In asparagus, there is a tight link between carbohydrate supplies and the metabolic activity of the spear (Irving and Hurst, 1993; King et al., 1990; Lill et al., 1990; Papadopoulou et al., 2001; Saltveit and Kasmire, 1985). In intact plants, the storage carbohydrates are primarily fructans, which are located in the roots (Shiomi, 1993). Mobilization in support of spear growth is initiated by hydrolysis of fructans to sucrose and subsequent transport of the sucrose to the above-ground portion of the plant (Bhowmik et al., 2001; Robb, 1984; Wilson et al., 2008). As the harvest season progresses, fructan depletion in the roots leads to reduced spear production (Morse, 1916; Shelton and Lacy, 1980). Spear shelf life, soluble solids and carbohydrates, carbon partitioning, and sucrose metabolism change throughout the harvesting season (Bhowmik et al., 2002; Hurst et al., 1993b; Wilson et al., 2008; Zurawicz et al., 2008). Harvest, which is accomplished by cutting or snapping the spear at its base, prevents access of the metabolically active spear to the stored carbohydrate of the root and ends the rapid preharvest growth (Kays and Paull, 2004). After harvest, the spear is forced to depend solely on energy resources it contains at harvest, which primarily take the form of sucrose, glucose, and fructose (Robb, 1984; Wilson et al., 2008).
Loss of carbohydrate supply in harvested asparagus spears causes soluble carbohydrate levels, especially sucrose, to be rapidly depleted (Hurst et al., 1993b; Irving and Hurst, 1993; Saltveit and Kasmire, 1985). This reduction in carbohydrate may limit respiration (Kays and Paull, 2004; King et al., 1990), which declines rapidly in the hours after harvest (Platenius, 1942). Loss of carbohydrate stores may, in turn, decrease the ability of the tissue to produce sufficient energy for some metabolic processes. Inability to maintain metabolic homeostasis is suggested to promote the spear’s rapid senescence (Irving and Hurst, 1993). As the level of carbohydrates in the spear declines, the transcription of genes related to senescence increases (Davies et al., 1996; Irving et al., 2001; King et al., 1995).
The tip section of asparagus spears is comprised of highly active meristematic tissues and is considered to be a strong sink that is particularly susceptible to deteriorative changes (Lill et al., 1996). The spear tip is usually the first portion to show symptoms of postharvest deterioration and physiological decline (Eason et al., 2002; King et al., 1990). Tissue deterioration then proceeds to the area just basipetal to the tip, which is the zone of cellular elongation (Robb, 1984). The spear base is comprised of more mature tissues. In the base, cell elongation has ceased and the vascular tissue has begun to lignify and phenolics accumulate (Rodriguez-Arcos, et al., 2002). The spear base is the tissue most resistant to deterioration (King et al., 1990; Lill et al., 1996) and may act as a source of metabolites for the tissues of the tip (Saltveit and Kasmire, 1985).
Although the general pattern of postharvest changes in the concentration of soluble sugars in asparagus spears has been documented (Hurst et al., 1993a; Irving and Hurst, 1993; Saltveit and Kasmire, 1985), the relationship between respiratory carbon use and the loss in carbohydrates along the longitudinal profile has not been investigated. The base of the spear has elevated levels of sugars relative to the more apical sections of the spear and may act as a carbohydrate source for the more apical sections of the spear. The objective of this research was to establish the carbon balance between respiration and hexose catabolism as a function of the longitudinal position of harvested asparagus spears during storage at 0 °C to determine if the spear base acts as a source of carbohydrate for the more apical portion of the spear.
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