Paul Holford

Ripening behaviour and responses to propylene in four cultivars of Japanese type plums

Abstract The aim of this study was to determine the physicochemical changes in highly coloured cultivars of plums that could be used as a guide to assessing optimum harvest maturity. The patterns of fruit growth and maturation were investigated in the cultivars: Gulfruby, Beauty, Shiro and Rubyred. Changes in the rates of respiration and ethylene production, skin colour, firmness, soluble solids concentration and titratable acidity were recorded at intervals from pit-hardening until the fruit were tree ripe. In order to evaluate the role of ethylene in the ripening process, propylene was applied to harvested fruit. Internal ethylene concentrations in the cv. Rubyred were also measured at intervals after pit-hardening either in harvested fruit or fruit attached to the tree. Studies of the changes in the physiological parameters associated with ripening showed that none were suitable for the assessment of harvest maturity in all cultivars of plums. However, this analysis revealed two distinct patterns of ripening behaviour in the cultivars studied. Gulfruby and Beauty showed a typical climacteric pattern of development, whilst Shiro and Rubyred exhibited a suppressed-climacteric phenotype. This phenotype appears to result from an inability of the fruit to produce sufficient quantities of ethylene to co-ordinate ripening. However, treatment with propylene showed that fruit displaying the suppressed-climacteric phenotype should still be placed in the climacteric class. This suppressed-climacteric character could be incorporated into plum breeding programs to produce new varieties with improved storage properties.

Culture conditions and detachment of the fruit influence the effect of ethylene on the climacteric respiration of melon

The respiratory rise associated with ripening has been considered a defining feature of climacteric fruit. However, recent work has suggested that the respiration rate remains constant in melon fruit ripening whilst attached to the plant. To clarify the relationship between attachment and the respiratory climacteric, ethylene production and O2 consumption were monitored during the ripening of melon fruits grown in France and Australia. All fruit allowed to ripen after being detached from the vine produced both respiratory and ethylene climacterics. However, although ethylene production increased in attached fruit, the concurrent rise in respiration was either eliminated or reduced in magnitude. A positive relationship was found between ethylene levels and respiration rate. However, for a given rate of ethylene production the stimulation of respiration was less in attached than detached fruit. The application of exogenous ethylene to melons antisensed for ACC oxidase stimulated O2 consumption only if they were detached from the vine. These data also demonstrate that attachment to the plant inhibits the effects of ethylene on respiration. In addition, it was noticed that transgenic plants were better able to tolerate infection by powdery mildew than wild-type plants. As ethylene production was greater in attached fruit, it is further suggested that feedback inhibition of ethylene synthesis is reduced while fruit remain attached to the vine. It is concluded that the presence or absence of a respiratory climacteric in fruit attached to the vine is affected by environmental factors influencing plant development. These effects may be related to changes in gas permeance, turgor, or the supply of ‘plant factors’ translocated through the phloem. The results support the hypothesis that the respiratory climacteric is not an essential part of ripening, but an artefact caused by stress or detachment.

Repellent effect of guava leaf volatiles on settlement of adults of citrus psylla, Diaphorina citri Kuwayama, on citrus

Abstract  The Asiatic citrus psyllid (Diaphorina citri Kuwayama [Hemiptera: Sternorrhyncha: Psyllidae]) is a vector of huanglongbing (citrus greening), a devastating disease of citrus caused by phloem‐limited bacteria. Growing guava (Psidium guajava) as an intercrop appears to be a successful means of reducing psyllid numbers within citrus orchards; however, the mechanism by which such a reduction is achieved is unknown. To determine the repellent effect of guava leaf and factors attributed to this activity, responses of adult psyllids to guava leaf and its odor were evaluated in cage tests and Y‐tube olfactometer test. The results showed that guava leaf possessed a repellent effect against the adult citrus psyllids. Fewer psyllids were found on citrus leaves in the presence of guava foliage than in its absence. Young and old guava leaf showed equal repellent activity. By covering the guava shoots with net cloth, it was revealed that the repellent effect of guava leaf against adult psyllids on citrus was attributed to the volatile compounds, rather than physical factors. The olfactometer response of adult psyllids to guava leaf odor was dosage‐dependent. Between guava odor and control, only 35.00%, 25.00% and 16.25% of the psyllids moved toward guava odor when presented with 5.0, 10.0 and 15.0 g of guava shoots, respectively. The olfactometer experiments also showed that both male and female psyllids responded similarly to the guava leaf odor.

The effects of short heat-treatments on the induction of chilling injury in avocado fruit (Persea americana Mill)

Abstract The ripening of avocado fruit and the development of chilling injury in relation to short heat-treatments has been studied in the cv. ‘Mass’. The minimal conditions needed to induce maximal production of heat shock proteins (HSP) in samples of mesocarp tissue were an exposure to 38 °C for 4 h. Short heat-treatments applied during the ripening process reduced the maximum rate of ethylene production during the climacteric period, but this was not correlated with lower levels of 1-aminocyclopropane-1-carboxylic acid (ACC). Heat-treatment also hastened the occurrence of the climacteric in fruit treated in the early pre-climacteric period. A similar regime applied to fruit immediately prior to this event delayed the onset of the climacteric. The use of short heat-treatments to overcome the effects of chilling injury was investigated by subjecting the fruit to 38 °C for 0, 6, 12, 24, 36 or 48 h prior to transferring them to 0 °C for 7, 14 or 21 days. Heating for 6–12 h provided a significant degree of protection from chilling injury and therefore may have potential for extending the period of cold storage.

Feeding behaviour of the Asiatic citrus psyllid, Diaphorina citri, on healthy and huanglongbing-infected citrus

Diaphorina citri Kuwayama (Hemiptera: Sternorrhyncha: Psyllidae) is a vector of huanglongbing, a disease of citrus that in Asia is caused by ‘Candidatus Liberibacter asiaticus’ (α‐Proteobacteria) (Las). Acquisition of Las by D. citri appears to be variable, and this variability may be due to the suitability of the host plants and their tissues for acquisition. Therefore, this study aimed to determine the effect of symptom severity of the disease on the feeding behaviour of D. citri. Use of an electrical penetration graph showed that the pathway phase of D. citri consisted of four waveforms, A, B, C, and D; waveforms A and B have not been reported for D. citri before. The remaining waveforms, E1, E2, and G, conform to those described before for D. citri. The duration of the non‐penetration period did not differ between healthy or infected plants. However, in moderately and severely symptomatic plants, the duration of the pathway phase increased, whereas the phloem phase was shorter. In all diseased plants, the times to first and sustained salivation in the phloem were longer than those in control plants, with the times being related to symptom severity. As symptom expression increased, the percentage of time spent by psyllids salivating during the phloem phase increased; however, the percentage of time spent in phloem activities reduced gradually from ca. 74% in the control plants to ca. 8% in the severely symptomatic plants. In contrast, the percentage of time spent on xylem activities increased, as did the proportion of psyllids feeding from xylem. The differences in the durations of the E waveforms on plants showing different levels of symptom expression may account for differences in acquisition found amongst studies; therefore, future work on the acquisition and transmission of Las needs to carefully document symptom expression.

Chitotriosidase and gene therapy for fungal infections

Abstract.Chitotriosidase secreted by activated human macrophages has been implicated in the defence against chitin-bearing pathogens. The antifungal properties of human chitotriosidase were investigated here following retroviral vector-mediated gene transfer of the open reading frame of the chitotriosidase gene into Chinese hamster ovary cells. A chitinase assay confirmed that the engineered cells secreted recombinant chitotriosidase constitutively. Two dimensional gel electrophoresis and western blotting indicated that the recombinant protein is the major, chitin-binding, fifty kilodalton isoform. Culture medium conditioned by the transduced cells inhibited growth of isolates of Aspergillus niger, Candida albicans and Cryptococcus neoformans. Furthermore, longevity was significantly increased in a mouse model of cryptococcosis when cells transduced with the chitotriosidase gene and encapsulated in alginate microspheres were implanted subcutaneously in the animals. Engraftment of microcapsules containing cells transduced with the chitotriosidase gene has the potential to combat infections caused by chitinous pathogens through the prolonged delivery of recombinant chitotriosidase.

Essential oil vapours control some common postharvest fungal pathogens

Diseases caused by fungal pathogens cause substantial postharvest losses to most perishable food crops. Fungal diseases are currently controlled with fungicides; however, reliance on this single control strategy leads to problems such as environmental damage and fungal resistance to fungicides. There is increasing pressure from consumers to find more natural methods of disease control. A possible alternative to fungicides may be essential oils, which have been shown to inhibit the growth of several fungi and are seen as natural compounds. The present study examined the effect of the vapour phase of spearmint, tea tree, pine and cinnamon oils and an antifungal blend on the growth of eight common postharvest fungal pathogens growing in vitro. It was found that the antifungal, spearmint and tea tree oils controlled the growth of Botrytis cinerea, Fusarium solani, Colletotrichum sp., Geotrichum candidum, Rhizopus oryzae, Aspergillus niger and Cladosporium cladosporiodes more effectively than pine or cinnamon oil but were less effective against Penicillium digitatum. Antifungal, spearmint and tea tree oils appeared to reduce sporulation in P. digitatum, A. niger and R. oryzae and inhibited spore germination by A. niger. This work shows that the antifungal, spearmint and tea tree oil vapours may provide an alternative means of controlling postharvest pathogens. All of the oils had a fungistatic mode of action and their use would require the development of commercial treatment methods applicable throughout the postharvest handling chain.

Application of two-dimensional gel electrophoresis to detect proteins associated with harvest maturity in stonefruit

Abstract The correct assessment of harvest maturity stage of stonefruit is an important determinant of their quality when they reach the consumer. However, the current indices of ripening used (e.g. skin colour, firmness and soluble solids concentration) vary with cultivar, rootstock, growing conditions and seasonal climatic factors. To develop an index that is independent of environmental factors, total proteins were extracted from fruit and separated by 2D-PAGE. Four proteins (Z1, Z2, Y and X) that were synthesised in fruit a few days before optimum harvest maturity were identified in Japanese-type plums ( Prunus salicina ), peaches and nectarines ( Prunus persica ), and European plums ( Prunus domestica ). These proteins are first expressed a few days before optimum harvest date and their expression increases as the fruit mature. The N-termini of the three most prominent proteins from plums (Z1, Z2 and Y) were sequenced. There appears to be close homology between these proteins and those from a range of species including almond, grape and rubber. It appears that these proteins belong to a family of allergens common to plants whose function may be to confer some form of protection to the plant during periods of stress. These proteins may be used for the development of a test kit that will assist growers to determine optimum harvest maturity for cultivars of plums, peaches and nectarines.

In vitro uptake of minerals by Gypsophila paniculata and hybrid eucalypts, and relevance to media mineral formulation

Despite the importance of mineral nutrition for plantlet growth in vitro, there have been few studies on mineral uptake from growth media or on optimising the media used in tissue culture. As plants in vitro experience abnormal growth conditions and may not possess roots, they may use different mechanisms of mineral uptake than plants growing ex vitro. To examine this possibility, plantlets of Gypsophila paniculata were grown on media in which the K or Ca concentration was varied. Mineral analysis showed a linear relationship between concentrations of K or Ca in the growth medium and plantlet tissues, suggesting uptake is by passive diffusion. However, interactions occurred between K, Ca and Mg uptake; therefore, other mechanisms are also likely to be involved in regulating mineral concentrations in tissue. The study also demonstrated that critical mineral concentrations could be estimated by using tissue-culture systems, as the concentration ranges of K and Ca in vitro correlated well with data for a related species ex vitro. This knowledge of critical concentrations, in conjunction with tissue analysis and ion speciation modelling, can be used to optimise in vitro mineral formulations through cycles of culture, tissue analysis and medium reformulation. To test this proposal, plantlets of Eucalyptus europhylla × grandis were grown on a proprietary medium formulation (SEM) and one modified as a result of tissue analysis (MEM). Plantlets cultured on SEM had chlorotic leaves and serious mineral imbalances. In contrast, plantlets cultured on MEM were not chlorotic, had more uniform growth and a more balanced mineral content. However, modification of mineral concentrations in the culture medium did not always result in similar changes in plant tissues. These differences in the proportions of minerals in the medium and those in the plant indicate that there are interactions between minerals in the medium and/or between minerals and the agar matrix that influence mineral availability and uptake.

A new technique for monitoring Trigona carbonaria nest contents, brood and activity using X-ray computerized tomography

SUMMARY Brood status in the Australian stingless bee Trigona carbonaria is difficult to monitor non-invasively as splitting the hive damages the nest, and the involucrum prevents observations of internal structures and evidence of queen activity. In addition, increases in hive weight, also used as a measure of colony health, can be attributed to increased pollen and nectar collection by foragers in times of colony stress rather than improved brood status. To assess brood status and queen activity, we performed helical two-dimensional and three-dimensional X-ray computerized tomography (CT) to measure brood chamber volume in a T. carbonaria hive. All previously reported nest structures including larval cells, pupal cocoons, involucrum, cerumen and batumen layers were clearly identifiable. In addition, the on-screen linear callipers enabled accurate estimates of brood chamber volume to be made. A batumen bridge was found that may secure the brood chamber to the base of the hive box; this structure has not been reported before. CT could be used to follow the life cycle of stingless bees, track the development of natural nests and observe nest morphology to distinguish between species of Trigona. In addition, sequential scans will be useful in non-invasively assessing changes in brood status.