Ross E. Lill

A method for measuring the juice content of mealy nectarines

Abstract Mealiness in nectarines is a symptom of chilling damage which develops during cool storage. Study of the disorder and methods for alleviating it have been hampered by the lack of an objective method for measuring the degree of mealiness. We describe a method of measuring the apparent juice content of ripe nectarines and demonstrate the inverse relationship between juice content and mealiness.

Regulation of Harvest-induced Senescence in Broccoli (Brassica oleracea var. italica) by Cytokinin, Ethylene, and Sucrose

Broccoli (Brassica oleracea var. italica) deteriorates rapidly following harvest. The two plant hormones ethylene and cytokinin are known to act antagonistically on harvest-induced senescence in broccoli: ethylene by accelerating the process, and cytokinin by delaying it. To determine the level at which these hormones influenced senescence, we isolated and monitored the expression of genes normally associated with senescence in broccoli florets treated with exogenous 6-benzyl aminopurine (6-BAP), 1-aminocyclopropane-1-carboxylic acid (ACC), a combination of 6-BAP and ACC, and sucrose, in the five days following harvest. Exogenous 6-BAP caused both a reduction (BoACO) and an increase (BoACS) in ethylene biosynthetic gene expression. The expression of genes used as senescence markers, BoCP5 and BoMT1, was reduced, whereas BoCAB1 levels were maintained after harvest in response to exogenous 6-BAP. In addition, the expression of genes encoding sucrose transporters (BoSUC1 and BoSUC2) and carbohydrate metabolizing enzymes (BoINV1 and BoHK1) was also reduced upon 6-BAP feeding. Interestingly, the addition of ACC prevented the 6-BAP-induced increase in expression of BoACS, but 6-BAP negated the ACC-induced increase in expression of BoACO. The culmination of these results indicates a significant role for cytokinin in the delay of senescence. The implication that cytokinin regulates postharvest senescence in broccoli by inhibiting ethylene perception and/or biosynthesis, thus regulating carbohydrate transport and metabolism, as well as senescence-associated gene expression, is discussed and a model presented.

Agrobacterium tumefaciens-mediated transformation to alter ethylene and cytokinin biosynthesis in broccoli

Broccoli (Brassica oleracea var. italica) deteriorates rapidly following harvest. Postharvest treatment of broccoli with 6-benzylaminopurine delays senescence, whilst exogenous ethylene has been shown to accelerate this process following harvest. To alter ethylene biosynthesis, broccoli was transformed, using Agrobacterium tumefaciens-mediated transformation, with an antisense ACC oxidase gene from broccoli driven by the asparagine synthetase promoter from asparagus. In addition, broccoli was transformed with the chimeric gene construct SAG12-IPT to alter cytokinin biosynthesis during harvest-induced senescence. Transformation was achieved using both hypocotyl and cotyledonary petiole explants. The presence of an antisense ACC oxidase gene enhanced transformation efficiency, but Ag+ incorporated into the medium did not. The transgenic nature of these plants was confirmed by PCR and Southern analyses.

Ultrastructural changes in the nectarine cell wall accompanying ripening and storage in a chilling-resistant and chilling-sensitive cultivar

Abstract Ripening of ‘Armking’ and ‘Fantasia’ nectarines at 20°C was characterised by rapid softening, extensive dissolution of the cell wall, and a high extractable juice content. After 4 weeks cold storage, cell walls of both cultivars had similar ultrastructure to that present at harvest. ‘Armking’ fruit still ripened normally after 4 weeks cold storage whereas ‘Fantasia’ fruit suffered abnormal textural changes associated with chilling injury, and had low extractable juice content. Ultrastructural observations support the hypothesis that changes in mechanisms of normal cell wall degradation are at least one component of the chiUing injury disorder.

Senescence-associated down-regulation of 1-aminocyclopropane-1-carboxylate (ACC) oxidase delays harvest-induced senescence in broccoli

To gain an in-depth understanding of the role of ethylene in post harvest senescence, we used broccoli (Brassica oleracea var. italica) as our model species. The senescence-associated asparagine synthetase (AS) promoter from asparagus was used to drive the expression of an antisense 1-aminocyclopropane-1-carboxylate oxidase (ACO) cDNA from broccoli, BoACO2, to reduce ethylene production following harvest. Physiological analyses revealed that transgenic broccoli lines harbouring the antisense BoACO2 gene construct (designated as AS-asACO) displayed delayed senescence in both detached leaves and detached heads as measured by hue angle. Harvested floret tissue from these plants also showed a delayed loss of chlorophyll, lower protease activity and higher total protein content, and changes in transcript levels of senescence marker genes when compared with wild type and transgenic lines transformed with an empty T-DNA. Genes that were down-regulated included those coding for cysteine protease (BoCP5), metallothionein-like protein (BoMT1), hexokinase (BoHK1), invertase (BoINV1) and sucrose transporters (BoSUC1 and BoSUC2). Northern analysis for BoACO1 and BoACO2, ACO assays and western analysis, revealed reduced ACO transcript, enzyme activity and protein accumulation, as well as reduced ethylene production in the transgenic AS-asACO lines when compared with controls, confirming that a key enzyme regulating ethylene biosynthesis was reduced in these plants. This, together with the changes observed in gene expression, confirm a significant role for ethylene in regulating the events leading to senescence in broccoli following harvest.

Distribution of selenoglucosinolates and their metabolites in Brassica treated with sodium selenate.

In Brassica species, hydrolysis of (methylthio)glucosinolates produces sulfur-containing aglycons which have demonstrated anticancer benefits. Selenized Brassicaceae contain (methylseleno)glucosinolates and their selenium-containing aglycons. As a prelude to biological testing, broccoli, cauliflower, and forage rape plants were treated with sodium selenate and their tap roots, stems, leaves, and florets analyzed for selenoglucosinolates and their Se aglycons. Two new selenoglucosinolates were identified: glucoselenoraphanin in broccoli florets and glucoselenonasturtiin in forage rape roots. A new aglycon, selenoberteroin nitrile, was identified in forage rape. The major selenoglucosinolates were glucoselenoerucin in broccoli, glucoselenoiberverin in cauliflower, and glucoselenoerucin and glucoselenoberteroin in forage rape roots. In broccoli florets, the concentrations of selenglucosinolates exceeded those of their sulfur analogues. Fertilization with selenium slightly reduced (methylthio)glucosinolates and aglycons in the roots, but increased them in the florets, the leaves, and sometimes the stems. These discoveries provide a new avenue for investigating how consumption of Brassica vegetables and their organoselenides may promote human health.

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.

Postharvest life of asparagus (Asparagus officinalis) under warm conditions can be extended by controlled atmosphere or water feeding

Abstract Green asparagus (Asparagus officinalis) is known to respond to controlled atmosphere storage (CA) at 20°C with increased shelf‐life, reduced respiratory rate, slower loss of soluble carbohydrates and protein, and slower accumulation of asparagine. This work investigated the likelihood of interactive effects of storage of spears in CA along with spear feeding solutions, including 2% sucrose. Standing the spears in a feeding solution extended shelf‐life in air but conferred little additional benefit in CA. Feeding the spears with 2% aqueous sucrose reduced asparagine accumulation and protein loss, but gave no visual benefit over water alone. Spears in the feeding solutions gained weight, particularly during the first 2 days after harvest, but weight gain was slower in CA than in air. Sensory assessment indicated that spears held in CA for 6 days had similar flavour and acceptability to spears held in air for 1 day. Spear quality was more strongly influenced by CA than by feeding solutions. Both approaches could assist in asparagus quality retention where a good refrigerated cool chain is not available, but these technologies are technically challenging to apply to air‐freighted asparagus.

Asparagus: Variation amongst plants in susceptibility to the postharvest disorder, tip breakdown

Abstract Tip breakdown is a serious postharvest disorder of asparagus (Asparagus offtcinalis) which can result in severe wastage and quality loss. We report significant variation amongst plants in their susceptibility to this disorder. This variation may be genetic in origin, but the results presented are equivocal on this. Plants with low levels of tip breakdown had higher levels of soluble carbohydrate in spear tips than plants susceptible to the disorder. Protein levels and postharvest accumulation of ammonia were not significantly different in plants with low levels of tip breakdown compared to plants with high levels. The possibilities for selecting resistant plants are discussed.

Mount Cook lily (Ranunculus lyallii)—a potential cut flower?

Abstract Five experiments were undertaken to assess the potential of Ranunculus lyallii Hook. f. (Mount Cook lily) as a cut flower. Three measures of vase life were calculated, and the most useful for this species was found to be the number of days until 30% of flowers plus buds on a stem had senesced. Eleven postharvest chemical treatments (including biocides, commercial preservatives, a wetting agent, and combinations of these) were applied to scapes as pulse or vase solutions, but none significantly extended vase life. However, cool storage (at 3°C) and scape maturity at harvest both significantly influenced vase life. The vase life of scapes cool stored for 6 days averaged 2.4 days longer than the vase life of unchilled controls. Scapes harvested when the terminal bud was starting to open lasted an average of 10.2 days, a gain of 4 days compared with mature scapes harvested with a fully open or senescent terminal bud. Bud opening on scapes occurred in two distinct flushes, 3.5 days apart, and no postharvest treatment significantly affected the length of this gap. Flower quality was higher in buds opening in the first flush. R. lyallii senesced atypi‐cally for the Ranunculaceae; petal wilt rather than abscission was the initial symptom of senescence. There was no evidence that ethylene accelerated senescence, and the ethylene antagonist, silver thio‐sulphate, also failed to delay senescence. R. lyallii has commercial potential as a cut flower.