Stress (continued from last month)
Stress starts in the roots. Stress causes all of our problems. Therefore our problems start in the roots. How does this happen?
When leaves are high in nitrate nitrogen, they form more organic acids. The higher the nitrogen, the more acids will be in the leaves. These acids want to be neutralized by either potassium, magnesium, or calcium. They prefer calcium.
They cause a gradient increase in all tree parts (a giant calcium drag) toward the leaves where the acids are created. The leaves take away calcium from the fruit and new root hair areas.
Boron will help maintain calcium in the cell structure by cross linking the calcium in cell walls. This is why Stoller’s SETT/CaB incorporates boron.
We all know that the vigorous shoot growth causes fruit disorders and poor storage problems. This is not, however, the worse part of the problem. The horrendous problem is underneath the soil surface.
When the strong sink for calcium is occurring in the leaves, causing a huge demand for calcium at the leaves, calcium is being diluted at the root hair surface—at a time when new root growth is struggling.
There is not enough calcium for new cell wall development. New root tissue falls apart and starts to ‘leak’.
The hormones supplied by the roots decrease. This is the beginning of stress. It occurs more rapidly under drought conditions or heavy rain.
No matter what causes stress, the tree or vine responds the same way:
- Proteins are hydrolysed to ammonia
- Ammonia becomes toxic
- This causes an increase in ethylene (the aging hormone)
- The ethylene encourages enzymes that act like ‘Pac Man’. They start ‘chewing’ on the cell walls. This causes more leakage of the roots, leaves, and wood.
The most damaging aspect of stress is the formation of ethylene. It is ethylene that causes flowering. It also is responsible for flower and fruit abortion. The greatest damage comes from increasing the enzyme that causes:
- Storage problems of the fruit
- Physiological disorders
- Disease susceptibility
- Tree death.
How does one control ethylene?
It is controlled by the hormones that are supplied by the roots.
We must keep roots alive and healthy. If roots become dysfunctional, it may be possible to spray the hormones directly to the leaves.
If we could control the ethylene formation caused by stress, we may be able to increase nitrogen use and reduce the amount of pruning.
Table 1 describes the role of various crop inputs. You will notice that nitrate has an important role but so does amine N, so it is the balance that matters.
(continued next month)
For information, see Tree Fruit Dec 2014
