Calcium & fruit growth: In early spring, while the cell walls in the fruit are being formed, calcium is mobilised and translocated from bark and woody tissues, where the calcium was stored the previous year, and is the main source of calcium for flowers, young fruit and emerging leaves.
Maximum concentrations are achieved when cells have stopped dividing, and then calcium moves slowly into the fruit.
Movement of calcium into the fruit slows down after cell division has been completed.
As the fruit expands, the same content of calcium is spread through a larger volume and the concentration in the fruit will actually decrease (i.e. the calcium is diluted as the fruit grows).
This is particularly the case in the outer layer (cortex) of the flesh of the fruit, which has the lowest calcium concentration, and which expands the fastest.
In contrast to calcium, fruit is continuously supplied with potassium, magnesium, phosphorus and nitrogen. The tree can transport these nutrients both in the xylem and in the phloem (food-conducting tissue under the bark of trunk and stems).
Calcium does not move in the phloem, probably because living cells maintain very low concentrations of calcium in their cytoplasm. It is only the pool of nutrients in the cytoplasm that is transported in the phloem.
This calcium continues to accumulate in the leaves and remains there, whereas potassium, for example, can move out of leaves into a developing fruit, along with the sugar needed for fruit fill.
Need to apply calcium directly to fruit
This explains why calcium sprays need to be applied directly onto the fruit to raise the calcium concentration of the fruit.
Calcium that lands on the leaves is not enough to raise the calcium concentration of the flesh of the fruit.
Effects of temperature on Ca movement
High temperatures and drought stress can shift calcium away from fruit toward leaves in the transpiration stream as the surfaces of leaves lose water about 10 times faster than fruit do.
During hot dry and windy weather (high water demand) calcium by-passes fruit altogether, or may even be removed from the fruit.
About twenty per cent of the calcium can move out of the fruit when there is hot weather three weeks before harvest.
Although other factors are involved, such as strong tree vigour and low crop load (high leaf : fruit ratios), fruit disorders can be triggered by abnormal weather that causes water or heat stress to the fruit. Thus, during unusually high temperatures in spring, summer or autumn, the stage can be set for fruit trees to develop various fruit disorders during storage.
Ca in stone fruit
Calcium concentrations in stone fruit are usually adequate, unless trees are very vigorous.
A light fruit set and/or nitrogen fertiliser applied in spring, induce strong tree vigour. Then the calcium that is absorbed by the roots can be diverted to growing shoots.
Effect of chill hours on apples
Apple varieties such as Cox’s Orange Pippin, Jonathan, Spartan, Golden Delicious and Granny Smith, which are sensitive to the amount of hours of cold (chilling) that trees receive in winter, appear to be more susceptible to disorders due to calcium than are less chilling-sensitive varieties such as Pink Lady™.
Calcium and water core
Low concentrations of calcium in fruit can cause water core in apples.
Water core is a disorder of apples in which some or most of the flesh and core area become hard and glassy. In severe cases, the glassy core can be seen from outside the apple.
Water core may itself downgrade fruit, or it may lead to water core breakdown during and after storage.
What you can do to ensure that adequate calcium reaches your fruit
Because disorders due to low concentrations of calcium in fruit are a direct result of the way calcium is distributed (partitioned) within the tree, you must do two things.
The first is through cultural practices, so more calcium goes to fruit and less to shoots and leaves.
The second is use of calcium sprays and drenches.
Cultural practices
Management of trees and irrigation which limit excessive vegetative growth have potential to improve fruit calcium concentrations of calcium in fruit. The following cultural practices are suggested.
Maintain good soil structure allowing water and oxygen to reach the feeding roots of your trees.
The tips of feeding roots are the only part of the roots that can take up calcium. The unsuberised part of a feeding root lasts about one to four weeks before the root tip becomes suberised and inactive.
For roots to take up enough calcium, it is therefore necessary that roots are active. Roots are active only when there is adequate water and air in the soil.
Mature fruit trees have two flushes of new root growth—the major flush is in early spring ahead of the main flush of shoot growth.
Another flush of new root growth occurs in late summer/autumn when shoot growth has stopped and fruit have been harvested.
Make sure that you monitor water in the soil correctly, because roots are not active in very dry soils or in soils that have been waterlogged.
Use renewal pruning to grow fruit on quality wood, and maintain a good balance between tree vigour and cropping.
To establish a good flow of calcium, maintain a satisfactory ratio of leaf to fruit. To do this:
- remove water shoots in summer
- encourage a large transpiring leaf area on young apple and pear spurs; trim plum, cherry and apricots spurs; and shorten peach and nectarine laterals.
Well-developed leaves close to the fruit help to drag a good transpiratory flow of water and calcium through the wood that bears fruit.
Remove old tired spurs and spent laterals.
Thin fruit at the correct time and cropping level, so that the remaining fruit do not need to share their potential supply of calcium with fruit you may thin off later.
Because size of fruit and crop are usually negatively correlated, a small crop usually consists of large fruit likely to have low concentrations of calcium.
Most of the calcium in the fruit enters during early fruit growth. When the fruit enlarges this dilutes this calcium, and excessive size may reduce the calcium concentration below that necessary for normal function of cells.
Do not prune excessively hard and do not cut upright shoots in half. When you prune your trees in winter, make thinning cuts instead of heading cuts.
continues next issue
See this article and images in Tree Fruit June 2013
