Fruit qualityand yield changes in pear (Pyrus communis L.) and peach (Prunus persica L. Batsch) trees as affected by iron deficiencychlorosis
Ana Álvarez-Fernández, Juan Carlos Melgar, Javier Abadía & Anunciaciόn Abadía*
Department of Plant Nutrition, Aula Dei Experimental Station, CSIC, P.O. Box 13034, E-50080 Zaragoza, Spain.
*Corresponding author: Telephone: +34 976 716057; Fax: +34 976 716145; E-mail:
Abstract
Iron deficiency induced chlorosis is a major constraint for pear and peach cultivation in calcareous soils. Effects of chlorosis on yield are well-known, but effects on fruit quality have been scarcely studied.The effects of different levels of chlorosison fruit quality and yield have been studied in field-grown pear and peach. Large yield reductions were the major effect of Fe deficiency in both, even when chlorosis was moderate; losses were mainly due to decreases in fruit number although size changes also occurred. Moderate and severe chlorosis caused fruit heterogeneity and maturity changes. Chlorotic pear treesyieldedfruits less green and firm with an increased sugars/acidsratio, indicating an advancedmaturity. Chlorotic peach trees had firmer fruits with higher acidity, phenolics and total organic acids, indicating adelayed maturity.Chlorosis caused common changes in pear and peach, including an increase in citric acid and a decrease in the malic/citric ratio.
Keywords: Acidity, fruit trees, fruit maturity, micronutrient deficiencies, peach, pear and sugars.
INTRODUCTION
Iron (Fe) deficiency is a major constraint for successful cultivation of fruit tree crops in calcareous or alkaline soils around the world, which account for one-third of the earth’s surface (1).Fruit treespecies differ in their susceptibilityto Fe deficiency,and it is widely accepted thatpear and peach are among the most susceptible fruit crops (2). In Spain, the second pear and peach producing country in Europe (3), Fe deficiency chlorosis is a major problem in the most important pear and peach production area, the Ebro valley basin (4). Typical symptoms of Fedeficiency include the development of interveinal chlorosis, starting from the apical leaves(5). Leaf chlorosis is the mostvisiblesymptomof Fe deficiencyin many high value crops, including pear (6), peach (7) and orange (8). In the case of pear, Fe chlorosis is atypical, since it usually occurs in the whole leaf lamina, including veins (9). Iron chlorosis hinders shoot growthand causes decreases in yieldand defoliation, as well as a 5-6 year shortening of the tree productive lifetime (4, 10).
Severe fruityield losses caused by Fe deficiency have been reported in peach, pear, Citrus, kiwifruit, olive and plum (10), generally associated to reductions in the number of fruits per tree(10, 11).Yield losses are often accompanied with reductions in fruit size (11) that may affect the percentage of commercially acceptable fruits (12), further decreasing the income of farmers. In peach, Fe deficiency resulted in marked reductions in fruit number per tree, which were dependent on the leaf chlorophyll (Chl) concentration; losses higher than 80% were found at Chl concentrations below 5 nmol cm-2 (11, 13). However, fruit size was much less affected than fruit number by the leaf Chl level (11). In green fruits such as pear, kiwi and olive, Fe deficiency can also lead to fruit yellowing (fruit chlorosis) (11).
The effects of Fe deficiency on fruit yield are so marked that the influence of Fe deficiency on fruit quality has been little explored so far. Some studies (see 11 for a review)have been carried out in Citrus (14, 15) and peach (4, 12, 16). Iron deficiency resulted in delayed ripening in Citrus, associated to decreases in total soluble solids and increasesin citric acid concentration (14). In peach, Fe deficiency also caused a delay in fruit maturity (4, 12, 16) associated to increases in organic acids concentrations and decreases in sugar/organic acid ratios(12). All these changes in fruit composition have been suggested to arise in part from metabolic disturbances resulting from changes in the activity of Fe-containing enzymes located in the fruit mesocarp(10, 17).Interestingly, fruit mesocarpFe concentration has been suggested to be a promising tool for diagnosing the Fe status of nectarine and kiwi (18).
Another aspect that has not been studied so far is the possible differential effects on fruit quality of different Fe deficiency degrees (e.g., chlorosis levels), in spite of the common occurrence of such variability at the tree and orchard levels (18). Previous works on the effectiveness of several foliar-applied Fe-compounds to control Fe-chlorosis in orange trees (14) suggested that differences in fruit quality could occur when leaf regreening proceeds to different extents. Therefore, the chlorosis degree may affect not only fruit yield but also fruit quality.
The objective of this work was to assess the changes induced in fruit quality and yield in two field-grown tree species, pear (Pyrus communis L.) and peach (Prunus persica L. Batsch), in response to different levels of Fe deficiency induced chlorosis.
MATERIALS AND METHODS
Plant material and growth conditions. The study was carried out in two field orchards located in the Ebro valley basin. A well-managed pear tree (Pyrus communis L. cv. Blanquilla, 8-year-old) orchard was located in Puigmoreno (Teruel, Spain; 41.11° N, 0.25° W). Trees were grafted on quinceBA2, with a frame of 3 x 5 m. The soil had a clay-loamy texture with 48% total CaCO3, 13.6% active lime, 0.7% organic matter and a pH in water of 7.8. Trees were not thinned. A peach tree (Prunuspersica L. Batsch cv. Babygold 5, 21-year-old) orchard was located in El Temple (Huesca, Spain; 41.97° N, 0.75° W). Trees were graftedon peach seedlings, with a frame of 4 x 5 m. The soil had a clay-loamy texture with 32% total CaCO3, 12.6% active lime, 1.9% organic matter, and a pH in water of 8.4. Fruit thinning intensity for peach trees was moderate (in the range of 3 to 21 kg per tree, depending on the tree leaf Chl level). Both orchards were flood irrigated as needed and well fertilized except for Fe (no Fe fertilizer was applied for 2 years). Drought stress or other nutritional disorders were not observed.Tree Fe status was monitored by estimating leaf Chl concentration with a hand-held Chl meter (SPAD-502, Minolta Corp., Ramsey, NJ), since leaf Fe concentration is not an adequate parameter for this purpose (19); Chl concentration is an accepted tool to monitor Fe status in fruit trees, provided other nutrient deficiencies are excluded (19).All SPAD data presented in the study were measured at harvest time.
Eleven pear trees were selected andgrouped into two classes:four trees without any chlorosis symptoms (SPAD 48-51; control trees, +Fe), and seven morewith Fe-deficiency symptoms in at least some branches (-Fe). These Fe-deficient pear trees had three types of branches,with: i) no chlorosis symptoms (SPAD 41-48), ii) moderate chlorosis symptoms (SPAD 22-28) and iii) severe symptoms (SPAD 8-10). Twelve peach trees were selected and grouped into three classes: four trees with no symptoms at all (control trees, +Fe; SPAD 39-43), four more that became moderately chlorotic during the season (-Fe; SPAD 24-44)and anotherfour trees with severe Fe-deficiency symptoms (-Fe; SPAD 18-24).
All fruits wereharvested by hand and at commercial maturity dates. In the case of pear trees, all fruits of branches showing severe, moderateand no chlorosis symptoms were collected separately. Yield was measured as total fruit weight per branch (only in pear) and per tree (both in pear and peach). The total number of fruits per branch (in pear) and per tree (in pear and peach) was also counted. Fruits were taken to the laboratory and analyzed within a few hours.
Fruit Physical Assays. Fresh weight (FW), size, color, and firmness were assessed individually, whenever possible, in 50-75 fruitsper tree (in all peach trees and in control pear trees) or per branch (in pear treeswith chlorosis symptoms). Some branches of pear deficient trees had less than 50 fruits, and in these cases all fruits were sampled.
Individual fruit size (width and length for pear, and maximum and minimum diameter for peach)was measured with a digital CD15DC Absolute Digimatic caliper meter (Mitutoyo Co., Kawasaki, Japan). Color was assessed inboth faces of each fruit using a tri-stimulus colorimeter (CR200, Minolta Co., Ltd., Osaka, Japan) with an apertureof 8 mm (only data for the greenest face are shown); measurements were made using the L, a, and b color space coordinates (22). In this system, L represents color brightness, low for dark colors and high for brightcolors; a is negative for green and positive for red; and b is negativefor blue and positive for yellow. Fruit firmness wasmeasured in the mature face, using a penetrometer, fitted with an 8-mm diameter probe for pear and a Durofel pressure tester (Copa-Technologie S.A., Tarascon, France) with a 0.10 cm2 tip for peach. The latter is a non-destructive method that provides a relative value of firmness (from 0 to 100 Durofel graduation, Dg).
Fruit Chemical Analysis. Chemical analyseswere carried out in a homogeneous sample of nine representative, commercially acceptable fruits per tree (in all peach trees and in control pear trees) or per branch (in pear treeswith chlorosis symptoms), selected from the large fruit-subsamples used for physical assays. An exception was total solublesolids (TSS), which was assayed in all 50-75 fruits whenever possible. The selection criteria to choose the representative nine fruits were size (74.7 ± 0.7 mmwidth in pear and 72.1 ± 0.4 mm maximum width in peach),color of the greenest face(b value of 31.3 ± 0.6 in pear and a value of 26.3 ± 0.8 in peach; bandaare the best color change indicatorsof maturity in pear and peach trees, respectively; 21, 22) and firmness (3.0 ± 0.1 kg cm-2in pear and 68.7 ± 0.4 Dg in peach). Fruits were peeled, and a portion of the mesocarp was removed fromeach opposite face and diced into 1 cm3pieces. A composite sample was built by mixing alldices from the nine fruits selected, and then divided into threealiquots for chemical analysis.
Titratable acidity (expressedas percentage of malic acid) andmesocarp juicepH wereimmediately measured following official methods as described in (12).The material needed for the rest of assays was frozen in liquid N2 and stored at -80 °C until analysis. Total soluble solids (TSS, in Brix degrees),total phenolic compounds,vitamin C, organic acids (malic, citric and quinic) and sugars (sucrose, glucose, fructose and sorbitol) were measuredas described in Álvarez-Fernández et al. (12).For mineral composition analyses, lyophilized, milled 2 g mesocarp samples weredry-ashed in a muffle furnace at 550 °C, and the residue was dissolvedin HNO3 and HCl (12). Calcium, Mg, Fe, Mn, Cu, and Zn were determined by atomicabsorption, K was determined by emission spectrometry, and P wasdetermined spectrophotometrically (12). Nitrogen concentrations were measureddirectly in lyophilized samples using an N analyzer (NA 2100,Thermoquest S.p.A., Milan, Italy).
Statistical Analysis. Values shown are means ± SE of four biological replicates (trees or branches) with the exception of Fe-deficient pear trees, where biological replicates were seven instead of four. Data for each biological replicate were averaged from three technical replications. Data were analyzed by one-way analysis of variance(ANOVA), and means were compared using Duncan’s test at p 0.10.
RESULTS
Changes induced by Fe deficiency chlorosis in yield and fruit physical properties.Large reductions in average yield per tree were found with Fe deficiency induced chlorosis in both fruit tree species (Table 1). Moderate chlorosis led to total yield losses of 64 and 83% in pear and peach trees, respectively. In severely chlorotic trees, occurring only in the peach orchard, yield losses were not significantly different from those found in moderately chlorotic trees.
Chlorotic pear trees had different types of branches, including some non-chlorotic ones and other branches with moderate or severe chlorosis symptoms. Although pear fruit yield was not significantly affected in branches without chlorosis symptoms (Table 2), yield reductions were found in moderately (only significant at p < 0.15) and severely chlorotic branches. Branches with severe chlorosis yielded from 0 to 8 fruits per branch (98% average yield reduction) and a representative sample for physical and chemical analysis could not be obtained.
Yield decreases were associated with reductions in the number of fruits per tree in both species (Table 1). However, yield decreases were not fully explained by the reduction in number of fruits per tree, since changes in fruit FW and size also occurred. In Fe-deficient pear trees, the reduction in yield (64%) was lower than the reduction in fruit number (78%), and this was associated to increases in average fruit FW and size (Table 3). In moderately chlorotic peach trees, the decrease in yield (83%) was similar to the decrease in fruit number (81%), with significant decreases in fruit FW and increases in fruit size (Table 3). Severe chlorosis in peach trees led to a 74% yielddecrease, whereas fruit number reduction was only of 60%; this was associated to decreases in fruit FW and size (Table 3).
Moderate chlorosis led to an increase in the number of large-sized fruits in both species, as shown by the fruit size distribution (Figure 1A and B). In Fe-deficient pear trees, more than 80% of the fruits had a diameter > 70 mm, whereas only 45% of the fruits reached that size in control trees (Figure 1A). When pear branches with different chlorosis degrees were examined separately (Figure S1A, supporting information), those showing moderate chlorosis had more large-sized fruits than green branches (in both Fe-deficient and control trees). Likewise, in moderately chlorotic peach trees, more than 80% of the fruits were >70 mm in diameter (Figure 1B), whereas in control trees only 40% of fruits reached that size. In severely chlorotic peach trees only 20% of the fruits were >70 mm in diameter, whereas 25% of fruits had a size <60 mm. This class of small peach fruits was not found in the rest of the trees (Figure 1B). The number of size classes was unaffected by chlorosis in pear (classes of 50-60/60-70/70-80/80-90/90-100 mm). However,fruits from control peach trees belonged to two size classes only (60-70/70-80 mm), whereas in moderately and severely chlorotic trees fruits belonged to three classes (60-70/70-80/80-90 and 50-60/60-70/70-80 mm, respectively).
Other fruit physical properties were also affected by Fe deficiency chlorosis (Table 3). Fruit color was differently affected by Fe chlorosis in pear and peach. In pear, Fe deficiency caused increases in fruit color coordinate values Land b, and decreases in a. In peach, moderate chlorosis did not change color coordinates, whereas severe chlorosis increased a values and decreased L values when compared to fruits of control trees. In both species, fruits in chlorotic trees were more heterogeneous in color than those of control trees (Figure 1C and D). In pear, the fruit population in Fe-deficient trees was distributed in four color classes (Figure 1C) with most of the fruits (70%) having b values >35, whereas the whole fruit population in control trees had b values <30. This effect in fruit color was also observed when pear branches with different chlorosis degrees were examined separately (Figure S1B, supporting information).Within Fe-deficient pear trees, fruits from branches with and without chlorosis symptoms showed a similar color distribution profile.In peach, moderate chlorosis increased the number of fruits with a values <20 by 15% when compared to the controls, whereas severe chlorosis increased the number of fruits with a color values >30 by 20% when compared to the controls.
Chlorosis affected differently fruit firmness in pear and peach (Table 3). In pear, Fe deficiency caused a decrease in fruit firmness, which was similar in fruits of branches with and without chlorosis symptoms. In peach, however, firmness was not affectedby moderate chlorosis, whereas an increase in firmness was found in fruits from severely chlorotic trees. Fruit population distribution for firmness did not change with chlorosis in both species (Figure 1E and F).
Changes induced by Fe deficiency in fruit chemical composition.Fruit TSS was affected by Fe chlorosisonly in peach (Table 4). Severely chlorotic peach trees had fruits with higher TSS content than those from both control trees and trees with moderate symptoms (Table 4). Moderate chlorosis in pear and severe chlorosis in peach slightly increased heterogeneity in TSS content (Figure 1G and H).
Changes in acidity, pH and total phenolic concentrations were only found in peach fruits, with moderate and severe chlorosis increasing acidity and total phenolic concentrations (Table 4). The fruit mesocarp pHwas lower in moderately chlorotic peach trees than incontrol and severely chlorotic trees. Fruit vitamin C was affected by Fe deficiency in both species (Table 4). In pear, Fe deficiency decreased fruit vitamin C concentrations, both in branches with and without chlorosis symptoms. In peach, moderate chlorosis also decreased fruit vitamin C, whereas severe chlorosis did not cause any effect.
Mesocarp organic acid concentrations were affected by Fe deficiency chlorosis in both species (Table 5). In pear, citric acid concentrations were markedly affected by Fe deficiency, being higher in fruits from Fe-deficient trees (both in branches with moderate chlorosis symptoms and in asymptomatic branches) than in fruits from control trees. Malic and quinic acid concentrations were decreased by Fe deficiency. These changes resulted in decreases in the malic/citric ratio and the total organic acid concentration in the mesocarp. In peach, moderate chlorosis increased the concentrations of citric acid, whereas malic and quinic acid concentrations were unchanged, resulting in decreases in the malic/citric ratio and no significant changes in total organic acids (Table 5). Severe chlorosis increased the concentrations of quinic acid and total organic acids (significantly only at p < 0.15), whereas citric and malic acid concentrations were unchanged.
Mesocarp total sugar concentrations were not affected by Fe-deficiency chlorosisin pear and peach (Table 5). However, Fe-deficiency caused a decrease in sucrose and glucose concentrations in pear. In peach, the only effect found was an increase in sorbitol concentrations. The fruit (total sugar)/(total organic acid) ratio was affected in pear but no in peach. In pear, this ratio increased with Fe deficiency only in fruits from branches with chlorosis symptoms (11.2 ± 0.4 in controls, 10.9 ± 0.3 in fruits from branches without symptoms from Fe deficient trees and 12.6 ± 0.5 in fruits from chlorotic branches). In peach, the (total sugar)/(total organic acid) ratio tended to decrease with Fe deficiency (6.2 ± 0.4 in control, 5.5 ± 0.2 in moderately chlorotic and 5.7 ± 0.5 in severelychlorotic trees), although differences were not statistically significant at p < 0.10.