Boron Deficiency in Coffee
Coffee is one of the most susceptible crops to boron deficiency. In coffee cultivation, boron is necessary for:
- Cell division and cell wall formation in interaction with calcium
- Root growth and water uptake
- Growth of internodes
- Fruit size and setting
- Calcium uptake
- Decreasing aluminum toxicity
- Drought and disease resistance
Boron deficiency causes the death of the terminal growing point. The later development of secondary branches (sometimes as many as seven at the same node) below the dead terminal bud has a typical fan-like effect. In severe cases, the secondary branches quickly die resulting in dieback of terminal sections of the new shoots.
In a study published in the Journal of Environmental Management, Muhammad Riaz, et al. demonstrated the importance of boron’s role in decreasing aluminum toxicity in citrus. Boron acted in the regulation of multiple physiological processes and has the potential to increase dry matter weight, plant height, root length, and number of leaves. This is applicable for coffee and other plant species.
Studies have also shown that with increased leaf boron content, there was a reduction in red mite infestation (Tetranychus pioroei). There was a correlation between boron and cyanidin production. Cyanidin is a polyphenol that is toxic to the mites. Another recent study showed that leaves with higher rates of boron concentration had lower rates of damage caused by coffee borer (Hypothenemus hampei).
What does boron deficiency look like?
When a plant is boron deficient, the leaves are typically misshapen and smaller in size. They are often narrow and twisted with irregular edges and have a leathery texture. The internodes are short, leaf tips may fail to develop properly, and the apical portion may then turn a pale olive/green color. In contrast, the basal portion of the leaf remains a deep dark green. It has been suggested that this apical chlorosis is due to a local calcium deficiency which may be the result of boron deficiency reducing calcium translocation.
Defoliation can occur. The underside of the midrib of both chlorotic and otherwise healthy older leaves may become suberized.
Production will be seriously reduced because of poor fruit formation. In nutrient solution studies, it has been demonstrated that boron deficiency does not seem to affect flowering but that, in contrast, fruiting is markedly reduced. In controlled experiments, no fruit was produced even though flowering was regular. This is in accord with work demonstrating that coffee shows a peak demand for boron (and for calcium) just after flowering and when the formed cherry is developing.
When do symptoms appear?
Terminal dieback and the development of crinkled leaves towards the end of a dry period and at the start of the rainy season (due to reduced boron absorption from the dry upper soil layers) are often the first signs that the coffee plant is suffering from boron deficiency.
Symptoms are also particularly noticeable at flowering and after liming due to the reduced availability of soil boron. Boron is mainly used on coffee to prevent the occurrence of the transient deficiency symptoms rather than to correct severe deficiencies which result in considerable branch dieback.