Protea flowers are beautiful, but their leaves often turn black soon after being cut, which hurts their sale. This article investigates why this happens and what can be done about it.
The Problem:
Within days of cutting, Protea leaves become limp, then develop black areas. Within a week, the leaves can be completely black. This happens because chemicals in the leaves called hydroxyphenols and tannins react when the cells inside the leaf break down.
Possible Causes:
Several stresses can damage plant cells and cause this reaction, including:
- Chilling injury: Damage from cold temperatures.
- Anoxia: Lack of oxygen.
- Ethylene: A gas that plants produce that speeds up aging.
- Water stress: Not enough water.
- Carbohydrate shortage: Lack of sugars for energy.
Scientists initially thought water stress was the main culprit. However, new evidence suggests that a lack of carbohydrates might be the real problem.
New Evidence:
Researchers found that treatments designed to reduce water loss didn't prevent the blackening. However, keeping the cut flowers in low light made the problem worse. This is because the leaves couldn't produce enough carbohydrates through photosynthesis in low light.
Further support for this theory comes from the fact that removing the flower head (the inflorescence) or interrupting the flow of nutrients below it (girdling) reduced blackening. This is likely because these actions reduced the demand for carbohydrates from the leaves.
Testing the Theory:
To confirm the role of carbohydrates, scientists tested the effects of supplying sugar directly to the cut stems and tracked the changes in the leaves' carbohydrate levels.
Results:
Adding sugar to the water extended the time it took for the leaves to blacken. Leaves kept in light maintained their carbohydrate levels and did not blacken. However, leaves kept in darkness quickly depleted their carbohydrate stores and turned black. This supports the idea that carbohydrate depletion is a key factor in leaf blackening.
What does this mean for Protea growers?
The study suggests that ensuring adequate carbohydrate supply to cut Protea stems is crucial for preventing leaf blackening. This can be achieved by:
- Adding sugar to the water.
- Keeping the flowers in well-lit conditions after cutting, at least for a few days.
- Exploring pre-harvest techniques to maximize carbohydrate stores in the leaves.
Other interesting findings:
The research also revealed that Protea leaves contain a unique sugar called polygalatol in high quantities. Unlike other sugars, polygalatol isn't used for energy and remains in the leaves even when they are starving. This might be an adaptation to their natural dry environment but doesn't help prevent blackening.
Future research:
The researchers suggest further studies to explore the best ways to manipulate carbohydrate levels in Protea before and after harvest to extend their vase life. This includes investigating optimal light conditions, sugar solutions, and other potential treatments.
