Changes in SH-containing compounds and catalase activity in apricot flower bud during the winter season
Introduction
Sulfhydryl groups (SH) occur in plant protoplasm mainly as components of proteins, essential amino acids (cysteine [CSH]) and peptide (glutathione [GSH]) (Levitt, 1980). Glutathione is the predominant low-molecular-weight sulfhydryl compound present in most living cells, maintaining protein thiol groups in the reduced state (Fahey et al., 1975). In plants, glutathione is involved in many metabolic detoxification processes (Levitt et al., 1961; Levitt, 1980). In addition, it represents the main reserve and long-distance transport form of reduced sulfhydryls, which are indispensable for protein synthesis (Rennenberg, 1982). Both in photosynthesizing and non-photosynthesizing tissue, glutathione is found in two forms: the oxidated form (GSSG), which is converted into the reduced form (GSH) by glutathione reductase (GR) (Foyer and Halliwell, 1976). The balance between GSH and GSSG maintains the SH of intracellular proteins in oxidated state (Halliwell and Foyer, 1978). GSH may protect membranes from free radical damage by trapping oxygen radicals in the aqueous phase (Barclay, 1988).
It has been hypothesized (Amberger, 1984) that an increase in glutathione could stimulate the breaking of rest. Catalase activity could play a role in this process. It has been shown that in peach and grapevine, catalase activity decreases during exposure to low temperatures and dormancy breaking (Kaminsky and Rom, 1974; Nir et al., 1986). The decrease in catalase activity is thought to cause an increase in peroxide content and favour a shift from the Embden–Meyerhoff Parnas system (EMP) to the pentose phosphate (PP) pathway, leading to an increase in reduced nucleotide production essential for intensified metabolism (Nir et al., 1984).
The role of glutathione in stone fruit flower buds has not been investigated.
The objective of this research was to study the changes in sulfhydryl compounds and catalase activity during apricot flower bud growth.
Two apricot cultivars, `S. Castrese' and `Portici' were compared. `S. Castrese' consistently yields well in all environments, whereas `Portici' can present scanty and inconstant yield. This irregular productivity is prevalently due to the appearance of elevated percentages (above 35%) of flower bud anomalies during the dormant period (Guerriero et al., 1991; Viti and Monteleone, 1991a).
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Materials and methods
Trials were carried out on container-grown (30 l) 5-year-old apricot trees of cultivars `S. Castrese' and `Portici', maintained in the open at sea level experimental station in Pisa (Tuscan). Trees were grafted to Myrabolan B, irrigated daily with tap water and complex nutrient mixture was added at periodic intervals.
Periodically, from November to February, 600–700 flower buds were randomly collected from 1-year-old mixed twigs of 10 plants per cultivar. The following parameters were measured:
Catalase activity and flower bud growth
The changes in catalase activity showed three distinct phases (Fig. 1). From November to January 9, the activity was initially low and rose gradually. During this first phase, the catalase activity levels in `S. Castrese' were almost double than in `Portici'. During the second phase, no change in catalase activity was observed. In this phase, the activity was highest and similar in the two cultivars.
A third phase was noted afterwards, in which catalase activity decreased gradually to the
Conclusions
The two cultivars showed a difference in flower bud response, which was detected as early as the first observations in autumn. In particular, `S. Castrese' was found to present constant and elevated catalase activity and a greater GSH content.
These results appear to be in agreement with the observations on production behaviour of the two cultivars (Guerriero et al., 1991).`S. Castrese' always shows elevated production while `Portici' can present scanty and inconstant yield, prevalently due to a
Acknowledgements
This research was supported through the contribution of the Research National Council (C.N.R.).
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