Calcium is a nutrient that is receiving more and more attention from orchardists and scientists because concentrations of calcium in fruit have been related to fruit firmness and post-harvest disorders of fruit.
While it is not only for nutrition, calcium is the principal key nutrient which determines the quality of fruit, controls the development and ripening of fruit, and keeps fruit relatively free of disorders in many orchards.
Fruit derive their firmness and lasting qualities from the structural integrity and composition of the cell walls. Calcium acts as a binding agent, cross linking and stabilising pectin/protein complexes in the cells of fruit flesh and increasing the rigidity of cell membranes.
Calcium is therefore an essential structural component of fruit. Calcium also inhibits certain processes which lead to premature softening and ripening.
Effects of low calcium concentrations
Low concentrations of calcium in fruit promote development of corking disorders such as bitter pit, cork spot, lenticel blotch and Jonathan spot.
In addition, superficial scald, lenticel breakdown, internal breakdown, low temperature breakdown, water core and rot may also be intensified when fruit calcium concentrations are low.
Low calcium concentrations in cherries can contribute to cracking and splitting when it rains towards harvest time.
Calcium in the soil
Forms of calcium
Calcium in the soil solution exists as cations (i.e. positively charged).
Potassium, magnesium, sodium and ammonium are also cations. Cations cannot exist in the soil solution without being counteracted by negatively-charged molecules (anions), such as phosphate, nitrate and sulphate.
Calcium is usually present in soils in ample amounts, but only the soluble fraction is readily available to the tree—provided the soil pH (soil extracted in water) is between 5.5 and 6.5; there are no major imbalances; and there is adequate water in the soil.
The insoluble fraction—which is found in minerals that contain calcium, and in calcium reversibly bound to various components of the soil (such as clay and organic matter)—is important to the tree in the longer-term.
Changes between these three fractions are usually slow.
Roots & calcium uptake
Before a tree can take up calcium, it is necessary for the calcium ions to come into contact with the surface of the root. This occurs through transport of calcium from water in the soil.
When trees transpire (lose water by evaporation from the leaves), water flows to the root surface, and calcium is transported in the mass flow.
Calcium is primarily taken up by the root tips of very young roots. Thus, it is important to ensure that trees have healthy root systems with new roots, especially in spring and autumn, when roots of mature trees are most active and must take up sufficient calcium.
Concentrated points of localised irrigation are major sites of calcium uptake by new roots.
Soil water and calcium uptake
The amount of water in the soil is important. Trees that suffer from continued water stress have trouble taking up enough calcium.
If there is too much water in the soil, the lack of oxygen in the soil affects root function and indirectly affects the uptake of calcium.
While calcium is one of the most abundant nutrients in the soil, and is found in higher amounts than other cations, its uptake is less than other cations.
This is because calcium can only be absorbed by the root tips. This problem is exacerbated by competition from other nutrients in the soil.
In this way, high concentrations of potassium, magnesium, or ammonium, all can further reduce the uptake of calcium by roots of fruit trees.
Calcium in the tree
The calcium content of a tree is genetically controlled, and the amount found in the tree is not affected by the supply of calcium in the soil, unless the soil is deficient in calcium.
Transportation & distribution
Calcium that is taken up by the roots is distributed throughout the tree principally with water.
The calcium moves up the trunk in the xylem (the water and nutrient-transporting vessels), and is taken to parts in the tree, principally the leaves, where the water mainly goes.
Any evaporative surface attracts water, but the water flow into a developing fruit is more complex than that into a leaf.
Upward translocation through the xylem appears to be mainly a passive process. During early development of fruit, the outermost layer of the skin, which has a waxy cuticle, is still relatively permeable to water mainly through stomata. But as the fruit grows and get larger, the outer fruit surface becomes more waxy, and with fewer stomata.
This decreases the transpiration pull by the fruit, and water movement into the fruit by the xylem system is reduced.
There is intense competition between growth of shoots and fruit for available calcium in the tree, and shoot growth is by far the stronger competitor.
Anything you do that stimulates excessive shoot growth will work against an adequate concentration of calcium in the fruit.
Effects of root pruning
When roots are pruned with a machine in spring, the concentration of calcium in leaves and fruit can decrease.
This is not surprising, since root pruning can cut a significant amount of roots, particularly when it is done on both sides of the tree, leaving fewer roots with root tips to take up calcium.
It is postulated that, although competition for calcium between shoots and fruit is reduced in root-pruned trees due to the decrease in vegetative vigour (fewer shoot tips), it is too late for the fruit to accumulate the all-important flow of calcium in early spring, by the time roots have fully recovered.
Calcium in the fruit
Calcium, as calcium pectate, is an important constituent of the cell wall.
Calcium is also involved in the functioning of membranes and regulates permeability of membranes. A membrane is the external wall of a cell.
The calcium concentration of fruit and of leaves differ greatly, with fruit having a lower concentration of calcium than leaves do.
Calcium concentration can also differ between fruit according to position on the tree. Fruit higher in the tree or at the extremities of branches have less calcium in the flesh than do fruit lower on the tree or near the trunk.
Fruit on old spurs or old fruiting wood have less calcium than fruit on young spurs and wood.
Within the fruit, distribution is also uneven—calcium being more concentrated in the skin and core than in the flesh.
A longitudinal gradient also exists between the stalk and calyx end of the fruit, with calcium lowest in the calyx end.
Concentrations of fruit calcium can vary from year to year.
(Continued next month)
For more information and images, see Tree Fruit May 2013
