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Massimo Tagliavini and Bruno Marangoni

Most deciduous fruit crops in Italy are grown in the north and especially in the eastern part of the Po River Valley (mainly in the Emilia Romagna and Veneto regions) and in the Adige River Valley (South Tyrol and Trento provinces). Soils in the wide Po River Valley, where pear (Pyrus communis), peach and nectarine (Prunus persica), kiwifruit (Actinidia deliciosa), plum (Prunus domestica and P. insititia), apricot (Prunus armeniaca), cherry (Prunus avium), and apple (Malus domestica) are grown, are alluvial, generally fertile, fine textured, alkaline, often calcareous and well enriched with Ca. Apple plantings are concentrated in the Adige Valley and located on a variety of soil types, including sandy loam, loamy sand soils or sandy clay, sometimes calcareous. Integrated fruit production is gaining importance and represents more than 80% of apple production in South Tyrol and about 60% of peach and nectarine production in Emilia Romagna. Under these conditions, the main objectives of mineral nutrition are to reconcile production and environmental concerns (minimize nutrient leaching, soil pollution, volatile emissions). In particular, fertilization aims to improve external and internal fruit quality and storage ability, reduce production costs, maintain soil fertility, avoid nutrient deficiency and excess and control tree vigor. Nitrogen applications have strongly decreased in recent years and there is a need to improve the efficiency of N fertilizers while avoiding deficiencies. Research is focussing on application technology, timing of N uptake, internal cycling of N and methods for assessing the need for N application (e.g., using estimates of native soil N availability). Early diagnosis of bitter pit is recommended for guiding applications of Ca sprays. Iron deficiency and chlorosis is a major problem in pear, peach and kiwifruit grown in alkaline and calcareous soils and Fe chelates are usually applied annually to the soil or to the canopy. Current research is focused on agronomic means for controlling the problem and on developing rootstocks tolerant to Fe deficiency.

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Moreno Toselli, James A. Flore and Bruno Marangoni

Low root-zone temperature is one of the potential causes of low rate of plant nutrient uptake in spring. In this period, fruit trees are frequently supplied with nitrogen and a delay in root absorption could lead to an increase of nitrate leaching. In this study we assessed the effect of low root temperature on kinetic of nitrogen absorption of apple trees. One-year-old rooted cuttings of `Mark' apple rootstocks were subjected to two root temperature: 8 ± 1°C (LT) and 23 ± 1°C (HT). Four days after treatment imposition, the potted plants were supplied with 20 mg of N as NH4N03, enriched with 10 atom% of 15N. One, 2, 4, and 8 days after fertilization, tree root system was inserted into a Sholander bomb where a 0.325-Mpa pressure was applied to collect the xylem sap from the stem cross section. The sap exudation rate was always depressed by low root temperature. Nitrogen flow through the xylem vessel was highest in HT plants the day after fertilization (10-fold higher than LT), then decreased constantly. In LT plants, N flow was low the first and the second day after fertilization then reached the maximum 4 days after fertilization, when it was significantly higher than in HT plants. The amount of fertilizer-N found in leaves reflected the different movement rate of N observed in the two treatments. In HT trees fertilizer-N reached a plateau 2 days after fertilization, while in LT it linearly increased over time. This results suggest that root zone temperature of 8°C, although causes a delay (2–4 days) in nitrogen uptake, does not represent a serious limiting factor for N nutrition of tested apple trees.

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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.