Sun Access The Secret When Pruning Avocados

Enjoying those avocados with your summer salads, hors d’oeuvres and other seasonal fare? They’re enjoying them in other parts of the world too, with the New Zealand industry exporting a million trays annually to such places as Australia, Japan and the United States. Their popularity is such that market analysts believe there’s the potential to lift exports to five million trays.

One of the major limitations to avocado production is, however, the large size that the trees ultimately reach. Any experienced avocado grower will tell you that as trees mature, they develop into a “shell” of leaves on the outside, with a large hollow space on the inside. That hollow space is wasted productive area—fine if you want to have a picnic, but not if you want to maximise yields.

Historically, one of two things has been done about the management of avocado trees: either they are left to grow, with no pruning (which results in the “shell”), or they are “stumped” to the trunk every so often and allowed to grow back. Both of these systems result in lost yield either through a decrease in production per tree, or through loss of yield as the tree grows back from the stump.

Pruning fruit trees should be done to maximise the productivity per unit area of the orchard, among other reasons. But how do we decide which pruning system we will use for a particular fruit tree?

It was only 10 to 20 years ago that growers and researchers contemplated developing pruning systems for avocado trees. Since that time there has been debate about which is the best system. Some people favour a vase-type approach (similar to that used for peaches), others recommend a central leader system (similar to that used for apples). So, how do we decide which is preferable?

At Lincoln University I have been involved in a collaborative programme with researchers at the University of California, Riverside, for the past few years that aims to answer this question, based on the physiology of the tree.

A vase-type pruning system relies on light penetrating the canopy. Short, intense bursts of light into the canopy are known as sun flecks, or light flecks. They are often the same intensity as full sunlight, but only last for anywhere from a few seconds, to a few minutes. Once the light has penetrated the canopy, the tree must be able to utilise that light for photosynthesis.

Our research has shown that avocado leaves respond only very slowly to these light flecks. In other words, a leaf on the inside of the canopy is carrying out photosynthesis at a very low rate, or maybe not at all. When a light fleck hits that leaf, the photosynthetic rate will increase, but the response time takes several minutes to almost an hour (depending on the age of the leaf). This response time is too slow for the leaf to effectively photosynthesize—by the time it starts, the light fleck is gone.

This is interesting physiologically because avocado leaves appear to be very slow to respond to changes in light level—much slower than most other species studied so far. The bottom line is that even though the leaves on the inside of the canopy are receiving some light, they are still not contributing to the overall productivity of the tree. The productive leaves are on the outside of the tree. They are exposed to light for a significant portion of the day, and contribute most of the photosynthate to the tree.

With this information, we can predict that a central leader training system—one in which a majority of the leaves on the tree are exposed to full sunlight—is best for avocados. A vase system that relies on light flecks will not result in optimal productivity. The problem is that avocado trees are not that amenable to pruning, so even though we know what we should be doing, based on the physiology of the tree, we still have to figure out how to do it.

This is just one of the practical outcomes that has come from our fundamental work on avocado physiology. We are also looking at how trees respond to temperature and water. This information will result in a model of canopy growth and provide growers with scientific information for making practical decisions.

• Dr Mike Mickelbart is a lecturer in the Plant Sciences Group at Lincoln University and he teaches Plant Science and Plant Physiology to horticultural and other students.

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