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Costanza Zavalloni, Adriana Nikoloudi and James A. Flore*

This study was conducted to determine whether standard and dwarfing sweet cherry rootstocks under water deficit conditions respond differently relative to plant growth and gas exchange parameters, water-use efficiency, and leaf carbon isotope composition. One-year-old potted sweet cherry cv. `Rainier' grafted on the standard rootstock `Mazzard' and on the dwarfing rootstock `Gisela 5' were compared under two different water treatments: 1) well-watered, which received daily 100% of the amount of water lost by ET, and 2) a water deficit treatment, which received 50% of the water applied to the control. Relative shoot growth rate, leaf emergence rate and cumulative leaf area were recorded every three to seven days during the experiment. Leaf net carbon dioxide assimilation rate, stomatal conductance, transpiration rate, internal CO2 concentration, and WUE were measured daily for the duration of the experiment. At the end of the experiment, leaf samples were collected to determine leaf carbon isotope composition. The growth parameters measured were affected similarly in the two rootstocks indicating a similar degree of sensitivity to water deficit in the genotypes tested. Cumulative leaf area was affected earlier by water deficit than relative shoot growth, and leaf emergence rate. Gas exchange parameters were affected earlier than growth parameters. Overall, WUE was not significantly different between dwarfing and standard rootstocks, and did not appear to increase under water deficit condition, indicating that irrigation should be considered as an important practice in sweet cherry orchards, especially when dwarfing rootstocks are selected.

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Moreno Toselli, Costanza Zavalloni, Bruno Marangoni and James A. Flore

15Nitrogen-ammonium nitrate was applied to four `Mutsu' apple (Malus ×domestica Borkh.) trees 40 days before harvest of 1996 (summer supplied nitrogen, SUN) and four others at full bloom in 1997 (spring supplied nitrogen, SPN) to evaluate the effect of application timing on N partitioning in mature trees. At leaf fall the largest amount of SUN was partitioned to roots and 2- to 4-year-old wood; the largest amount of SPN was partitioned to fruit and leaves and only a small amount detected in the roots. SUN did not increase N concentration in fruit or modify fruit firmness and soluble solids concentration, although it contributed to building up N reserves in the perennial woody organs. In 1997, as a result of the different timings of N supply, two sources of labeled N were distinguished and monitored in the vegetative organs: 1) the remobilized N, taken up in summer of 1996, stored in winter and then translocated to the growing tissues; 2) the newly absorbed N, taken up and moved to the canopy after the 1997 spring supply. Both fractions of remobilized and newly uptaken labeled N contributed to leaf and fruit N. Remobilized 15N was provided principally by roots which, from August to leaf fall, decreased their percentage of 15N by ≈18%, replacing the labeled with unlabeled N to maintain a constant concentration of total N.

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Costanza Zavalloni, Jeffrey A. Andresen and J.A. Flore

A simulation model for determining flower bud phenological stages and fruit growth as a function of daily maximum and minimum temperatures was developed for `Montmorency' sour cherry (Prunus cerasus L.). The models were developed and tested with observations collected in the three major sour cherry production areas in Michigan located in northwestern, western central, and southwestern sections of the lower peninsula. Observations of flower bud phenology and fruit diameter were collected at 3- to 7-day intervals, in spurs and terminal shoots across multiple years. Nonlinear equations using accumulation of growing degree-days (base 4 °C) as an independent variable were fitted to observed flower bud phenological stages and fruit diameter, expressed as percentage of final fruit diameter. Simulated bud phenology stages were in agreement with observed data. Mean differences of simulated vs. observed dates of early phenological stages in the three production areas were between 4 and 1 days for side green and near 0 days for tight cluster, while during later stages (e.g., first bloom and full bloom) mean differences ranged from -2 to 0 days. Means differences of predicted fruit diameter were in the range of 0 to -3 days. Needing only daily temperature data, these simulation models have potential applicability in improving the timing and efficiency of management decisions related to crop phenology, such as pest control, fertilization, and irrigation.