Carbohydrate changes during dormancy release in floral, vegetative buds and bark tissues of pear cultivar 'Wonhwang' cuttings following dormancy breaking agents treatment

Temperate perennial woody plants annually pass through a period of active growth, growth cession, dormancy and growth resumption. Dormancy is defined as the temporary suspension of visible growth of any plant structure containing a meristem [1]. Dormancy helps to protect the plant tissues from harsh environmental conditions and secure their proper architecture. Bud dormancy in perennial fruit trees, including pear, is an adaptive developmental process for their survival under the adverse environmental conditions. Bud dormancy in perennial woody plants can be divided in three phases as para-dormancy (regulated by physiological factors within the plant but outside the dormant structure), endo-dormancy (regulated by physiological factors within the dormant structure) and eco-dormancy (regulated by environmental factors) [1]. Bud dormancy in temperate plants is well studied at the physiological level [2,3]. In winter, the short photoperiod and low temperature cause shoot extension growth to cease and the initiation of apical to protect the apical meristem [4,5]. The buds must receive an amount of chilling to resume growth which is genetically controlled and varies among genotypes [6]. 

Abnormal pattern of budbreak is mainly attributed to lack of chilling in temperate fruit trees in mild winter regions [7]. Due to warm winter temperature dormancy release become one of major limited factors for pear production in warm regions [8]. The pear industries in those regions may be adversely affected because of irregular blossoming and delayed flowering behavior. The amount of chilling hours of exposure to temperatures <7.2°C is required to break dormancy and induce floral and vegetative bud break. Artificial means to break dormancy is needed for maintaining economic production of pear crop in particular regions lack of natural chilling and also for protected pear cultivation in some regions of China [9]. Artificial chilling exposure and bud breaking agents with different concentration interact in breaking dormancy of fruit trees. Different scientists from the beginning of the previous century until recent times attempted to break the endodormancy and stimulate the budbreak of deciduous fruit trees planted in warm regions using the dormancy breaking agents such as thiourea, cyanamide and potassium nitrate [10]. Many researchers have been conducted the research to artifcially interrupt dormancy in areas lacking sufficient chilling units with synthetic chemicals [11,12,13,14]. A number of chemicals have been used to break the dormancy, including hydrogen cyanamide (HC) [15] and thiourea [12]. Hydrogen cyanamide increases the percentage of budbreak and improves regularity in budbreak of fruit trees under low chilling conditions [16]. Synthetic application of 2% thiourea provided 360 hour of chilling (CH) which caused early budbreak in pear [17]. The effectiveness of these chemicals mostly depends upon the application dates, quantity and cultivars. Investigators determined and calculate the chilling requirements or accumulations of deciduous fruit species based on different ways. The common method used on a large scale is to force cuttings or seedlings into growth chamber, laboratory or greenhouse under room temperature (2025°C or more) for two to three weeks or more then recording the percentage of budbreak [18].

 Significant changes are found in carbohydrate contents in floral and vegetative buds in fruit trees submitted to chilling deprivation as compared to natural chilling conditions [19]. Carbohydrates are the main source of energy for the metabolic changes that occurred during the dormancy release stage. Carbohydrate accessibility is most probably of major relevance to the control of bud growth and development during dormancy induction and release [20]. During earlier summer, starch is accumulated in reserve tissues and then converted to soluble sugars during dormant season. There are two enzymes concerned in sucrose metabolism as sucrose synthase and invertase. In plants, various kinds of invertases are found such as alkaline and acid invertase. It was observed that invertases play an important role in sink initiation, growth and cell expansion while sucrose synthase is linked with the metabolism of carbohydrates storage, fruit maturation, and polysaccharides synthesis [21]. Likewise, soluble sugars are also documented as important signaling molecules involved in many processes of plant life-cycle, including dormancy [22,23,24]. However, budbreak pattern appear to be more correlated with the capacity of bud to use soluble sugars than with sugar abundance in dormant tissues [25,19]. The acts of artificial dormancy breaking agents, such as HC and thiourea have been associated with the onset of sub lethal stress which leads to budbreak [26,27,12]. Therefore, the present work was planned to study the effect of exogenous application of hydrogen cyanamide and thiourea on floral and vegetative budbreak. To better understand the starch and soluble sugars concentrations would be effected by HC and thiourea application to endo-dormant cuttings of Pyrus pyrifolia Nakai cv. 'Wonhwang.