Boron Deficiency in Crops for Beverages

Boron deficiency in agave

The first signs of boron deficiency in agave are yellow spots, most numerous near the tip, on both surfaces of the leaf. These are followed by the formation in the epidermis of ramifying finger-like depressions from the leaf margin, which may later become suberized. In sand culture experiments, symptoms have included hooked leaf tip and leaf spine that is absent or reduced to a white hair.

In cases of severe deficiency, the growing point becomes disorganized and the leaves are short, narrow, twisted, and sometimes split. The plants have a flat-topped appearance.

Boron deficiency in cocoa

One of the first signs of boron deficiency is profuse chupon formation and the appearance of a few curled leaves which are almost normal green in color. When the deficiency is mild, normal and boron-deficient flushes may alternate. As the deficiency progresses, leaves on the new shoots become very chlorotic and are distorted and twisted. Most of the leaves that are formed under conditions of acute boron deficiency fall before they harden. The leaves that mature become brittle and rough; they remain green but the formation of necrotic patches at the leaf tips is common and is the most typical symptom. Suberization of the veins also occurs.

Profuse flowering on the main stems and branches can be expected and the flower cushions may swell. Ultimately, the terminal buds die as may the axillary shoots which subsequently develop. The growth habit becomes bushy. The trunk and branches may crack and tend to bleed.

Boron is essential for normal flower formation and especially for seed set. Pollen viability and growth of pollen tubes can be reduced by boron deficiency resulting in parthenocarpy and distorted pods; the vascular traces in the pods may become brown and necrotic. Care must be taken to confirm a boron deficiency that is suspected on the basis of pod distortion and to eliminate from consideration the insect Bathycoelia thalassina, which can also cause pod malformation.

Boron deficiency is more readily shown in Amazon cocoa than Amelonado. This may be associated with the more rapid growth and high yields of Amazon cocoa which, being a more shallow feeder, may be more susceptible to drought-induced boron deficiency.

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.

Boron deficiency in hops

Boron deficiency can cause the growing points to turn brown and potentially die when they are only a few inches long. The root stocks produce many shoots with short internodes, many of which will die, giving the plant a stunted, bushy appearance. Axillary buds on the shoots which survive may also become necrotic and die although some of the stunted lateral shoot eventually develop.

Leaves produced on such shoots tend to be small, distorted, and double-toothed rather than lobed. Stipules develop normally at first but quickly become necrotic from the tip downward as they age. Many inflorescences turn brown and die. The cones that form are small, loose, and have a scorched appearance which starts at the tip and progresses to the base. The yield is severely reduced. Root systems of deficient plants develop poorly.

Boron deficiency in tea

The first sign of boron deficiency in tea plants is the restriction in growth of the terminal bud, which becomes dormant. The leaves become dark green, thick, and leathery; and they are frequently misshapen and crinkled. The growing point ultimately dies and, as a result of the loss of apical dominance, many axillary buds try to grow but these also die back if boron is in short supply.

Clusters of small shoots fill the upper axils after a succession of abortive attempts at shooting. Translucent oil spots on the lower surface of mature leaves have also been reported but such spots do not persist.

As the deficiency progresses, excess cork develops—first on the upper side of the petiole, but later extending to the main and lateral veins outlining them with a corky streak on both upper and lower surfaces. The veins crack as the cork develops. Corky streaks may develop on the stem rather like elongated lenticels.