Massimo Tagliavini

Iron deficiency and chlorosis in orchard and vineyard ecosystems

Abstract Several perennial, deciduous, as well as evergreen fruit crops develop symptoms of iron deficiency—interveinal chlorosis of apical leaves—when cultivated in calcareous and alkaline soils. Under these conditions fruit yield and quality is depressed in the current year and fruit buds poorly develop for following year fruiting. This paper reviews the main fundamental and applied aspects of iron (Fe) nutrition of deciduous fruit crops and grapevine and discusses the possible development of sustainable Fe nutrition management in orchard and vineyard ecosystems. Cultivated grapevines and most deciduous fruit trees are made up of two separate genotypes the cultivar and the rootstock, providing the root system to the tree. The effect of the rootstock on scion tolerance of Fe chlorosis is discussed in terms of biochemical responses of the roots to acquire iron from the soil. Symptoms of iron chlorosis in orchards and vineyards are usually more frequent in spring when shoot growth is rapid and bicarbonate concentration in the soil solution buffers soil pH in the rhizosphere and root apoplast. Since the solubility of Fe-oxides is pH dependent, under alkaline and calcareous soils inorganic Fe availability is far below that required to satisfy plant demand, so major role on Fe nutrition of trees is likely played by the iron chelated by microbial siderophores, chelated by phytosiderophores (released into the soil by graminaceous species) and complexed by organic matter. As most fruit tree species belong to Strategy I-based plants (which do not produce phytosiderophores in their roots) Fe uptake is preceded by a reduction step from Fe3+ to Fe2+. The role of ferric chelate reductase and proton pump activities in Fe uptake and the possible adoption of these measurements for screening procedure in selecting Fe chlorosis tolerant rootstocks are discussed. In a chlorotic leaf the existence of Fe pools which are somehow inactivated has been demonstrated, suggesting that part of the Fe coming from the roots does not pass the leaf plasmamembrane and may be confined to the apoplast; the reasons and the importance for inactivation of Fe in the apoplast are discussed. The use of Fe chlorosis tolerant genotypes as rootstocks in orchards and vineyards represents a reliable solution to prevent iron chlorosis; in some species, however, available Fe chlorosis resistant rootstocks are not very attractive from an agronomic point of view since they often induce excessive growth of the scion and reduce fruit yields. As most fruit tree crops and grapes are high value commodities, in many countries growers are often willing to apply synthetic Fe chelates to cure or to prevent the occurrence of Fe deficiency. The application of iron chelates does not represent a sustainable way to prevent or cure iron chlorosis because of to their costs and of the environmental risks associated with their use. Since Fe chelates were introduced, little research on alternative means for controlling the chlorosis has been performed. Sustainable management of Fe nutrition in orchards and vineyards should include all genetical and agronomical means in order to naturally enhance Fe availability in the soil and in the plant. Special attention should be given to soil analysis and to prevention measures carried out before planting. Alternatives to iron chelates are being developed and in the future they should be included into the routine practices of managing fruit trees and grapevine under Integrated Production and Organic Farming.

Agronomic means for the control of iron deficiency chlorosis in deciduous fruit trees

Abstract Iron deficiency induced chlorosis represents the main nutritional disorder in fruit tree orchards grown on calcareous and/or alkaline soils. Until rootstocks tolerant to Fe deficiency chlorosis are available for most susceptible fruit species, the agronomic means of preventing or curing Fe deficiency chlorosis will be considered of utmost importance by fruit growers. Chlorosis of fruit trees has been successfully controlled through foliar or soil applications of Fe chelates, which are expensive and have to be applied annually. In this paper results of research carried out within an EU joint research project are reported, where the effectiveness of alternative, low‐input, environmentally friendly management techniques to control Fe deficiency chlorosis has been tested in established kiwifruit, peach and pear orchards located in the Po Valley (Italy), in the Ebro Valley (Spain) and in the area of Imathia (Greece). Iron sulphate supply to the soil proved to be effective only if applied together with high amounts of organic matter such as compost or manure. Promising results in preventing chlorosis were obtained by sowing a mixture of graminaceous species along the tree row and supplying them with Fe sulphate. Laboratory tests indicated that long lasting decreases of pH in calcareous soils are difficult to achieve. We have also followed two approaches using foliar sprays: 1) testing a variety of compounds which may activate the Fe pool likely present in chlorotic leaves (citric, sulphuric, ascorbic and indole‐3‐acetic acid) and 2) applying Fe sources alternative to synthetic Fe chelates. Sprays aiming to activate the Fe pools in a chlorotic leaf were generally effective, although rarely caused a full recovery. This suggests that inactivation of Fe occurs outside the mesophyll cells. Sprays of Fe sulphate in all the crops tested showed similar or even higher regreening effect than FeDTPA.

Influence of root pruning and water stress on growth and physiological factors of potted apple, grape, peach and pear trees

Abstract Apple, grape, peach and pear trees were grown with roots divided between two pots for 2 months and then assigned to the following treatments: (1) control, receiving 100% of total plant transpiration (TPT) distributed equally between both pots; (2) pruned, receiving 100% of TPT in one pot with removal of roots in the second pot; (3) stressed, receiving 100% of TPT in one pot with water withheld from the other. Shoot growth of all species except peach was reduced only by root pruning (20%, 30% and 40% less than control in grape, pear and apple, respectively). New root growth of pruned and stressed plants was generally less than control (25% on average). During the first 15 days after treatment, both root pruning and water stress depressed transpiration and net photosynthesis. Thereafter, stressed plants did not differ from control; transpiration and net photosynthesis of pruned plants approached those of control only after 50 days. At the end of the experiment unchanged shoot:root ratios were found in stressed trees of each crop compared with control, while root pruning caused a variable shoot:root ratio depending on the degree of canopy reduction. The findings indicate the ability of the examined crops to adapt to drastic manipulations of the root system.

Iron source affects iron reduction and re‐greening of kiwifruit (Actinidia deliciosa) leaves

Abstract Among deciduous fruit plants, kiwifruit (Actinidia deliciosa) is one of the most susceptible to iron (Fe) chlorosis. To develop effective means for overcoming Fe chlorosis, it is of upmost importance to gain information about the reduction of Fe by leaf tissues, especially under conditions that lead to chlorosis. In the present study we have characterised the leaf Fe‐chelate reductase (FCR) in Fe sufficient and Fe deficient kiwifruit leaves and for the first time tested the hypothesis that FeIII‐malate is a suitable source of Fe for FCR, in addition to FeIII‐citrate. Under field conditions, we have also tested the re‐greening effects caused by the foliar application of different Fe sources, including FeIII‐malate, FeIII‐citrate, FeIII‐DTPA and an FeII source (FeSO4 + aminoacid‐polypeptide mixture) on chlorotic leaves. The results demonstrated that, similarly to other species, mesophyll tissues of A. deliciosa leaves are able to perform an enzymatic Fe reduction prior to Fe uptake. Plasma membrane enriched material extracted from Fe sufficient leaves reduced FeIII‐malate and FeIII‐citrate. The pH optimum was 6.0–6.2 for FeIII‐malate and 6.5 for FeIII‐citrate. The substrate‐dependence showed higher affinity for malate than for citrate. In contrast to the root level, the activity of the FCR of kiwifruit leaves was not enhanced under Fe deficiency. On the contrary, after two weeks of Fe depletion, the reduction of FeIII‐citrate was 4.5‐fold lower in the Fe deficient plants than in the Fe sufficient ones, while the reduction of FeIII‐malate was not significantly affected. Under field conditions, the Fe solutions caused regreening of chlorotic leaves, whose intensity and duration varied according to Fe source and Fe concentration. Among the treatments, the highest re‐greening effect was caused by FeIII‐DTPA and especially by the FeII source. FeIII‐citrate and FeIII‐malate were less effective in stimulating chlorophyll formation. All treatments increased leaf Fe concentration and content. Although less Fe from malate than from citrate penetrated into the leaves, the re‐greening effect from FeIII‐malate was intermediate between that of FeIII‐DTPA and the one caused by FeIII‐citrate. The results suggest that if FeIII‐malate can reach the plasmamembrane it provides a good source of Fe for leaf Fe uptake.

Remobilised nitrogen and root uptake of nitrate for spring leaf growth, flowers and developing fruits of pear (Pyrus communis L.) trees

Both uptake of fertiliser N and remobilisation of stored N were quantified for the early growth of spur and shoot leaves, flowers and fruit development of pear trees. One-year old Abbé F. trees grafted on quince C rootstocks were fertilised with a generous N supply for one year and while dormant during the winter, transferred to sand cultures. Each tree received 3 g of labelled nitrate-N at the end of winter and in early spring. Leaves, flowers and fruit were sampled on 5 separate occasions and the recovery of labelled N used to distinguish the remobilisation of N and the root uptake of nitrate. Remobilisation of stored N accounted for most of the N present in leaves and flowers during blossoming. Remobilisation of nitrogen stopped between petal fall and the beginning of fruit development. Root uptake of nitrate linearly increased over time and at the last sampling, 55 days after bud burst, fertiliser N contributed approximately half of the total N recovered in both spur and shoot leaves, the remainder coming from remobilisation. Flowers and fruits based their N metabolism more on remobilisation as compared to the leaves. This pattern of internal cycling of N is discussed in relation to fertilisation strategies for pear trees.

Technologies for the diagnosis and remediation of Fe deficiency

Abstract The objective of this paper is to review the developments in the last few years in two important issues related to Fe deficiency in plants. First, the current knowledge on the possible ways to carry out the diagnosis and prognosis of Fe deficiency in plants is discussed. This includes discussion on the best ways to carry out a meaningful analysis of Fe-containing compounds in different plant parts. We will also discuss other measurement techniques that can permit to assess the Fe nutritional status in plants, including leaf chlorophyll concentrations and others. Second, the new developments in management techniques to control and remediate iron deficiency are discussed. This includes possible improved ways to supply Fe compounds available to plants, both to the soil and to the irrigation water. We also discuss possible ways to supply directly the plant with Fe containing compounds, either to the foliage or to the stem. A particular emphasis is given throughout the paper to fruit tree crops growing in Mediterranean areas.

Nitrogen fertilization management in orchards to reconcile productivity and environmental aspects

Nitrogen fertilization in orchards of Emilia-Romagna Region, (Italy) was based in the past on excessive, not split, applications often supplied late in winter; the NUE (Nitrogen Use Efficiency) was therefore low and the risk of nitrate leaching was high. This paper summarizes the studies conducted in the last 10 years at the Department of Horticulture and Forestry of the University of Bologna aimed to develop a more rational use of nitrogen in orchards and vineyards. Root escavation of mature trees revealed that the use of localized irrigation (drip or microjet) causes a concentration of roots in the area wetted by the emitters. In such a situation, band applications of N to the tree row may allow a reduction of amounts of N fertilizer, while widespread applications, especially if the orchard soil is tilled, lead to an accumulation of nitrates in the alley. Results of several field trials where increasing N rates were applied indicate that the kind of response to N supply depends on the presence in soil of natural sources of nitrogen. This fact clearly stresses the necessity of evaluate the N status of an orchard before N fertilization. Rapid estimation of leaf chlorophyll by portable instruments is a promising index of leaf N concentration, only provided that calibration is made for each cultivar. A method, currently under testing in orchards and vineyards of Emilia-Romagna, is proposed here to adjust N fertilizer rates to the demand of the crop and to the level of available N in soil as determined in soil or soil solution samples.

Bulk soil pH and rhizosphere pH of peach trees in calcareous and alkaline soils as affected by the form of nitrogen fertilizers

One-year old nectarine trees [Prunus persica, Batsch var. nectarina (Ait.) Maxim.], cv Nectaross grafted on P.S.B2 peach seedlings [Prunus persica (L.) Batsch] were grown for five months in 4-litre pots filled with two alkaline soils, one of which was also calcareous. Soils were regularly subjected to fertigation with either ammonium sulphate or calcium nitrate providing a total of 550 mg N/tree. Trees were also grown in such soils receiving only deionized water, as controls. Rhizosphere pH, measured by the use of a microelectrode inserted in agar sheet containing a bromocresol purple as pH indicator and placed on selected roots, was decreased by about 2–3 units compared to the bulk soil pH in all treatments. This decrease was slightly less marked when plants were supplied with calcium nitrate rather than ammonium sulphate or control. Measurements conducted during the course of the experiment indicated that ammonium concentration was similar in the solution of soils receiving the two N fertilizers. During the experiment, soil solution nitrate-N averaged 115 mg L−1 in soil fertilized with calcium nitrate, 68 mg L−1 in those receiving ammonium sulphate and 1 mg L−1 in control soils. At the end of the experiment nitrate concentrations were similar in soils receiving the two N sources and bulk soil pH was decreased by about 0.4 units by ammonium sulphate fertigation: these evidences suggest a rapid soil nitriflcation activity of added ammonium. Symptoms of interveinal chlorosis in apical leaves appeared during the course of the experiment in trees planted in the alkaline-calcareous soil when calcium nitrate was added. The slightly higher rhizosphere pH for calcium nitrate-fed plants may have contributed to this. The findings suggest that using ammonium sulphate in a liquid form (e.g. by fertigation) in high-pH soils leads to their acidification and the micronutrient availability may be improved.

Response to iron‐deficiency stress of pear and quince genotypes 1

Abstract Roots of iron (Fe)‐efficient dicots react to Fe‐deficiency stress by strongly enhancing the ferric (Fe3+)‐reductase system and by lowering the rhizo‐sphere pH. In this study, we tested whether such adaptation mechanisms characterize pear and quince genotypes known to have differential tolerance to calcareous and alkaline soils. Two trials were performed using micropagated plants of three quince rootstocks (BA29, CTS212, and MC), three Pyrus communis rootstocks (OHxF51 and two selections obtained at the Bologna University: A28 and B21) and of two pear cultivars (Abbe Fetel and Bartlett, own‐rooted). In the first trial, plants were grown in a nutrient solution with [Fe(+)] and without [Fe(‐)] Fe for 50 days. Their root Fe‐reducing capacity was determined colorimetrically using ferrozine and FeEDTA, and Fe uptake of Fe(+) plants was estimated. In the second trial, the rhizosphere pH of plants grown in an alkaline soil was measured by a micro‐electrode. With the only exception of pears OHxF51 and A28...

Iron content in vegetative and reproductive organs of nectarine trees in calcareous soils during the development of chlorosis

Abstract We investigated for 2 years (1995–1996) the time course development of chlorosis and the variation of iron (Fe) content in vegetative and reproductive organs in two nectarine orchards planted with cv Spring Red and cv Stark Redgold on calcareous soils of the Po valley (Italy) with the final aim to evaluate possible tools for the early prognosis of Fe chlorosis and a more efficient fertilization management. Due to the withdrawal of Fe supply, floral Fe concentration significantly decreased in 1996 as compared with 1995 in cv Spring Red, but not in Stark Redgold. Correlation coefficients between Fe and chlorophyll (Chl) from the same leaves were always higher when Fe was considered as amount present per leaf or per unit of leaf area than as leaf dry weight. The fact that chlorotic and green leaves had similar Fe concentration could be explained by an overestimate of Fe in the chlorotic leaf as a consequence of a reduction of its size. However, the decrease of Chl concentration between 60 and 120 days after full bloom (DAFB) occurred while leaf Fe content generally increased, indicating that even during chlorosis development leaves were supplied with some iron. We therefore suggest that the development of chlorosis was associated with an inactivation of Fe in the leaf apoplast. In 1995, regardless the cultivar, floral Fe concentration and leaf Chl were never correlated. In 1996 floral Fe concentration was linearly related to leaf Chl recorded 60 and 120 DAFB in cv Spring Red only. Floral Fe concentration at full bloom 1996, regardless the variety, was linearly related to leaf Chl determined in spring of the previous year, suggesting that flower Fe concentration might be used for assessing the storage of iron during the previous season.