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Compositional changes during the postharvest senescence of broccoli branchlets held at 20C in the dark were investigated, particularly during the first 24 hours. Major losses of sugars, organic acids, and proteins from floral, middle, and base sections of branchlets were detected during the first 6 hours. Between 12 and 96 hours, free amino acid pools increased (especially the amides glutamine and asparagine) for all sections of branchlets, while ammonia accumulated only in floral sections. Results are discussed in relation to the nature of the processes that set the tissues on the pathways leading to postharvest senescence. Studies of postharvest senescence in horticultural crops have usually concentrated on mature tissue (e.g., fruit pericarp tissue) in which changes occur relatively slowly. In contrast, organs contain- ing immature tissue that is actively growing at harvest (e.g., asparagus and broccoli) senesce rapidly. Major physiological, biochemical, and molecular changes have been demonstrated within 3 h after harvesting asparagus spears (Davies and King, 1993; Hurst et al., 1993b; King et al., 1990; Lill et al., 1990). Since all crops initially spend a period at ambient temperature after harvest (while the crop is graded and packed), and then frequently during display and sale, an understanding of early changes follow- ing harvest is essential for accurate interpretation of subsequent changes, which may include storage at other temperatures. Fresh broccoli is highly perishable with a storage life of 3 to 4 weeks in air at 0C (Makhlouf et al., 1989; Smith, 1940) and 2 to 3 days in air at 20C (Wang, 1977). As broccoli deteriorates, the head yellows due to degradation of chlorophyll (Wang, 1977). Decay is common on florets during advanced stages of senes- cence (Aharoni et al., 1985). Ethylene plays an important role in regulating the yellowing of broccoli florets. Application of exogenous ethylene or propy- lene hastens yellowing (Aharoni et al., 1985; Tian et al., 1994). Conversely, application of anti-ethylene agents (aminoethoxyvinyl- glycine, CO 2 , silver ions, rhizobitoxine, sodium benzoate, benzyladenine, and gibberellic acid) retards yellowing (Aharoni et al., 1985; Wang, 1977). The sensitivity of broccoli florets to exogenous propylene may also increase after harvest (Tian et al., 1993). However, the relationship between ethylene and other physiological changes during postharvest senescence of broccoli has not been characterized. We have previously described physiological changes occurring early after harvest of broccoli, including decreased CO 2 produc- tion and a reduced respiratory quotient (RQ) (King and Morris, 1994). In this paper, we detail compositional changes during the senescence of broccoli held at 20C in the dark for up to 96 h, particularly those changes occurring during the first 24 h. Branchlets were used as they are a useful model system for investigating changes during senescence of broccoli (King and Morris, 1993). Materials and Methods Plant material and holding conditions. 'Green Belt' broccoli (Brassica oleracea L. var. italica ) heads were harvested at maturity from a commercial field at dawn, stripped of remaining leaves, and transported to the laboratory within 1.5 h of harvest. Heads were surface-sterilized with 100 ppm sodium hypochlorite (Rushing, 1990). Three branchlets (≈40 g each; Fig. 1) were excised from each head using a sterile scalpel. The branchlets were obtained from the second layer above the head base. These were considered to represent branchlets of an average size and developmental age on the head. Branchlets were placed in sterilized glass jars (550-ml capacity) and held at 20C in the dark, except for brief periods of light when measurements were being taken. The containers were aerated continuously with humidified, ethylene-free air at a flow rate of ≈1.5 liters·h -1 . Containers were equilibrated for 1 h before initial measurements.

Early Compositional Changes during Postharvest Senescence of Broccoli