Getting Boron right in Avocado – more about physiology than deficiency

Boron is a tricky nutrient to get right, as there is significant variation between crops in the way they transport and metabolise it. On avocado it is more complicated than on many crops due to its production of two types of sugar one of which transports boron and one of which doesn’t. Here we discuss how boron works in avocado, and how we can learn from this to improve yields.

To better understand how boron works in avocado we need to understand the effects of transpiration and photosynthesis on how it is distributed.

Boron is transported from the roots as boric acid, which is not phloem mobile. This means that the plant relies on xylem to move boron with water upwards through the plant via transpiration stream. The foliage gets the majority of boron taken from roots, and it is preferentially distributed to the outer canopy which receives more sun rather than the inner parts of the tree.

Any conditions that limit water transport, limit boron uptake from roots. This includes heat stress, and salinity, which cause the crop to restrict water loss from transpiration via closure of stomata.

Once in the foliage boron is redistributed from leaves to developing flowers and fruit using sugars.

Avocado plants produce two kinds of carbohydrate from photosynthesis, simple C6 sugars (glucose, stored as starch in the fruit), and complex C7 sugars (D-mannoheptulose, transported as the sugar alcohol perseitol). Avocado plants use perseitol to translocate boron to the developing flowers and fruit.

The production of perseitol in the leaves is necessary for moving boron to developing avocado, and that if the plant reduces production of complex C7 sugars boron supply to the crop is restricted.

Agronomy and the weather have a greater effect on boron supply to the developing crop than the levels of boron applied to the crop.

To get adequate boron levels into the developing fruit farmers must manage water, stress and canopy light.

Water supply and weather has a larger influence on boron status than boron levels in soil, as it dictates how much transpiration occurs and how much boron (and calcium) move through the plant and where they accumulate.

 High temperatures, drought and salinity all reduce transpiration and the plants capacity to move boron from soil to leaf. This is why foliar applications of boron are an important tool for avocado growers to maintain fruit development. However on their own they do not solve the problem as light levels also have a role to play in boron transport in the crop.

Growing conditions also effect the relative production levels of C6 sugars, and C7 (D-mannoheptulose) produced in the leaf. If leaves are shaded they produce lower levels of C7 sugars, and switch to producing predominantly C6 sugars. This is thought to be a functional adaptation to low light to save carbon and allow continued supply of sugars to the developing fruit.

This creates a practical problem for growers of avocado, as the more shade in the canopy the less boron is available for fruit set and development. Making use of shade to avoid sunburn, planting density and pruning a larger influence on boron supply to the crop than the level or form of boron supplied.

If foliage is producing low levels of perseitol, no matter how much boron is applied to roots or leaves very little will move into the fruit.