Paul L. Hurst

Respiration, soluble carbohydrates and enzymes of carbohydrate metabolism in tips of harvested asparagus spears

We held asparagus (Asparagus officinalis L.) spears at 20°C for up to 5 days after harvest and examined changes in respiration rate, soluble carbohydrates and the activities of enzymes concerned with glycolysis and carbohydrate breakdwon in the tips. Respiration rate declined after harvest, to reach 25% of its initial rate after 2 days. The gas exchange (respiratory) quotient was 1.05 at harvest and declined in parallel with respiration rate for 2 days. There was a rapid loss of sucrose from spear tips over the first 6 h, and a more gradual loss after that. Glucose concentration declined gradually throughout, but fructose concentration barely changed over 3 days. We conclude that a major change in the metabolism of asparagus spear tips occurs soon after harvest. The change is characterised by a switch from sucrose-derived hexose phosphate to another primary respiratory substrate, probably derived from protein and/or lipid. The activities of hexokinase (EC 2.7.1.1), fructokinase (EC 2.7.1.4), phosphofructikinase (EC 2.7.1.11), sucrose synthase (EC 2.4.1.13) and alkaline invertase (EC 3.2.1.26) were generally low compared with the activities of the other enzymes assayed. Overall, enzyme activities declined over 5 days of starvation. The exceptions were hexokinase, phosphofructokinase and pyruvate kinase (EC 2.7.1.40), where activities were reasonably steady.

Sugar Regulation of Harvest-Related Genes in Asparagus

The signals controlling the abundance of transcripts up-regulated (pTIP27, pTIP31, and pTIP32) or down-regulated (pTIP20 and pTIP21) after harvest in asparagus (Asparagus officinalis L.) spears were examined. pTIP27 and pTIP31 are known to encode asparagine synthetase (AS) and a [beta]-galactosidase ([beta]-gal) homolog, respectively. The nucleotide sequences of pTIP20, pTIP21, and pTIP32 were determined, and they encode histone 3, histone 2B, and an unknown product, respectively. Changes in respiration, soluble sugars, and abundance of the five mRNAs were similar in the tips stored as 30-mm lengths or as part of 180-mm spears. We previously hypothesized that sugars may regulate the level of AS transcripts in asparagus tissue. Asparagus cell cultures were used to test the role of sugar status in regulating gene expression. Transcript abundance for AS, [beta]-gal, and pTIP32 was low in cells in sugar-containing medium but increased within 12 h after transferring cells to a sugar-free medium. Histone 3 and histone 2B transcripts were, in general, abundant in cells on sugar-containing medium but declined in abundance when transferred to sugar-free medium. When cells were returned to sugar-containing medium the abundance of transcripts for histone 3 and histone 2B increased, whereas that for AS, [beta]-gal, and pTIP32 decreased. Soluble sugar levels are known to decline rapidly in the tips of harvested spears. Metabolic regulation by sugar status may have a major influence on gene expression in asparagus spears and other tissues after harvest.

Analysis of acid invertase gene expression during the senescence of broccoli florets

Broccoli (Brassica oleracea var. italica) kept at ambient temperatures after harvest senesce rapidly and sucrose levels decrease. However, if exogenous sucrose is supplied back to the plant, senescence symptoms can be delayed for up to 48 h. Acid invertase enzyme activity increases after harvest in broccoli stored at 20 °C and an investigation was undertaken to see if this enzyme has a role in the senescence process. Soluble acid invertase enzyme was extracted from fresh broccoli (0 h) and after 96 h of storage. There were two peaks of invertase activity, one at pH 5–5.5 and the other at pH 8. Broccoli stored in water and sucrose had reduced levels of acid invertase enzyme activity after 48 h compared to those stored dry. Similarly, broccoli kept in a controlled atmosphere (CA) of 10% CO2 and 5% O2 senesced at a slower rate and had significantly less acid invertase enzyme activity than air controls after 48 h. Two broccoli acid invertase cDNAs (BoINV1 and BoINV2) were isolated and these had 85 and 90% identity at the nucleotide and amino acid level. RNA blot analysis with BoINV1 and BoINV2 showed that transcripts of approximately 2.3 kb accumulated during senescence of the broccoli florets. The accumulation pattern was similar for both acid invertases but the level of accumulation was greater for the BoINV2 encoded transcript. The accumulation of these transcripts was delayed by water, sucrose and CA treatments compared to the controls. Southern analysis experiments demonstrated that the two cloned acid invertases are part of a small gene family. These findings suggest that acid invertase may be one of the key enzymes responsible for the reduction in sucrose levels that accompanies the rapid deterioration of harvested broccoli.

Winter squash (Cucurbita maxima) texture: Sensory, chemical, and physical measures

Abstract Significant differences were found in a wide variety of textural attributes in eight winter squash cultivars (Cucurbita maxima Duchesne) assessed at 4–5 and 9–10 weeks after harvest using both a trained sensory panel and an Instron Universal Testing Machine. The Instron texture profile analysis force‐displacement curves were distinctive for each of the eight cultivars tested, with behaviour under deformation forces indicating differences in the structure and composition of the tissue. Starch and logged dry matter contents varied significantly between cultivars, and were strongly correlated with all the sensory textural attributes rated, and with gumminess and hardness (dry matter only) measured by the Instron. Simple linear regressions indicated significant relationships between textural attributes as measured by the sensory panel and the Instron.

Sensory characteristics and consumer acceptability of ‘Pink Lady’ and other late‐season apple cultivars

Abstract ‘Pink Lady’ is a late maturing ‘Lady Williams’ × ‘Golden Delicious’ apple (Malus domestica Borkh.) cultivar developed in Western Australia and imported into New Zealand by the Ministry of Agriculture and Fisheries in 1986. We used trained and consumer panellists, and chemical and physical analyses, to compare the sensory quality of ‘Pink Lady’ with that of four standard late‐harvest apple cultivars. If acceptable, the new apple could usefully extend the harvest season of New Zealand apples. Trained taste panellists gave ‘Pink Lady’ apples similar ratings to ‘Braeburn’ and ‘Fuji’ apples for texture, sweet‐sour balance and flavour, but lower ratings for juiciness. ‘Pink Lady’ was a firm apple as measured by penetrometer, and Instron texture measurements indicated it was a hard, crisp apple. ‘Pink Lady’, ‘Braeburn’, and ‘Fuji’ apples were rated higher for acceptability than ‘Granny Smith’ and ‘Red Dougherty’ by the consumer panel, who said they would buy these cultivars and pay more for them. Consum...

Postharvest inhibition of glutamine synthetase activity with phosphinothricin reduces the shelf-life of asparagus

Abstract Harvested spears of asparagus ( Asparagus officinalis L.) accumulate ammonia in their tips towards the end of shelf-life. However, the accumulation of ammonia is not due to limitations in the activity of the ammonia assimilating enzyme, glutamine synthetase (GS). When GS was inhibited by postharvest treatment with phosphinothricin (PPT), the ammonia content of the tips increased almost immediately and the shelf-life of the spears was severely reduced with symptoms of deterioration identical to those commonly ascribed to chilling injury. PPT treatment also inhibited the pattern of glutamine and asparagine accumulation seen during shelf-life. These observations emphasize that during the postharvest life of asparagus GS has a pivotal role in reassimilating considerable amounts (3% of dry weight) of ammonia in the tips.

Sucrose synthase, invertases, and sugars in growing asparagus spears

Abstract Asparagus (Asparagus officinalisL.) spear tips lose sucrose rapidly after harvest and this probably contributes to spears’ short postharvest life. As the first step towards controlling this loss of sucrose and possibly improving postharvest life, the activities of sucrose‐cleaving enzymes, and levels of sugars were measured in tips and 3 cm sections of spears from tip to butt. Sucrose was the major sugar in tips but not in the rest of the spear. Glucose and fructose were almost equimolar throughout the spear and increased fivefold from tip to butt. Fructose was always slightly more abundant than glucose. Sucrose synthase, bound acid invertase, and neutral invertase activities were similar in both tips and spear sections. Soluble acid invertase activity in the middle section of the spear was 4—5 times that of tips and butts. Sugar composition and enzyme activities in spear sections and tips were not closely correlated.

Influence of Cultivar, Cooking, and Storage on Cell-Wall Polysaccharide Composition of Winter Squash (Cucurbita maxima)

Changes in the cell-wall polysaccharides (CWP) of the edible tissues of four winter squash cultivars during storage and after cooking were investigated. A procedure for isolating cell walls of tissues containing high levels of starch was used. The starch-free CWP were sequentially fractionated using CDTA, dilute Na(2)CO(3), and 4 M KOH. Cellulose made up 40-42% of the total CWP for three cultivars (Delica, CF 2, and CF 4) at harvest but was 35% in the softer Red Warren. The pectic polysaccharides of Delica, CF 2, and CF 4 cell walls are more branched than those from Red Warren squash. The higher proportion of uronic acid in the pectic polysaccharides of Red Warren squash correlates with its lower firmness. Cooking resulted in an increase in the water-soluble pectins and a decrease in the pectins associated with cellulose. The total CWP content of the squash cultivars remained unchanged for up to 2 months of storage and then markedly decreased between 2 and 3 months of storage. The galactose content of Delica and Red Warren cell walls remained relatively constant from harvest to 2 months of storage and then decreased markedly during 2-3 months of storage.

Towards a freshness test for asparagus : spear tip asparagine content is strongly related to post-harvest accumulated heat-units

Abstract The shelf-life of asparagus (Asparagus officinalis L.) is strongly related to the post-harvest accumulated heat-units; the greater the number of accumulated heat-units (degree-h >0°C) experienced by the spears the lower the residual shelf-life. The potential of using asparagine and/or amino acid content as markers of freshness for asparagus spears was assessed by determining the relationship between the concentrations of these metabolites in the spear tips of two cultivars (‘Limbras 10’ and ‘Jersey Giant’) and the post-harvest age of the spears in terms of accumulated heat-units. There was a strong quadratic relationship between spear tip asparagine content and degree-h (R2 = 0.878) that was independent of cultivar (P = 0.16). Free amino acid content was also correlated quadratically with degree-h (R2 = 0.788 and 0.813 for ‘Limbras 10’ and ‘Jersey Giant’, respectively) but this was cultivar dependent (P = 0.002). Spear tip asparagine concentration has potential as a marker of freshness for asparagus but requires the development of an asparagine assay suitable for use in packhouses or by exporters.

Controlled atmospheres and sugar can delay malate synthase gene expression during asparagus senescence

A cDNA clone encoding malate synthase (MS; EC 4.1.3.2) was isolated from a 48-h postharvest asparagus (Asparagus officinalis L.) spear cDNA library using a MS clone from Brassica napus. The asparagus MS (AoMS1) cDNA hybridized to a 1.9-kb transcript that increased in abundance preferentially in spear-tip tissue during postharvest storage. The AoMS1 transcript also accumulated during natural foliar senescence of asparagus fern. The cDNA consists of 1960 nucleotides with an open reading frame of 1665 nucleotides or 555 amino acids, and encodes a deduced protein with a predicted Mr of 63 kDa and a pI of 8.1. The deduced amino acid sequence of AoMS1 showed high identity with the B. napus MS clone (77.2%) used to isolate it, and with MS from cucumber (77%). Genomic Southern analysis suggests that a single gene in asparagus encodes AoMS1. Controlled- atmosphere treatments aimed at reducing deterioration of harvested asparagus spears reduced the expression of AoMS1. The reduction was correlated with the reduced oxygen level, and reduced MS enzyme activity was also observed. Asparagus cell cultures were used to test the role of sugar status in regulating AoMS1 gene expression. In cultures without sucrose there was an accumulation of AoMS1 transcript that was absent in cultures containing sucrose.