Calcium and postharvest physiology in sweet cherry

Ripe sweet cherries on the tree — Figure 1. The calcium that will decide whether these fruit reach the consumer firm has already entered — or has failed to enter — **before ripening**. At the stage in the photograph the pathway that carries it has all but closed.

Savvas Pastopoulos, Agronomist MSc — Pastopoulos Agronomics, Neos Mylotopos, Pella, Greece

Calcium in sweet cherry is the element with the widest gap between what the market promises and what the experiments show. There is, however, a physiological mechanism that explains why half the studies show an effect and the other half show nothing — and once it is understood, it becomes clear which sprays are worth making and which are money thrown away.

The mechanism that explains everything

Calcium moves within the plant exclusively in the xylem. It is not translocated in the phloem [2].

Up to the third stage of fruit development, the xylem contributes more than 80% of total inflow to the cherry. From the third stage onwards, however, that share declines progressively and at ripening reaches almost zero, as xylem conductance falls [2].

In practice: a calcium spray close to harvest has no road to follow. Whatever calcium is going to enter the fruit must enter early.

A note on units This English edition uses hectares. The Greek edition uses the Greek stremma; 1 ha = 10 stremmata.

600 ppmthe minimum fruit calcium for quality cherries [1]

62%of applied calcium washed off by just 5 mm of rain [1]

r² = 0.88correlation between flesh calcium and firmness [2]

80%fewer cracked fruit with calcium through overhead sprinklers during rain [1]

Contents

Why calcium struggles to reach the fruit
How much calcium the fruit needs
Foliar sprays — what the experiments show
Fruit cracking
Postharvest — where the numbers are cleaner
Nitrogen, the competitor
The measurement trap
What actually makes the difference
The key points
Sources

1. Why calcium struggles to reach the fruit

Three reasons, and they add up.

1.1 It travels only with transpiration water

Calcium moves with the xylem stream and is not redistributed through the phloem [2]. Whatever does not enter the fruit while the xylem is running will not enter later from somewhere else.

1.2 The leaves win the competition

Soil solution ≈ 0 MPa → apoplast of a transpiring leaf ≈ −0.8 MPa

Flow follows the potential gradient [2]. Leaves have a far greater surface-to-volume ratio and far higher transpiration than fruit, so they draw most of the calcium to themselves. The fruit gets what is left.

1.3 The door closes exactly when it is needed

Sweet cherry fruit growth follows a double sigmoid curve and is divided into three stages [1]. The third stage is the rapid swelling before harvest — and precisely then xylem conductance collapses and inflow falls to zero [2].

The consequence that overturns common practice Most calcium sprays are made close to harvest, when the grower sees the fruit swelling and worries about cracking. That is exactly the worst possible moment as far as calcium entry into the fruit is concerned, because the transport pathway has already closed. Whatever benefit exists at that point comes from action on the skin, not from nutrition.

Sources: 1 2

2. How much calcium the fruit needs

The threshold is clear: at least 600 ppm in the fruit for the production of quality cherries [1].

The relationship with firmness is among the cleanest in the literature. Across a range of flesh calcium from 400 to 2,400 ppm and firmness from 1.5 to 4.5 kg/cm, the correlation was found to be linear and strongly significant, with r² = 0.88 [2].

Correction to an earlier publication

In the article "The physiology of calcium in cherry" [1] this range is given as "400–2800 ppm". The original source, the review by Winkler and Knoche from which the table is drawn, gives 400 to 2,400 ppm [2].

The difference does not change the conclusion — the correlation remains strong — but the correct range is 400 to 2,400 ppm. The correction is recorded here explicitly.

The quantitative significance for the orchard. Annual removal of calcium oxide from an orchard has been estimated in kilograms per hectare: pear 200, apple 180, peach 150, grapevine 60 to 130, olive 35 to 70, kiwifruit 55 to 60 [9]. These are quantities that cannot be covered by foliar sprays — they are covered from the soil.

Sources: 1 2 9

3. Foliar sprays — what the experiments show

Here lies the most honest statement in the whole literature, and it comes from the review itself: "for every parameter examined there are studies that showed a positive effect and studies that showed no correlation at all" [1]. Preharvest sprays and postharvest dips sometimes raise fruit calcium and sometimes do nothing [2].

Application	Result	Source
3 applications of 0.5% CaCl₂, three-year trial, 'Merton Premier'	+26% fruit calcium	Wojcik et al. [1]
3 applications, 'Van'	+65% calcium concentration	via [1]
CaCl₂ 58.5 mM, 2nd and 3rd stage, 'Vogue'	Increase in calcium	via [1]
1% Ca(OH)₂ at fruit set, two cultivars	Increase in only one of the two	Landi et al. [1]
Ca(NO₃)₂ 0.75 g/L at 21, 35 and 42 days after full bloom	No increase	Nagy et al. [1]
1 kg/ha Ca(NO₃)₂, 12 replicates, 'Grace Star'	No increase	Measham et al. [1]
Various forms, 'Lambert', 'Ron's Seedling', 'Supreme', 'Van'	No effect	Koffmann et al. [1]
6 calcium applications	From 450 to 650 ppm — that is, past the 600 threshold	Wang et al. [1]

The factor that cancels half the sprays It has been documented that a rainfall of just 5 mm washes off 62% of the applied calcium [1]. In cherry, where calcium sprays are made precisely in the season of spring rain, this explains much of the inconsistency in results: the calcium did not fail, it left.

Warning for every foliar spray Spraying trees carrying fruit can cause fruit marking and leaf burn. With calcium products the risk is higher: in a trial with calcium caseinate, leaves and fruit were covered with a white film [4]. A prior trial on a limited number of trees is required, with the same product at the same rate, followed by 5 to 7 days of observation before general application. Spray in the late afternoon or early morning, never in a heatwave and never on foliage under water stress.

Sources: 1 2 4

4. Fruit cracking

Cracked sweet cherry after rain — Figure 2. Fruit cracking from rainfall before harvest. The loss is total — the fruit cannot even be sold as second grade — and it can happen within a few hours.

4.1 How it is measured

Christensen (1996) established a measurement protocol, the Cracking Index. Harvested fruit are immersed within one hour in deionised water. At two, four and six hours they are removed, checked for cracks, and the sound ones returned to the solution [1]:

CI = (5a + 3b + c) × 100 / 250

4.2 What the experiments achieved

Treatment	Reduction in cracking
2% calcium acetate	From 80% to 32% cracked fruit [1]
3 applications of 0.7% Ca(OH)₂	Index reduced by 82% [1]
Ca(OH)₂ foliar, '0900 Ziraat'	66% — the most effective in the trial [4]
CaCl₂ foliar, '0900 Ziraat'	62% [4]
Calcium caseinate	About 50%, but with a white film on leaves and fruit [4]
CaCl₂ through overhead sprinklers, during the rain	80% fewer cracked fruit [1]
Mean of trials with moderate baseline damage	Only 13%, range 4 to 31% [1]

The two numbers that must be read together

The impressive figures — 82%, 66%, 62% — come from trials with very high baseline damage. Where damage was moderate, the mean reduction fell to 13% [1].

This is the same pattern that appears with every nutrient: the intervention pays where there is a problem, not where things are already going well. In an orchard that does not crack, calcium will not deliver a 60% improvement.

4.3 The mechanism in the cell wall

Calcium holds the pectins of the cell wall together. In experiments with EGTA, a chelator with high affinity for calcium, removing calcium from the wall increased the cracking index and the release of soluble pectin, while CaCl₂ reduced both [2]. A wall without calcium swells and loses its cohesion.

Sources: 1 2 4

5. Postharvest — where the numbers are cleaner

After harvest the problem is no longer transport within the tree. The fruit is immersed in a solution and calcium enters through the skin. That is why results here are far more consistent.

Three problems limit postharvest quality [2]: loss of firmness, wilting and browning of the stem, and decay.

5.1 Calcium in the hydro-cooling water

In work with CaCl₂ added to hydro-cooling water at 0 °C for 5 minutes, in two commercial cultivars [3]:

Measurement	Result
Tissue calcium, 'Sweetheart'	Increase of 29 – 85%, logarithmically with dose
Tissue calcium, 'Lapins'	Increase of 39 – 188%, linearly with dose
Respiration rate	Reduced
Ascorbic acid degradation	Reduced
Membrane lipid peroxidation	Reduced
Total phenolics and antioxidant capacity	Increased
Firmness and resistance to pitting	Increased
Loss of acidity and decay	Reduced, in both cultivars

The finding that overturns "the more the better"

Stem browning was inhibited by CaCl₂ at 0.2% and 0.5%. At the higher doses, however — 1.0% and 2.0% — it INCREASED, probably through modification of membrane lipid peroxidation [3].

In other words the curve reverses. A grower who doubles the dose "to be safe" gets the opposite result in exactly the character that decides the commercial appearance of a cherry — the green, turgid stem. It is the strongest practical finding in this whole text.

5.2 Firmness — what the consumer wants

Firmness is unquestionably the most important quality attribute at the point of sale [2]. Consumers want firm fruit, with firmness above 4.75 N. The range 2.52 to 4.75 N is regarded as acceptable [2]. Below that, the fruit is rejected.

5.3 Decay

After artificial inoculation with Botrytis cinerea, the proportion of affected fruit was significantly reduced following a dip in a 1% solution of various calcium forms — CaCl₂, Ca(NO₃)₂, Ca(OH)₂ — and five days of incubation [1].

5.4 The stem

It is common, after removal from cold storage, for the stems to have browned and dried out [1]. The stem is the first thing the buyer looks at and the last thing the grower notices. The calcium dose in hydro-cooling is the direct lever, subject to the reversal described above.

Sources: 1 2 3

6. Nitrogen, the competitor

Excessive nitrogen fertilisation acts antagonistically towards the supply of calcium to the fruit [1]. The mechanism follows from what was described in the first section: more nitrogen means more vegetative growth, more and larger leaves, stronger transpiration — and therefore stronger competition by the leaves against the fruit for the calcium in the xylem.

6.1 Autumn foliar urea

Autumn urea sprays, alone or combined with zinc sulphate, have been tested for their effect on the following spring's bloom development in 'Bing', alongside manual defoliation [7]. The logic is that the nitrogen stored in autumn is what will feed the spring.

Seasonal uptake, storage and remobilisation of nitrogen is critical in cherry, where new growth and fruit development occur simultaneously, within a short period from bloom to ripening [5]. Trees on dwarfing rootstocks such as Gisela 5 and 6 tend to give heavy crops but smaller fruit when the crop load is not balanced by sufficient leaf area [5].

And a negative result, worth as much as the positive ones In a trial at Cluj-Napoca, Romania, over two years and in four cultivars, with urea at 10 litres per hectare per application, no differences in fruit firmness were found relative to the unsprayed control [6]. Better results came from two applications rather than one.

The conclusion is not that urea does not work. It is that it does not work for everything: replenishing nitrogen reserves is one thing, fruit firmness — which is decided by calcium — is another.

6.2 Nitrogen, potassium and fruit minerals

In a two-year trial with '0900 Ziraat' on 'Gisela 5', nitrogen and potassium rates of 0, 50, 125 and 250 g per tree were tested [8]. The main conclusion of the work is that unbalanced fertilisation adversely affects the nutrient content of the fruit — that is, more fertiliser does not mean a richer fruit.

Sources: 1 5 6 8

7. The measurement trap

Calcium can be measured 50% lower than it really is

In calcium analysis by spectrophotometry, phosphorus in the sample interferes and binds the calcium. The interference is suppressed by adding lanthanum. When this is omitted, calcium is underestimated by about 50%.

In ripe 'Staccato' fruit the measurements were 0.65 ± 0.07 mg/g dry weight with lanthanum against 0.32 ± 0.03 without [2].

What this means for the grower: an analysis result showing calcium deficiency may be a method error. Before a calcium programme is decided on the basis of an analysis, the laboratory should be asked for explicit confirmation that lanthanum was used. The same interference has led to significant underestimation in published work as well.

8. What actually makes the difference

The order of priority

The soil first. Corrections are made before planting: drainage, pH, incorporation of calcium where needed [10]. Annual calcium removal from an orchard is measured in hundreds of kilograms per hectare [9] and cannot be covered by sprays.
Early, not late. Whatever calcium enters the fruit enters while the xylem is functioning — that is, before the third stage [2].
Control the nitrogen. Excessive nitrogen fertilisation actively competes with the calcium supply to the fruit [1].
Watch for rain after spraying. Five millimetres is enough to remove 62% [1].
Hydro-cooling is the most reliable opportunity. There the calcium genuinely and measurably enters — but at a low dose [3].
The rootstock matters. It affects calcium uptake from the soil and its concentration in the scion [9], although further research is needed [1].

The one intervention with a consistently large effect The 80% fewer cracked fruit did not come from a sprayer. It came from applying CaCl₂ through overhead sprinklers, during the rain [1]. The reason is simple: instead of spraying calcium that the next rain will wash off, the calcium is in the rainwater that reaches the fruit. It requires an installation, but it is the only approach that consistently gives large percentages.

9. The key points

What to take away

Calcium travels only in the xylem. Inflow to the fruit exceeds 80% up to the third stage and falls almost to zero at ripening [2]. Late sprays have no road.
Threshold of 600 ppm in the fruit, with a strong correlation between calcium and firmness, r² = 0.88 over the range 400 to 2,400 ppm [1] [2].
5 mm of rain washes off 62% of the applied calcium [1].
In postharvest hydro-cooling the dose response reverses. At 0.2 and 0.5% the stem is protected; at 1.0 and 2.0% it gets worse [3].
The large reductions in cracking apply to orchards with a large problem. With moderate damage the mean falls to 13% [1].
Nitrogen competes with calcium through increased leaf transpiration [1].
Analysis without lanthanum underestimates calcium by 50% [2]. Ask the laboratory explicitly.
The honest picture: for every parameter there are studies with a positive result and studies with none at all [1] [2].

10. Sources

The numbers in the text refer to the list below. Sources [1] to [4] and [5] to [8] were read in full.

[1] Pastopoulos, S., Kazantzis, K. and Sotiropoulos, T. (2020). The physiology of calcium in cherry and its relation to fruit quality parameters (in Greek). Georgia – Ktinotrofia 11/2020:40–45. ELGO "DIMITRA", Institute of Plant Breeding and Genetic Resources, Department of Deciduous Fruit Trees, Naoussa. Source for the 600 ppm threshold, the 62% washed off by 5 mm of rain, the cracking index and its formula, the reduction from 80% to 32% with calcium acetate, the 82% reduction with three applications of 0.7% Ca(OH)₂, the mean reduction of 13% with a range of 4 to 31%, the 80% reduction with sprinklers during rain, the double sigmoid growth curve, the antagonism of nitrogen, the results of Wojcik, Landi, Nagy, Measham, Koffmann, Ippolito and Wang, and the statement that for every parameter there are studies with a positive and studies with a nil result.
[2] Winkler, A. and Knoche, M. (2019). Calcium and the physiology of sweet cherries: A review. Scientia Horticulturae 245:107–115. Source for calcium transport exclusively in the xylem, for the xylem contributing more than 80% up to the third stage and falling almost to zero at ripening, for the 0 MPa of the soil solution against −0.8 MPa in the apoplast of a transpiring leaf, for the correlation r² = 0.88 over the range 400 to 2,400 ppm and 1.5 to 4.5 kg/cm, for the firmness limits of 4.75 N and 2.52 to 4.75 N, for the role of calcium in the pectins and the EGTA experiment, and for the 50% underestimation without lanthanum, 0.65 against 0.32 mg/g dry weight in 'Staccato'.
[3] Wang, Y., Xie, X. and Long, L. E. (2014). The effect of postharvest calcium application in hydro-cooling water on tissue calcium content, biochemical changes, and quality attributes of sweet cherry fruit. Food Chemistry 160:22–30. Source for the application of CaCl₂ in hydro-cooling water at 0 °C for 5 minutes, for the increase in tissue calcium of 29 to 85% logarithmically in 'Sweetheart' and 39 to 188% linearly in 'Lapins' as the dose rose from 0.2% to 2.0%, for the reductions in respiration, ascorbic acid degradation and lipid peroxidation, for the increases in phenolics, antioxidant capacity, firmness and pitting resistance, and above all for the reversal of the stem response: browning inhibited at 0.2 and 0.5% but increased at 1.0 and 2.0%.
[4] Gholinezhad, E. et al. (2014). Effects of different foliar calcium compounds on fruit cracking and quality of sweet cherry cv. '0900 Ziraat'. Notulae Botanicae Horti Agrobotanici 42(1). Source for the reduction of the cracking index by 38 to 66%, for the superiority of Ca(OH)₂ at 66% and CaCl₂ at 62%, for the roughly 50% reduction with calcium caseinate, and for the white film that covered leaves and fruit.
[5] Foliar Applications of Urea Affect Nitrogen Reserves and Cold Acclimation of Sweet Cherry. Source for the importance of seasonal uptake, storage and remobilisation of nitrogen in cherry, where vegetative growth and fruit development occur simultaneously within a short period, and for the tendency of trees on the dwarfing rootstocks Gisela 5 and 6 to carry heavy crops with smaller fruit when crop load is not balanced by leaf area.
[6] Bulletin UASVM Horticulture 69(1)/2012. Effect of urea applications at 10 litres per hectare in four cherry cultivars, Cluj-Napoca, Romania, 2010–2011. Source for the negative result: no difference in fruit firmness relative to the unsprayed control, and for the superiority of two applications over one.
[7] Effects of fall applications of urea and zinc sulfate to 'Bing' sweet cherry spring budbreak (2005). Source for the trial of autumn urea sprays with zinc sulphate and manual defoliation, and for their effect on the following spring's bloom development.
[8] Effect of nitrogen and potassium rates on the nutrient content of the fruit, '0900 Ziraat' on 'Gisela 5', two-year trial. AcademicPres. Source for the rates of 0, 50, 125 and 250 g of nitrogen and potassium per tree, and for the conclusion that unbalanced fertilisation adversely affects the nutrient content of the fruit.
[9] Calcium in fruit trees. Source for annual removal of calcium oxide in kilograms per hectare — pear 200, apple 180, peach 150, grapevine 60 to 130, olive 35 to 70, kiwifruit 55 to 60 — and for the effect of the rootstock on calcium uptake and its concentration in the scion.
[10] Cherry nutrition — agronomic principles. Source for the principle that soil corrections, drainage, pH and calcium incorporation are done before planting.

Disclaimer

This text is informative and is not a fertilisation programme nor a recommendation to apply. The concentrations, rates and timings cited come from experimental trials on particular cultivars, rootstocks, soils and climates and do not transfer unchanged to every orchard. The contradictory evidence in the literature is stated explicitly in the text and should not be disregarded.

For every foliar spray: application to trees carrying fruit can cause fruit marking and leaf burn, a risk that is higher with calcium products, which may leave a visible white film. A prior trial on a limited number of trees is required, with the same product at the same rate, followed by 5 to 7 days of observation before general application. Spray in the late afternoon or early morning, never in a heatwave and never on foliage under water stress.

Plant protection and fertiliser products may be used only in accordance with the approvals of the competent authority of the country of use and with the product label. The approvals listed at fyto.plantprotect.gr are issued by the Greek Ministry of Rural Development and Food and are valid in Greece only.

Every nutrition programme is drawn up after leaf and soil analysis and an on-site assessment by a licensed agronomist. Pastopoulos Agronomics accepts no liability for the use of the information in this text.