Gerard Jacobs

Effect of harvest maturity on pectic substances, internal conductivity, soluble solids and gel breakdown in cold stored ‘Songold’ plums

Abstract ‘Songold’ plums (Prunus salicina Lindl.) were harvested at optimum and post-optimum maturity with a flesh firmness of 64 and 44 N, respectively. After cold storage for 10 days at −0.5 °C, followed by 18 days at 7.2 °C, the plums were ripened for 14 days at 10 °C. The fruit was analysed for soluble solids, internal conductivity, pectic composition and viscosity of water soluble pectin at eight examination stages during storage and ripening. Insoluble pectin levels declined during cold storage and ripening while soluble pectin levels increased to make up the difference. Neither harvest maturity nor storage time had a significant effect on the concentration of calcium pectate, and this pectic fraction did not appear to influence development of gel breakdown (GB). Water soluble pectin and availability of cell fluids, as indicated by viscosity and internal conductivity, respectively, indicated a high gel potential in plums of both maturities, but at different stages during storage and ripening. Significant levels of GB developed only in plums harvested at post-optimum maturity. In this fruit, significantly higher sugar levels and loss of cell membrane integrity early in storage probably enhanced formation of pectin-sugar gels as cell fluids bind with pectins in cell walls.

Progression of apple (Malus × domestica Borkh.) bud dormancy in two mild winter climates

Summary One year old, ca. 500 mm long shoots of ‘Golden Delicious’ and ‘Granny Smith’ apple were selected randomly from commercial orchards in either Elgin (34°S, 305 m, ca. 900 chill units (CU) in 1995) or the Bokkeveld (33°S, 945 m, ca. 1500 CU in 1995) regions of the Western Cape, South Africa. Twenty shoots were harvested weekly from 29 March 1995 until 23 August 1995 and forced at a constant 25°C with continuous illumination until budburst was observed on four out of 20 shoots per bundle, i.e. days to 20% budburst. The progression of bud dormancy in Elgin differed from that observed in the Bokkeveld. Both cultivars reached maximum dormancy in the Bokkeveld before any considerable chilling accumulated (<100 CU), but in Elgin, ca. 600 CU accumulated before a maximum was attained. In Elgin temperatures that normally promote chill requirement satisfaction enhanced dormancy. The chilling models used in this study were inadequate in considering climatic conditions involved in the entrance to bud dormancy. In the Bokkeveld, temperatures other than those used to calculate CU induced dormancy – possibly freezing temperatures or frost conditions.

Physiological factors associated with overripeness, internal breakdown and gel breakdown in plums stored at low temperature

Plums (Prunus salicina cv. Songold) were stored at -0.5°C for up to 50 d. On the day of harvest and thereafter at ten-day intervals, sub-samples were stored at 15°C and allowed to ripen for eight days, with evaluations every two days. Low temperature storage prior to ripening at 15°C was a prerequisite for plums to ripen. Overripeness was inhibited by low- temperature storage but developed when fruit was ripened at, or before, 30 d of storage. Longer storage resulted in the development of gel and internal breakdown at both -0.5°C and during ripening. The transition from overripeness to gel and internal breakdown was associated with significant increases in electrolyte leakage and internal conductivity at a time that viscosities of water soluble pectins were high. The resulting decrease in extrac- table juice suggested that cell fluids which leaked through cell membranes had bound with pectic substances. This was associated with development of gel and internal breakdown.

Endogenous cytokinin distribution patterns at budburst in Granny Smith and Braeburn apple shoots in relation to bud growth

Abstract The possible relationship of branching habit to cytokinin content of apple shoots ( Malus × domestica Borkh.) was investigated. One-year old apple shoots are acrotonic (distal branching), more strongly so in Granny Smith than in Braeburn. In the first trial, long, 1-year old Granny Smith and Braeburn apple shoots were sprayed on 29 August 1995 to break rest with dinitro- o -cresol (DNOC) oil (5%). The cytokinin contents of the xylem sap, the combined bark and buds, and the wood were determined in distal and proximal shoot halves over the next 6 weeks. Budburst (terminal and lateral buds) was first visible (green tip) in both cultivars on 20 September 1995. A greater increase in cytokinin content of distal xylem sap, coupled with elevated cytokinin in the distal wood, reflect the overall acrotony of both cultivars. The strong acrotony of Granny Smith is reflected in the higher cytokinin concentration in distal portion 1 week before the proximal portion of the shoot. The differential distribution of cytokinin reflects the pattern of budburst and may be correlated with growth habit. In a subsequent trial, Granny Smith shoots chilled and forced in the absence of roots showed an increase in cytokinin content of the bark and buds, and the wood as growth resumed. This was roughly comparable in magnitude to the increase observed under field conditions. The cytokinin increase in rootless shoots and differential distribution of cytokinin prior to sprouting, support the hypothesis that shoot-derived, rather than root-derived, cytokinins act to trigger spring budburst.

Effect of storage regimes on pectolytic enzymes, pectic substances, internal conductivity and gel breakdown in cold stored ‘Songold’ plums

SummaryPlums (Prunus salicina cv. Songold) were cold stored according to a single-temperature regime comprising 28 d at −0.5°C, and a dual-temperature regime comprising 10 d at −0.5°C followed by 18 d at 7.2°C. After cold storage, the plums were ripened for 8 d at 10°C. Pectolytic enzyme activity, pectic composition, internal conductivity and gel breakdown were determined at seven stages during storage and ripening. Although not exposed to chilling temperatures prior to harvest, approximately 10% of the plums exhibited gel breakdown at harvest, indicating that the disorder cannot be classified solely as a cold-storage chilling disorder. The higher temperatures of the dual-temperature regime resulted in higher polygalacturonase activity than with the single-temperature fruit. Consequently, protopectin degradation and the concomitant production of water- soluble pectins were greater in the dual-temperature fruit. Single-temperature storage resulted in higher pectinmethylesterase activity during the latter s...

Influence of sampling date and position in the tree on mineral nutrients, maturity and gel breakdown in cold stored 'Songold' plums

Abstract Plums ( Prunus salicina cultivar ‘Songold’) were harvested on three dates, 7 days apart, from top and bottom sectors of trees trained on a palmette system. Fruit maturity at harvest, assessed in terms of skin ground colour, flesh firmness, soluble solids, titratable malic acid and soluble solids to acid ratio, was significantly affected by harvest date and canopy position. Delayed harvest was associated with advanced maturity, and, within trees, bottom fruit were more mature than top fruit. Similarly, these parameters measured after storage at −0.5°C for 10 days, followed by 18 days at 7.2°C and 7 days at 10°C, indicated that fruit quality was also affected by harvest date and canopy position. Other quality indices affected were gel breakdown, overripeness and decay. Best overall fruit quality was obtained in earlier harvested fruit with a ground colour of 2.6 as measured with an apple colour meter and a flesh firmness of 5.2 kg. More mature fruit developed the most gel breakdown, and in fruit harvested last, concentrations of N, P, K, Ca and Mg were lower than in earlier harvested fruit, and were least optimal for good fruit quality. Susceptibility to post-harvest decay was also highest in fruit harvested at the later date.

Chilling period influences the progression of bud dormancy more than does chilling temperature in apple and pear shoots

Summary One year old, ca. 50.cm long shoots of ‘Doyenne du Comice’ pear (Pyrus communis L.) and ‘Granny Smith’ apple (Malus × domestica Borkh.) were selected randomly in autumn and winter 1999 from commercial orchards in either Elgin (348S, 320.m) or in Somerset West (34°S, 80.m), South Africa. Shoots were cold-stored at 1, 4, 7 or 10°C for periods of 0, 1, 2 or 3 months after a daily 12/12.h freezing temperature pre-treatment of 21/13°C (supposedly nonchilling temperatures) for periods of 0, 1, 2 or 3 weeks. In 2000, ‘Granny Smith’ apple and ‘Packham’s Triumph’ pear shoots were harvested in autumn from orchards in Elgin and cold-stored, without a freeze treatment, at 1, 4, 7, 10 or 13°C for periods of 0, 1, 2, 3 or 4 months. After the different treatments the shoots were forced to budburst at 25°C with continuous illumination. To determine the progression of bud dormancy, the rate of budburst, final percentage budburst, and the synchronization of budburst between shoots were used in 1999, but only the rate of budburst in 2000. In all the trials the storage period was the most important factor influencing the progression of dormancy. While in some cases the effects of both storage temperature and the freeze treatment were significant, the contribution to differences in the progression of dormancy was negligible. When our data were fitted to the chilling models currently used in South Africa the difference in temperatures between –1 to 13°C was over-emphasized relative to the period of exposure to these chilling temperatures.

Influence of the number of calcium sprays on the distribution of fruit mineral concentration in an apple orchard

SummaryThree treatments – six, nine and twelve Ca sprays – were applied to four year old ‘Braeburn’ apple trees to establish the effect of the number of sprays on the distribution of fruit mineral concentration in the orchard. The P, K, Ca and Mg concentrations of 150 fruit and N concentration of another 150 fruit were determined two weeks before the estimated optimum harvest date and each fruit was analysed separately. At the same time a second sample of fruit was picked and stored at –0.5°C for seven weeks. After another two weeks’ storage at 17°C, fruit was examined for bitter pit development. In a second single fruit analysis the P, K, Ca and Mg concentration of 37 bitter pit and 29 bitter pit-free fruit was determined, as well as the N concentration of 27 bitter pit and 27 bitter pit-free fruit. Results of the pre-harvest analysis showed that the number of sprays affected the fruit Mg and Ca concentration, but there were no differences in N, P and K concentration between treatments. Frequency distrib...

Physiological and anatomical changes associated with ripening in the inner and outer mesocarp of cold stored ‘Songold’ plums and concomitant development of internal disorders

Plums (Prunus salicina cv. Songold) were cold stored for 10 d at -0.5°C followed by 18 d at 7.2°C and 7 d at 10°C. On the day of harvest and after 15,30 and 35 d of storage, the physiology, anatomy and development of internal disorders in inner and outer mesocarp tissue were assessed. Soluble solids, juice pH, internal conductivity, viscosity of soluble pectin and membrane leakage were significantly higher in the inner tissue, while titratable malic acid levels were lower, indicating that fruit was riper on the inside than the outside. Changes as indicated by the same variates showed that the rate of ripening in inner tissue was more rapid than in the outer mesocarp tissue over storage time. Gel breakdown in inner tissue was associated with high viscosities of water soluble pectin, permeable membranes, and as a result, binding of cell fluids which led to low levels of extractable juice. In outer tissue where extractable juice levels were higher, overripeness developed. At the ultrastructural level, it was...

Quantification of bearing habit on the basis of lateral bud growth of seven pear cultivars grown under conditions of inadequate winter chilling in South Africa

Abstract The bearing habit of seven pear cultivars was quantified according to the ontogenetic development from axillary buds, i.e. developmental changes in the terminal position of laterals on fruiting branches. Ten non-pruned branches of seven pear cultivars, i.e. Forelle (on Quince A and BP1 rootstocks), Abate Fetel, Flamingo, Packham’s Triumph, Golden Russet Bosc, Rosemarie and Beurre D’Anjou (all on BP1 rootstock) were observed. Description started with the development of the main fruiting branch, forming several leaves in the first year of growth (designated year Y), with meristems developing in the leaf axils. In the following season (year Y+1), these buds had five developmental alternatives: to remain as a latent bud (L), to develop as a vegetative bud (V), to become a flower bud without fruit (F), to become a flower bud setting a fruit (P), or to abort and leave a scar (S). Each year the development of these buds was observed and classified anew, giving rise to sequences describing up to 5 years of development. From 44% (‘Beurre D’Anjou’) to 79% (‘Flamingo’) of laterals remained in the growing phase (phase G comprising of V, F or P buds). This coincided with low proportions of buds remaining in the latent phase (L), with even fewer buds moving to the ending state (S; extinction mechanism). A relatively large proportion of growing laterals went from the growing phase back to the latent phase (up to 21% in the case of ‘Beurre D’Anjou’), probably due to inadequate winter chilling. The predominant bud type in the growing stage was vegetative (V). Although flowering was generally low, ‘Flamingo’ and ‘Abate Fetel’ became reproductive (F or P) in year Y+3. ‘Packham’s Triumph’ and ‘Rosemarie’ displayed F and P buds in year Y+1, corresponding to the ability of these cultivars to bear fruit on longer 1-year-old shoots. Although the occurrence of the extinction mechanism was low, the higher proportion of latent buds may aid in reducing the number of growing buds, thereby increasing the allocation of assimilates to fruiting structures. This, in combination with the tendency of ‘Packham’s Triumph’ and ‘Rosemarie’ to develop longer, more autonomous shoots, may explain their higher productivity and the occurrence of bourse-over-bourse bearing.