So why does this matter to fruit growers? farmers in high UV regions also have a harder time correcting physiological disorders. Lets discuss why photo-oxidative stress can affect this.
Stress (any stress) including UV stress triggers movement of calcium from the cell wall into the interior of the cell where it is involved in triggering production of the stress hormone ethylene, this is sometimes termed ‘second messenger’. This means that fruit grown where there is high UV has a higher metabolism of calcium. It moves away from cell walls and must be replaced more often if cell wall strength is to be maintained.
This is nicely illustrated by looking at research the level of tissue calcium needed to prevent physiological disorders. Studies on apples show that in the same apple variety (Cox) the level of tissue calcium needed to prevent bitter pit is higher in South Africa (where UV is high) than the UK. This difference is due to the higher metabolism of calcium in fruit crops grown in the Southern Hemisphere.
The take home message from this research is that you need higher calcium levels to prevent physiological disorders like bitter pit in apples and cracking in cherries in countries with high UV. It is also the case that other factors affected by calcium levels such as shelf life and firmness in soft fruit require higher levels of calcium, as metabolism is faster.
Unfortunately the effect of UV on calcium depletion in fruit crops are most apparent late season as the calcium gradually depletes. This is the time when fruit is least able to replace calcium as there is no active cell division, and the main driver for transport (auxin produced in the seed) is small, meaning transport of calcium into fruit from foliar sprays is relatively inefficient with much of the calcium applied unable to get to where it is needed.
So what can growers do to help?
- Early sprays of foliar calcium on young developing fruit, this is the best time to load the level of fruit calcium up using conventional products.
- Levity’s scientists have developed Albina, a product that can help maintain quality even in late season applications. This is a good way of holding quality in crops grown under high UV in South America, New Zealand and South Africa.