the underlying physiological and biochemical responses have been intensively studied over the past decades