Sheryl D. Somerfield

Cytokinin treatment delays senescence but not sucrose loss in harvested broccoli

Abstract We describe details of the physiological changes that both precede and accompany visual deterioration in harvested broccoli floral tissues ( Brassica oleracea L.), and show that some of these changes are altered after treatment with the cytokinin, 6-benzylaminopurine. Cytokinin treatment did not prevent the rapid loss of sucrose that occurs in florets after harvest. During the first 6 h after harvest, sucrose concentration declined by approximately 50% in both controls and treated tissues. However, after cytokinin treatment the large increases in asparagine and glutamine concentration were delayed by more than 48 h. Cytokinin treatment also altered the physiological changes which usually accompany the yellowing of broccoli florets. The decline in amino acids and soluble proteins, and the late increase in ammonia concentration were all delayed after cytokinin treatment. We discuss these results in relation to the effects of cytokinin treatment on the possible mechanisms regulating postharvest senescence.

Physiological changes associated with Sandersonia aurantiaca flower senescence in response to sugar

Abstract Sandersonia aurantiaca is a liliaceous cut flower in which senescence is not regulated by ethylene. We stood flower stems in solutions of deionized water (control) or sucrose (2%) and monitored the pattern of senescence of individual flowers attached to the stems and quantified the amount of carbohydrate and protein present in the flowers. Treatment with sucrose extended the postharvest life of flower stems by delaying the senescence of individual flowers attached to the flowering stem. Flowers on sucrose-treated stems were larger, firmer and brighter orange than the control flowers. Flowers that were treated with sucrose also contained greater quantities of carotenoids, soluble and storage carbohydrates and soluble protein than the control flowers. The first visible signs of senescence occurred prior to any net loss of soluble carbohydrate or protein.

Vase solutions containing sucrose result in changes to cell walls of sandersonia (Sandersonia aurantiaca) flowers

It is well established that vase solutions containing sugar can improve the vase-life of many cut flower crops. Since cut sandersonia flowers supplied with 2% sucrose are firmer during wilting compared to water-fed controls, we have examined whether the effects of sucrose treatment extend to alterations in cell wall structure in the floral tissues, which may influence the wilting-related flower softening. Mature but not fully opened individual flowers were removed from the stems of sandersonia plants and were fed continuously with either 2% sucrose solution or water for up to 10 days. Sucrose supplementation resulted in decreased amounts of chelator-soluble pectin and increased amounts of Na2CO3-soluble pectin per individual flower, and also changed the molecular size profiles of both these pectin fractions compared to the water-fed controls. The molecular size differences were obvious after 3 days in vase solutions, and diminished with subsequent vase time. Senescence-related galactose loss was delayed in sucrose-fed flowers but there was no difference in the levels of β-galactosidase activity present in these flowers compared to controls. The observed differences in cell wall pectins due to sucrose feeding were not reflected in differences to the overall firmness of pre-senescent flowers (up to day 3). High levels of galactose persisted into the wilting phase when sucrose-fed flowers were firmer than water-fed controls. We conclude that while sucrose induced significant quantitative and qualitative differences in pectin fractions and galactose content, firmness of floral tissue, particularly during senescence, was not governed by these events alone.

Developmental and Ripening-Related Effects on the Cell Wall of Pepino (Solanum muricatum) Fruit

Several cell wall components in ripening pepino fruit have been quantitatively and qualitatively characterised, with the aim of identifying their contributions to the loss of tissue firmness. Pepinos were graded into nine groups based on progressive, characteristic skin colour changes, previously shown to correspond with decreasing fruit firmness. While fruit softening began when the pepinos were still green but with newly acquired purple stripes, the first significant quantitative signs of cell wall modification (total pectin and hemicellulose content declining and CDTA-soluble pectin content increasing, on a fresh weight basis) were detectable later in ripening, when the fruit began to acquire yellow skin pigmentation. Gel fractionation studies demonstrated that there were increased levels of low-molecular-weight pectin and xyloglucan during pepino ripening. The change in molecular weight distribution of CDTA-soluble pectin occurred as fruit started to acquire yellow pigmentation, while xyloglucan polymers were modified at an earlier stage that coincided with the initial loss of firmness.

Asparagine synthetase gene expression increases as sucrose declines in broccoli after harvest

Abstract Broccoli (Brassica oleracea L.) floral tissues rapidly differentiate and grow before harvest and then senesce rapidly after harvest. The factors regulating this rapid postharvest senescence are currently under investigation. We show that within 6 h of harvest sucrose concentration in florets declines by c. 50%, and between 24 and 72 h asparagine levels increase more than 7‐fold. This increase in asparagine parallels an earlier increase in asparagine synthetase (AS) gene expression in florets. Northern analyses show that AS transcript abundance increases from 2 to 24 h after harvest, and then declines. AS transcript abundance also increases in harvested leaves as they turn yellow, although to a level lower than that seen in florets. In other plant systems, including asparagus, increases in AS gene expression occur as a result of a decline in sucrose status. We note the considerable similarities between broccoli and asparagus postharvest physiology and discuss our results for broccoli AS in terms o...

Characterization of the harvest-induced expression of β-galactosidase in Asparagus officinalis

Abstract The harvest-induced senescence of asparagus spears is accompanied by both up-regulated and down-regulated gene expression. The expression of pTIP31, coding for asparagus β -galactosidase (EC 3.2.1.23) is temporally associated with removal of the asparagus spear from the main body of the plant — neither wounding or compression treatments induce up-regulation of transcripts corresponding to pTIP31. Harvest-induced pTIP31 transcripts appear initially in cells below the meristems of the side branches, and in inner bracts and developing flowers of the spear tip, and in the cells of the outer cortex and conjunctive tissue lower down the spear. Transcripts are also located in vascular tissues 24 h after harvest. Enhanced levels of β -galactosidase activity are found in the branches and bracts of the spear tip within 12 h of harvest, while the rate of increase in activity in the middle zone of the spear was comparable to that of the branches and bracts after 24 h. The proportion of galactose in spear cell walls decreases after harvest. We propose that galactosidase activity releases galactose residues that may be used as respiratory substrate for the rapidly deteriorating asparagus spears.

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.

Galactosidases in opening, senescing and water-stressed Sandersonia aurantiaca flowers

Three glycosyl hydrolase family 35 β-galactosidase-encoding cDNAs, SaGAL1 (full-length), SaGAL2 and SaGA3L (both partial), have been isolated from Sandersonia aurantiaca (Hook.) SaGAL1 protein was functionally expressed in E. coli and β-galactosidase identity confirmed by activity assay. All three clones are primarily expressed in tepal tissues of senescing sandersonia flowers. In order to identify relationships between tepal texture and galactose metabolism, cut sandersonia flowers were treated with sucrose, periods of dryness or PEG and parameters associated with galactose metabolism and firmness were monitored. Sucrose supplementation, known to increase tepal firmness, delayed expression of SaGAL1 and SaGAL3 in opening (stage 5) flowers, whereas the response to periods of dryness followed by rehydration depended on the maturity of the flower. These treatments also tended to hasten the onset of processes associated with programmed cell death, monitored by PRT5 (a senescence-associated protease) expression. Galactosidase activity and cell wall galactose content were also affected but in an inconsistent manner. PEG supplied to opening flowers for 1 d followed by water, induced a long period of wilt, and intensive PRT5 expression. However, β-galactosidase gene expression and activity was delayed in these flowers, and cell-wall galactose content changed apparently independently of galactosidase activity. We have not been able to demonstrate a causal connection between the change in petal texture and concurrent induction of galactose mobilisation in sandersonia during normal development and senescence.

Xyloglucan endotransglycosylase: a role after growth cessation in harvested asparagus

Little is known about the mode of xyloglucan endotransglycosylase (XET) activity in cell walls once the turgor, which drives expansion, is reduced. Such a situation exists when growing shoots are excised from the parent plant, and is the case for many commercially valuable vegetable crops, e.g. asparagus, Asparagus officinalis L. XET activity was present in all zones of rapidly growing, immature asparagus spears, but with highest levels at the spear base where elongation growth had ceased. Activity increased in all parts of the spear for up to 72 h after harvest. Two members of the XET-related gene family in asparagus (AoXET1 and AoXET2) were isolated and mRNA corresponding to these clones accumulated at low levels, particularly in the basal zone during spear growth. Transcript levels increased in all parts of the asparagus spear after harvest, but this increase did not coincide with the increase in XET activity. The harvest-related changes to xyloglucan molecular weight were restricted to slight, segment-specific, up- or down-shifts. However, this may hide strategic alterations to linkages leading to a more rigid wall without major changes in overall molecular weight. The initial postharvest surge in XET activity could be related to harvest stresses such as water deficit, but we propose that the later induction of AoXET1 and AoXET2 is linked to the development of lignified secondary cell walls.