LINCOLN, Neb. -- It all started during a study of leaf senescence -- the color changes or loss of tree leaves in the fall or the decay of leaves and transfer of nutrients to seeds in crops.

Ellen Paparozzi, University of Nebraska-Lincoln horticulturist, found that when the common Swedish ivy plant was exposed to nitrogen stress it would go into a suspended state without leaf loss and then grow again when nitrogen was reintroduced.

"The plants turned yellow, but six months later the leaves were still on," the Institute of Agriculture and Natural Resources scientist said. "They were yellow with purple stems, but they were still there."

After nitrogen was added back to the plants the leaves re-greened in about three weeks. Paparozzi and her team of UNL graduate students, UNL UCARE Program students and a research technologist found they could do this repeatedly and the plants would start and stop growing but not lose any leaves.

"So we have a plant that repeatedly re-greens and also goes into a suspended state when nitrogen is taken away and other elements remain present," she said. "It was just such a fascinating physiological phenomenon, and a mechanism I never thought plants had, so I had to continue to work on it."

Now, the researchers are trying understand why this happens, isolate it in the plants' physiology and some day replicate it in other plants.

Experiments growing Swedish ivy were done hydroponically with two levels of nitrogen, 0 or 150 milligrams per liter.

"If this suspended state could be induced into any plant and that plant could be brought back by just adding nitrogen, you could ship those plants anywhere, even to Mars," Paparozzi said. "I talked to a colleague at NASA who said this is a great idea and asked when we could have this in fruit plants. That is the potential he saw. So this offers a huge avenue for shipping plants."

Paparozzi has spent years trying to unravel this mystery and find out just what exactly happens.

She and her team are exploring a number of factors including: chlorophyll concentration, photosynthetic rate, protein and nutrient content. They're making both microscopic and visual observations.

For example, photosynthetic rates dropped when nitrogen was taken away, but after a lag did come back. Chlorophyll concentration never fully recovered.

Electron micrographs indicate huge starch grains in the chloroplasts, so Paparozzi thinks the plant is a starch accumulator, meaning it stores starch, which might have something to do with the suspended state.

Scientists also are looking into the effect of cytokinins inside the plant, as just spraying the plant with cytokinins had no effect. Cytokinins are the hormone known for reversing leaf yellow in plants.

The project just got a collaborator whose lab specializes in cytokinin analysis. This lab in the Czech Republic is one of only four labs in the world that does this type of procedure.

One of the UCARE Program students also found another species of Swedish Ivy that doesn't respond the same way. Without nitrogen, the leaves turn yellow and fall off.

"So the cytokinin analysis comparing both plants will be especially valuable at this time," she said. Paparozzi also plans to look more at the anatomy of the plants, particularly the abscission layers, the layer that forms to detach the leaf from the stem. They also hope to find a way to study the genetics of this plant.

This research is conducted in cooperation with IANR's Agricultural Research Division and the Hatch Act and National Foliage Foundation.

SOURCE: University of Nebraska.