Examining key constraints to cucumber quality in subtropical protected cropping systems

The shape of the fruit is a major quality determinant of cucumbers. Consumers desire straight, evenly cylindrical fruit and uneven, misshapen pieces are considered second grade or waste within the industry. Large scale cucumber producers can lose a commercially significant proportion of crops due to misshapen fruit at harvest and have thus identified addressing formation of misshapen fruits as being a high priority issue in their operations. Strategies to manage crops to produce a high proportion of fruit with the desired shape profile are therefore of great economic importance to the industry, however the mechanisms driving the formation of misshapen fruit are not well understood and therefore the capacity to manage the issue is limited. This thesis was initiated with support from an industry partner, a major greenhouse cucumber producer in Australia, and sought to develop crop management strategies to decrease crop losses due to misshapen fruit. The research approach was to identify environmental and production practice factors that influence cucumber fruit shape, examine anatomical and physiological changes that occur during fruit bending, and combine this knowledge to generate crop management recommendations for industry. Four research questions were developed in consultation with local growers to address this issue: 1- Does location within the greenhouse affect the distribution of misshapen cucumber fruit? 2- What impact is the formation of misshapen fruit having to the grower’s marketable and total yield? 3- What physiological differences are responsible for the development of misshapen fruit? 4- What management practices might be implemented to address this issue in low-medium tech greenhouses? Environmental conditions within the greenhouse did not significantly affect the formation of misshapen cucumber fruit despite this hypothesis being a common belief within the industry. While no relationships were found, the temperature and light level ranges within the greenhouse production system were not extreme and the possibility of significant environmental effects over a bigger range of conditions could not be ruled out. There was however evidence that production zones within the greenhouse that generated higher fruit load also produced greater numbers of misshapen fruit, but only trends between fruit curvature and both increased crop load and decreased assimilate supply was discovered. A critical observation made during the environmental and fruit load experiments was that degree of fruit curvature can change during fruit development, with curved fruit able to straighten and revert to first grade fruit standard by harvest maturity. Anatomical investigation of curved fruit revealed a differential in average mesocarp cell size as well as shape between the concave and convex sides of the curved fruit. Smaller and more rectangular shaped cells were found in the mesocarp tissue on the concave compared to convex side of the fruit. No size or shape differentials between different sides of the fruit were recorded in straight fruit. No significant differences in mesocarp osmotic potential between the two sides of the fruit curvature were present in misshapen fruit. When sections of epicarp and mesocarp tissue from the concave and convex sides of misshapen fruit were soaked in distilled water, significantly higher epicarp expansion was recorded in the concave compared to convex region section while no significant differences were recorded in mesocarp expansion. The concave side of the curved fruit also required a significantly higher force to penetrate the fruit compared to the convex side. The degree of curvature was recorded to change rapidly during fruit development, with curved fruit able to fully straighten over a 3–4-day period. Based on the speed in which rate of expansion on the different sides of the fruit must be occurring to cause this rapid change in fruit shape, it was proposed that small differences in assimilate partitioning to different sides of the fruit may occur within a diurnal growth cycle. Differences in assimilate distribution over the night period where rapid fruit growth occurs would explain the observed differences in average cell size recorded at midday on the following day. By this time, transpirational water losses from the plant may lead to an equilibrium water potential, including the osmotic potential as was observed in the mesocarp tissue. Compressive stress on the epicarp tissue on the concave side of the curved fruit caused by the more rapid expansion of the convex compared to concave side would explain the differential in expansion capacity of the epicarp tissue on the concave compared to convex side of the fruit. This compressive stress would also explain the higher force required to penetrate the flesh. The development of curvature, and also straightening of curved fruit, in cucumber over relatively short periods of time means that strategies for management of fruit shape must consider both short term factors that may cause bending and straightening of fruit during development, and longer-term factors that may maintain the curved shape through to commercial harvest maturity. Fruit load was the only factor assessed in this project that may have contributed to sustained fruit curvature during development and given the effect of fruit load on individual fruit sink strength this factor would be consistent with small changes in assimilate distribution contributing to misshapen fruit development. The capacity of small misshapen fruit, where curvature was concluded to be induced by short term variation in assimilate distribution within the fruit, to go through a “recovery” phase in which they straighten if left to mature on the plant is of significant commercial value. Current industry practice is to remove small misshapen fruit as soon as they appear. However, an increase in yield with no significant effect on quality was obtained in trials completed in this study where these fruit were not removed. Under production conditions where only the short-term variations in growth within fruit are causing curvature, the majority of the curved fruit will straighten and be of first grade standard at harvest. A cost benefit analysis demonstrated a significantly greater economic return if growers were to adopt the management practice of not removing the small misshapen fruit in comparison to conventional practice where this fruit is removed.