A biophysical model of kiwifruit (Actinidia deliciosa) berry development.

A model of kiwifruit berry development is presented, building on the model of Fishman and Génard used for peach fruit. That model has been extended to incorporate a number of important features of kiwifruit growth. First, the kiwifruit berry is attached to the stem through a pedicel/receptacle complex which contributes significantly to the hydraulic resistance between the stem and the fruit, and this resistance changes considerably during the season. Second, much of the carbohydrate in kiwifruit berries is stored as starch until the fruit matures late in the season, when the starch hydrolyses to soluble sugars. This starch storage has a major effect on the osmotic potential of the fruit, so an existing model of kiwifruit starch dynamics was included in the model. Using previously published approaches, we also included elasticity and extended the modelling period to cover both the cell division and cell expansion phases of growth. The resulting model showed close simulation of field observations of fresh weight, dry matter, starch, and soluble solids in kiwifruit. Comparison with continuous measurements of fruit diameter confirmed that elasticity was needed to adequately simulate observed diurnal variation in fruit size. Sensitivity analyses suggested that the model is particularly sensitive to variation in inputs relating to water (stem water potential and the humidity of the air), and to parameters controlling cell expansion (cell wall extensibility). Some limitations in the model structure were identified, suggesting that a revised model including current apoplastic/symplastic concepts needs to be developed.

Temperature influences growth and maturation of fruit on 'Hayward' kiwifruit vines.

The responses of fruit and shoot growth of 'Hayward' kiwifruit vines to changes of temperature were determined during spring, summer or autumn. Mature vines were warmed 2-5°C above ambient temperatures by enclosing them in temperature-controlled tunnel houses for 34-89 d. Increasing temperature during spring advanced the date of flowering by 17 d and increased the rate of shoot elongation by 6 mm d-1 °C-1. The fruit on these early-flowering vines were larger and had a higher dry matter concentration than control fruit during the first part of the season. Increasing temperature during summer increased the rate of shoot elongation but reduced fruit growth, accumulation of dry matter in fruit and fruit firmness. In contrast, increasing temperature during late autumn increased fruit growth but reduced the soluble solids concentration (SSC) of fruit and thus, delayed commercial maturity. When fruit growth data for summer and autumn were combined the variation in fruit growth with temperature could be described by a single quadratic curve. Maximum fruit growth occurred at 17°C and temperatures above or below this optimum reduced fruit growth. Consequently, during summer when ambient temperatures averaged 17°C, warming vines decreased fruit growth, while during late autumn, when ambient temperatures had fallen to 13°C, warming vines increased fruit growth. Warming vines during summer reduced both the SSC of ripe fruit and the vitamin C concentration. Warming vines during autumn increased SSC but reduced the vitamin C concentration.

Air volume measurement of 'Braeburn' apple fruit.

The radial disposition of air in the flesh of fruit of Malus domestica Borkh., cv 'Braeburn' was investigated using a gravimetric technique based on Archimedes' principle. Intercellular air volume was measured by weighing a small tissue sample under water before and after vacuum infiltration to remove the air. In a separate procedure, the volume of the same sample was measured by recording the buoyant upthrust experienced by it when fully immersed in water. The method underestimates tissue air volume due to a slight invasion of the intercellular air spaces around the edges of the sample when it is immersed in water. To correct for this error, an adjustment factor was made based upon an analysis of a series of measurements of air volume in samples of different dimensions. In 'Braeburn' there is a gradient of declining air content from just beneath the skin to the centre of the fruit with a sharp discontinuity at the core line. Cell shape and cell packing were observed in the surface layers of freshly excised and stained flesh samples using a dissecting microscope coupled to a video camera and a PC running proprietary software. Tissue organization changed with distance below the skin. It is speculated that reduced internal gas movement, due to the tightly packed tissue of 'Braeburn' and to the potential diffusion barrier at the core line between the cortex and the pith, may increase susceptibility of the flesh to disorders associated with tissue browning and breakdown.

Vessel differentiation in the pedicel of apple and the effects of auxin transport inhibition.

The growth dynamics of vessel formation and the effect of auxin transport inhibition on vessel differentiation were investigated in the pedicel of developing apple (Malus domestica Borkh.). Quantitative microscopic analysis showed that a majority of vessels were differentiated pre-bloom with the commencement of pedicel extension but that the full conducting capacity of the xylem was attained shortly after bloom. The effect of a post-bloom application of N-(1-Napthyl)phthalamic acid showed that an auxin-like signal emanating from the young fruit not only stimulated vessel differentiation in the pedicel but also controlled fruit abscission and the development of the seed and fruit.

Causes and effects of changes in xylem functionality in apple fruit.

BACKGROUND AND AIMS: The xylem in fruit of a number of species becomes dysfunctional as the fruit develops, resulting in a reduction of xylem inflow to the fruit. Such a reduction may have consequential effects on the mineral balance of the fruit. The aim of this study was to elucidate the dynamics and nature of xylem failure in developing apples (Malus domestica) showing differing susceptibilities to bitter pit, a calcium-related disorder. METHODS: Developmental changes in xylem functionality of the fruit were investigated in "Braeburn" and "Granny Smith" apples by using a dye infusion technique, to stain the vasculature along the path of dye movement. The vascular bundles were clearly visible in transverse section when fruit were sectioned equatorially. The intensity of staining of the vascular bundles in the fruit was recorded at regular intervals throughout the season. Tissue containing dysfunctional bundles was fixed and embedded in wax for subsequent sectioning and examination. KEY RESULTS: As the season progressed, an increasing proportion of vascular bundles failed to show any staining, with the most marked change occurring in the primary bundles, and in nearly all bundles with increasing distance from the stalk end of the fruit. Decreased conductance in the primary bundles of "Braeburn" occurred earlier than in "Granny Smith". Microscopy revealed that the xylem in vascular bundles of the fruit suffered substantial damage, indicating that the mode of dysfunction was via the physical disruption of the xylem caused by expansion of the flesh. CONCLUSIONS: Results support the view that the relative calcium deficiency of apple fruit is due to a progressive breakdown of xylem conductance caused by growth-induced damage to the xylem strand in the bundle. The earlier onset of xylem dysfunction in the cultivar more susceptible to bitter pit suggests that the relative growth dynamics of the fruit may control the occurrence of calcium-related disorders.

Frost hardening of Pinus radiata seedlings: effects of temperature on relative growth rate, carbon balance and carbohydrate concentration.

Pinus radiata (D. Don) seedlings were grown for 100 days at day/night temperatures of 10/1, 15/1, 20/1 and 25/1 degrees C, to determine whether temperatures above a threshold of 5 degrees C influence frost hardiness development. Relationships between hardening and relative growth rate, carbohydrate concentration and net carbon balance were also investigated. Seedlings hardened at a nearly constant rate in each treatment, although the rate of hardening was strongly temperature dependent. It increased as the temperature declined, but in a curvilinear fashion. Temperatures below 9.5 degrees C were effective in hardening the seedlings. During the daily temperature cycle, dehardening occurred at temperatures above the threshold, whereas hardening occurred at temperatures below the threshold. The net difference between the two processes determined the development of frost hardiness. The development of frost hardiness was negatively correlated with relative growth rate and positively correlated with the accumulation of starch and sugars. We conclude that frost hardening is a complex process that is causally linked to carbohydrate concentrations.