WHICH NUTRIENTS IN THE LEAF DECREASE LINEARLY AS FRUIT LOAD INCREASES IN APPLES? A PRELIMINARY STUDY

Ersin ATAY TAGEM, Fruit Research Institute, Eğirdir, Isparta, Turkey

atayersin@yahoo.com

ABSTRACT: The effects of fruit load on leaf nutrient concentration was examined in a 5-year-old ‘Golden Delicious’/‘M.9’ apple orchard located in the West Mediterranean Region of Turkey. The aim was to explore which nutrients in the leaf decrease linearly as fruit load increases. Following physiological fruit drop in the last week of May, trees were hand-thinned to obtain three different fruit load densities corresponded to 3.4, 3.9 and 5.1 fruit per cm2 of trunk cross-sectional area (TCSA) at harvest. Leaf samples for nutrient contents analyses were collected in the last week of July. The concentrations of macronutrients and micronutrients were determined. As fruit load increased, potassium and manganese concentrations decreased correspondingly. The obtained results, while preliminary, suggest that the potassium level (1.39±0.02%) at the highest fruit load density as adjusted here was nearing its severe deficiency thresholds (~1%).

Key words: apple, fertilization, fruit density, Malus x domestica, nitrogen

INTRODUCTION

The ultimate goal of fruit growers is to achieve maximum yield of better quality crops at minimum cost (Westwood, 1995). Fruit load management is a very effective tool to reach this target (Wünsche et al., 2000; Bustan et al., 2016). Fruit skin colour shows superiority on low cropping trees in comparison to high cropping trees (Robinson et al., 2009). Too high fruit loads penalize the fruit quality especially in size and soluble solids content (Wünsche et al., 2000; Serra et al., 2016). Fruit load management is commonly used to maintain the regular annual cropping and moderate vigour of trees. Apple growers set fruit load densities to bend the main leader and lateral branches down, especially in high-density apple orchards, to achieve the balance between vegetative growth and fruiting (Lauri, 2009; Atay and Lauri, 2013). Fruit load management itself determines the orchard profitability. Therefore, for a long time, horticulturists have sought to identify effects of fruit load to promote orchard efficiency.

From a physiological point of view, fruit load density has a close relationship with water consumption and carbohydrate metabolism (Wünsche et al., 2000; Bustan et al., 2016). It also has an influence on nutrient concentrations in both fruit and leaves (Ferguson and Watkins, 1992; Blanco et al., 1995). Nutrient allocation at the whole-tree scale is crucial for orchard performance regarding fruit quantity and quality (Blanco et al., 2002). It is also important to not forget, nutrient deficiency in the leaf can lead to yield loss in fruit production. This study aimed to find an answer to the following question: Which nutrients in the leaf decrease linearly as fruit load increases?

METHODS

The research was conducted in a 5-year-old ‘Golden Delicious’/‘M.9’ apple orchard planted at 3.5 x 1 m distances at Fruit Research Institute located in the West Mediterranean Region of Turkey. Trees were trained to the vertical axis system and were supported by a three-wire trellis up to 2.2 m high. The soil in the orchard (0-30 cm) is a clay-loam, with average values of 38.86% clay, 36.12% silt, 25.04% sand, 3.67% lime and 4.90%organic matter. The salinity and pH are 0.14 mS cm-1 and 8.24 (1:2.5), respectively.

Following physiological fruit drop in the last week of May, trees were hand-thinned to obtain three different fruit load densities (low, medium and high). The fruit load was assessed at harvest and corresponded to 3.44, 3.90 and 5.07 fruit per cm2 of TCSA for low, medium and high fruit load, respectively. Apart from the fruit load arrangement, all trees in the orchard received the same regular field practices including irrigation, weed control, and fertilisation.

Leaf samples for nutrient analyses were collected in the last week of July. Initially, leaves were decontaminated and rinsed with tap water, 2 N HCl and distilled water. Then, samples were dried at 70oC till constant weight and then ground for sieving. The total nitrogen was determined using Kjeldahl method with a distillation unit

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(Gerhardt, Königswinter, Germany). Concentrations of phosphorus, potassium, calcium, magnesium, iron, manganese, copper, boron and zinc were determined with an inductively coupled plasma spectrometer (Perkin-Elmer, Optima 2100 DV Optical Emission Spectrometer, Shelton, CT 06 484, USA). The concentrations of nutrients in leaf tissues were expressed on a dry-mass basis.

The experiment was conducted in a randomized block design with three replicates. Each replication comprised four experimental trees (i.e. 4 x 3 = 12 trees in total for each fruit load density). The relationships between nutrient concentration and fruit load density were revealed through a linear regression model.

RESULTS AND FINDINGS

The result showed that potassium was the only macronutrient decreasing linearly as fruit load increases (Figure 1). Fruit are strong sinks for potassium allocation at the whole-tree level. Therefore low cropping trees have higher leaf potassium than high cropping trees (Tough et al., 1998; Neilsen and Neilsen, 2003). The potassium values were 1.51±0.04%, 1.49±0.01% and 1.39±0.02% for low, medium and high fruit load, respectively. The normal level of potassium in apple leaves oscillates between 1.5% and 2.5%. Apple leaves can exhibit severe potassium deficiencies when potassium is less than 1% (Neilsen and Neilsen, 2003). Apple growers with a naked eye can diagnose severe potassium deficiency symptoms in the orchard typified by purple spots and yellowing between leaf veins as well as dead tissues in leaf tips.

Figure 1. Relationship between Leaf Macronutrient Concentration and Fruit Load. Nitrogen (N), Phosphorus (P), Potassium (K), calcium (Ca) and magnesium (Mg). The fruit numbers per cm2 of TCSA

were 3.44, 3.90 and 5.07 for low, medium and high fruit load, respectively. The Values Shown are Means ±Standard Deviation (SD) (n=3).

Manganese was the only micronutrient that decreased linearly as fruit load increases (Figure 2). The manganese values were 70.09±1.26, 69.52±12.13 and 57.32±11.70 for low, medium and high fruit load, respectively. In general, manganese deficiency is unlikely when leaf magnesium concentrations measure between 25 ppm and 120 ppm, while leaves with concentrations below 25 ppm are likely to display the manganese deficiencies (Neilsen and Neilsen, 2003).

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Figure 2. Relationship between Leaf Micronutrient Concentration and Fruit Load. Iron (Fe), Copper (Cu), Manganese (Mn), Zinc (Zn) and Boron (B). The fruit numbers per cm2 of TCSA were 3.44, 3.90 and

5.07 for low, medium and high fruit load, respectively. The Values Shown are Means ±SD (n=3).

CONCLUSION

Potassium and manganese in the apple leaf decreased linearly as fruit load increases. When considering the concentrations of manganese that showed by far higher values in all fruit load densities than the threshold level of manganese deficiency, it can be said that there are no disastrous consequences of the high fruit load density as adjusted here for fruit growers. However, on the other hand: the potassium level at the highest fruit load density in the study was nearing its severe deficiency thresholds. This study supported the idea that fertilization, especially for potassium needs to be adapted to the fruit load present.

RECOMMENDATIONS

Potassium fertilization in apple orchards needs to be adapted to the fruit load present.

REFERENCES

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Lauri, P.E. (2009). Developing a new paradigm for apple training. The Compact Fruit Tree, 42: 17-19. Neilsen, G.H., & Neilsen, D. (2003). Nutritional requirements of apple. In Ferree, D.C., & Warrington, I.J.

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Robinson, T., Lopez, S., Lungerman, K., & Reginato, G. (2009). Crop load management for consistent production of ‘Honeycrisp’ apples. New York Fruit Quarterly, 17 (1): 24-28.

Serra, S., Leisso, R., Giordani, L., Kalcsits, L., & Musacchi, S. (2016). Crop load influences fruit quality, nutritional balance, and return bloom in ‘Honeycrisp’ apple. HortScience, 51 (3): 236-244.

Tough, H.J., Park, D.G., Crutchley, K.J., Bartholomew, F.B., & Craig, G. (1998). Effect of crop load on mineral status, maturity and quality of ‘Braeburn’ (Malus domestica Borkh.) apple fruit. Acta Horticulturae, 464: 53-58. doi: 10.17660/ActaHortic.1998.464.4.

Westwood, M.N. (1995). Temperate-zone pomology: Physiology and culture (Third Edition). Portland, Oregon: Timber Press.

Wünsche, J.N., Greer, D.H., & Palmer, J.W. (2000). Effects of crop load on fruiting and gas-exchange characteristics of ‘Braeburn’/M.26 apple trees at full canopy. Journal of th e American Society for Horticultural Science, 125:93-99.

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