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Accumulation of Ca in several fleshy fruit is often supposed to depend, among others, by climatic variables driving fruit transpiration. This study tests the whole causal chain hypothesis: VPD → fruit transpiration → Ca accumulation. Also there are evidences that relationship between fruit transpiration and Ca content is not always clear, hence the hypothesis that low VPD reduces the fraction of xylemic water destined to transpiration was tested by examining the water budget of fruit. Attached fruits of Actinidia deliciosa were subjected to Low (L) and High (H) VPD. Their transpiration was measured from early after fruit-set to day 157 after full bloom (DAFB). Fruits were picked at 70, 130 and 157 DAFB for Ca and K determinations and for water budget analysis. Cumulative transpired water was ∼ 70 g and ∼ 16 g H2O f(-1) in HVPD and LVPD, respectively. Calcium accumulated linearly (R(2) = 0.71) with cumulative transpiration when VPD was high, while correlation was weaker (R(2) = 0.24) under LVPD. Under low VPD the fraction of xylem stream destined to transpiration declined to 40-50%. Results suggest that Ca accumulation is coupled to cumulative transpiration under high VPD because under that condition cumulative transpiration equals xylem stream (which carry the nutrient). At LVPD, Ca gain by fruit is uncoupled from transpiration because ∼ 60% of the xylemic water is needed to sustain fruit growth. Results will apply to most fruits (apples, tomatoes, capsicum, grapes etc.) since most suffer Ca deficiency disorders and grow in changing environments with variable VPD, also they could be supportive for the implementation of fruit quality models accounting also for mineral compositions and for a reinterpretation of certain field practices aimed at naturally improve fruit Ca content., (Copyright © 2015 Elsevier GmbH. All rights reserved.)

Background and Aims: Xylem flows into most fruits decline as the fruit develop, with important effects on mineral and carbohydrate accumulation. It has been hypothesized that an increase in xylem hydraulic resistance (RT) contributes to this process. This study examined changes in RT that occur during development of the berry of kiwifruit (Actinidia deliciosa), identified the region within the fruit where changes were occurring, and tested whether a decrease in irradiance during fruit development caused an increase in RT, potentially contributing to decreased mineral accumulation in shaded fruit., Methods: RT was measured using pressure chamber and flow meter methods, the two methods were compared, and the flow meter was also used to partition RT between the pedicel, receptacle and proximal and distal portions of the berry. Dye was used as a tracer for xylem function. Artificial shading was used to test the effect of light on RT, dye entry and mineral accumulation., Key Results: RT decreased during the early phase of rapid fruit growth, but increased again as the fruit transitioned to a final period of slower growth. The most significant changes in resistance occurred in the receptacle, which initially contributed 20 % to RT, increasing to 90 % later in development. Dye also ceased moving beyond the receptacle from 70 d after anthesis. The two methods for measuring RT agreed in terms of the direction and timing of developmental changes in RT, but pressure chamber measurements were consistently higher than flow meter estimates of RT, prompting questions regarding which method is most appropriate for measuring fruit RT. Shading had no effect on berry growth but increased RT and decreased dye movement and calcium concentration., Conclusions: Increased RT in the receptacle zone coincides with slowing fresh weight growth, reduced transpiration and rapid starch accumulation by the fruit. Developmental changes in RT may be connected to changes in phloem functioning and the maintenance of water potential gradients between the stem and the fruit. The effect of shade on RT extends earlier reports that shading can affect fruit vascular differentiation, xylem flows and mineral accumulation independently of effects on transpiration.

The external heat ratio method is described for measurement of low rates of sap flow in both directions through stems and other plant organs, including fruit pedicels, with diameters up to 5 mm and flows less than 2 g h(-1). Calibration was empirical, with heat pulse velocity (v(h)) compared to gravimetric measurements of sap flow. In the four stem types tested (Actinidia sp. fruit pedicels, Schefflera arboricola petioles, Pittosporum crassifolium stems and Fagus sylvatica stems), v(h) was linearly correlated with sap velocity (v(s)) up to a v(s) of approximately 0.007 cm s(-1), equivalent to a flow of 1.8 g h(-1) through a 3-mm-diameter stem. Minimum detectable v(s) was approximately 0.0001 cm s(-1), equivalent to 0.025 g h(-1) through a 3-mm-diameter stem. Sensitivity increased with bark removal. Girdling had no effect on short-term measurements of in vivo sap flow, suggesting that phloem flows were too low to be separated from xylem flows. Fluctuating ambient temperatures increased variability in outdoor sap flow measurements. However, a consistent diurnal time-course of fruit pedicel sap flow was obtained, with flows towards 75-day-old kiwifruit lagging behind evaporative demand and peaking at 0.3 g h(-1) in the late afternoon.