Researcher finds key to winter tomatoes

By Karen Ball, Guelph Mercury

Summer's over, but there's still time left to enjoy Ontario tomatoes this time of year. Local tomatoes are only available for the summer months, except when farmers grow tomatoes in the greenhouse, thereby extending the harvest into the winter.

However, farmers face challenges with greenhouse production in the winter, including low light levels. University of Guelph researchers are creating new ways to help farmers grow delicious and nutritious tomatoes for the entire year.

Prof. Barry Micallef is teaming up with Profs. Bernard Grodzinski, Department of Plant Agriculture, and Mike Dixon, from the School of Environmental Sciences, to develop new growing techniques that help greenhouse producers extend the tomato growing season into the winter.

Normally, there isn't enough natural light in the winter for tomatoes to grow. To compensate, greenhouse producers can use supplemental lighting.

Tomato plants need exposure to long periods of artificial light to make the practice economically feasible in Ontario. But this lengthy exposure (called "photoperiod") comes with a cost — the leaves turn yellow and die, a disorder termed photoperiodic injury. And no leaves available for photosynthesis means no tomatoes.

"Long periods of light are needed for adequate productivity in the winter when using supplemental lighting, but greenhouse vegetables such as tomatoes respond poorly to it," said Micallef. "The question is how can we improve the tomatoes' response to long photoperiods."

To help, Micallef and his team are developing a new technique for greenhouse growers, called time-of-day fertigation.

Here's how it works. Nitrate is the primary form of nitrogen used in hydroponic tomato production. The uptake and capacity to use nitrate by tomato plants fluctuates throughout the day. This is a classic biological rhythm. Long photoperiods disrupt these natural uptake rhythms, causing an imbalance between nitrate uptake and utilization by the plant.

If too much nitrate is provided during the plants' natural low nitrogen phase, photoperiodic injury occurs.

To solve this problem, researchers are reducing the amount of nitrate provided throughout the day, as well as changing the temperature in the greenhouse, to better mimic natural day and night rhythms of nitrate uptake and utilization by the plant.

"This isn't something people have done before in greenhouse production," said Micallef. "We can really make a big improvement if we go to time-of-day fertigation."

Using this technique will help tomatoes grow under longer periods of light — a critical requirement for winter greenhouse production. This will allow growers to extend the growing season by four months and boost their revenue during the winter.

Current greenhouse trials using the new tomato-growing technique at Great Northern Hydroponics and Erieview Acres Inc. are promising profitable results. They have shown that altering the plant's nitrogen levels during long periods of light produces healthy, green plants — which will get farmers a high price for their produce.

"One of our greenhouse growers is getting three times more revenue per box in the winter than he would get for his tomatoes in the summer," Micallef said.

The new time-of-day fertigation growing technique will provide great economic benefits for greenhouse growers across Ontario.

Next, Micallef plans to perfect the growing technique with his team and continue to develop other methods of extending the greenhouse vegetable growing season.

This research is funded by the OMAF and MRA-U of G partnership and the Natural Sciences and Engineering Research Council.

For more: http://www.uoguelph.ca/plant/faculty/bmicallef/