By David R. Sandrock
Limiting nitrogen runoff while maximizing nutrient uptake are goals of all ornamental plant growers.

At Oregon State University's Department of Horticulture, research is being conducted under the direction of Leslie H. Fuchigami to reduce nitrogen pollution and enhance plant growth by manipulating nitrogen fertilization practices.

Researchers are interested in improving nitrogen use efficiency in growing systems by understanding the plant's physiological responses to nitrogen. The goal is to develop ways of optimizing nitrogen in the plant while at the same time minimizing the amount of nitrogen released into the environment. Some approaches under investigation include:

* Foliar applications of various nitrogen forms.

* Timing of nitrogen applications.

* Development of a computer model to reduce spring nitrogen application based on soil temperature and growth stage of the plant.

* Efficiency of foliar vs. soil applications of nitrogen and their effects on plant growth.

* Using N15 isotopes to monitor the allocation of nitrogen in the plant and soil.

* Using defoliants to enhance nitrogen mobilization out of the leaves and into storage tissues.

* Breakdown and mobilization of Rubisco (a storage form of nitrogen in leaves).

Several success stories have emerged from this research. For example, studies conducted by Lailang Cheng at Cornell University, Shufu Dong at Oregon State and SungHee Guak at Pacific Agri-Food Research Center in British Columbia, Canada, members of Dr. Fuchigami's lab, have shown that reserve nitrogen is more important than reserve carbohydrates for spring re-growth of ornamental and fruit trees.

Reserve nitrogen levels are highly correlated with spring regrowth, whereas reserve carbohydrate levels are not. This is not surprising considering that late-season reserve nitrogen storage requires carbohydrate "skeletons" for construction of amino acids. Therefore, reserve carbohydrate levels in the spring are reduced and less correlated with spring re-growth.

Foliar sprays and acclimation

In another study, the researchers have shown that nitrogen applied to the foliage (2 applications at 3 percent urea, one week apart) after bud set does not delay cold acclimation, as does soil-applied nitrogen.

Building nitrogen reserves in plants with late-season soil applications is sometimes difficult for growers since it can reduce cold acclimation. Fall foliar nitrogen applications allow for the buildup of strong nitrogen reserves without risking cold damage.

Results of these studies are having an impact on how and when growers apply nitrogen to their trees and streamlining the fertilization process. For example, growers have relied upon soil-applied nitrogen to build reserve pools and early spring soil applications of nitrogen to help the first push of growth in the spring.

Now foliar nitrogen applications in the fall allow growers to use less nitrogen than traditional fertilization practices while producing plants with strong nitrogen reserves.

In a third study, the researchers have been able to use 1 percent copper chelate, a chemical defoliant, in combination with foliar urea sprays to achieve early defoliation and maintain plant nitrogen reserves. It is important to note that defoliation does not induce dormancy. Therefore, it is mandatory that the applications be made after bud set and prior to defoliation.

In the past, early defoliation has often resulted in trees with lower nitrogen reserves because the leaves are unable to transport their nitrogen to storage tissues before being severed from the plant. However, by applying foliar urea just prior to defoliation, the leaves are able to mobilize adequate nitrogen (as amino acids) to the storage tissues before abscising.

This allows growers to harvest trees with optimum nitrogen reserves earlier and avoid problems brought on by the inevitable wet winters, such as increased disease pressure, longer harvest period and muddy employees and equipment.

Several Oregon nurseries have incorporated these practices into their production plans and report that it has changed the way they grow trees. All of these practices allow nurseries to produce a quality tree with optimum nitrogen while avoiding the release of excess nitrogen into the environment.

What's next?

Investigations into nitrogen use efficiency are essential. As concerns about nitrogen pollution increase, so will the demands on nurseries and greenhouses. With continued support from the industry, researchers at Oregon State will continue to investigate ways of optimizing plant nitrogen while minimizing the amount released to the environment.

Working together with government agencies, researchers and the public, nurseries and greenhouses can continue to strive for more efficient use of nitrogen in their production plans. Decreasing nitrogen pollution while producing quality plants is a goal that will benefit the consumer, the grower, and the environment.

© 2001 Branch-Smith Publishing