24 September 2004 Research Gives 'Added Value' to Qualifications – Dr Mike Mickelbart

Research-informed teaching sets horticultural study at Lincoln University apart from many other types of horticultural education and gives the qualifications 'added value'.

"A background of studying in a research-active environment and in research-informed programmes, gives holders of Lincoln University horticultural qualifications special appeal in the job market," says Lincoln University's Ian Collins.

"Research plus other factors such as collaborative partnerships with national and international organisations and the 'real world' relevance of Lincoln's teaching to particular industries all contribute to the added value of the University's qualifications."

Among the lecturing staff whose teaching offers the benefits of 'research-informed teaching', collaborative links with international institutions and relevance to industry is Dr Mike Mickelbart, who teaches Plant Science and Plant Physiology to horticultural and other students.

Here Dr Mickelbart tells of important work he is doing in the area of avocados and the scientific basis for pruning. Its ‘real world' research relevant to the significant avocado sector of the New Zealand horticultural industry.

One of the major limitations to avocado production is 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 with regards to avocado pruning: either the trees 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 unit tree, or through loss of yield as the tree grows back from the stump.

Pruning fruit trees should be done to, among other things, maximise the productivity per unit area of the orchard. 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. During that time, there has been some debate as to which is the best system. Some people favour a vase-type system (similar to what is used for peaches), whereas others recommend a central leader system (similar to what is used for apples). So how to decide?

Here 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 into the canopy. The 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.

We have shown that avocado leaves respond 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 on a physiological level, 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.

Lincoln University offers the following horticultural qualifications –

Bachelor of Science, Plant and Horticultural Science stream,; Bachelor of Commerce (Agriculture) in Horticultural Management; Diploma in Horticultural Management; Master of Horticultural Science; Diploma in Horticulture; Graduate and Postgraduate Certificates and Diplomas in Applied Science; PhD.

For further information see www.lincoln.ac.nz

Ian Collins, Journalist, Lincoln University, Canterbury, New Zealand