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New Water Sensor Tested on Tomato Petioles

The Tomato Magazine
August 2005

By Lisa Lieberman

While this has been a relatively good water year for Fresno tomato growers—most have received 85 to 90 percent of their federal water allocations—many are feeling more pressure than ever to conserve water. At the same time, in order to make ends meet, higher Brix levels and tonnage are also very important.

In an effort to address these issues, Dr. Bill Gensler, a retired professor from the University of Arizona electrical engineering department, has come up with a new water sensing system for tomatoes. It is designed to enable growers to better determine how much water a plant really needs by measuring water directly from plant petioles.
Traditionally, growers have used soil moisture measurements and relied on historical weather patterns to help determine their irrigation scheduling. But now, according to Gensler, his system of measuring water directly from the plant should help remove much of the guesswork from irrigation scheduling.

How It Works
In the system developed by Gensler, a tiny filament, about three times the thickness of a single hair, is inserted into the petiole of a tomato plant.

“It acts like a water dipstick—just like the oil dip stick you use in your car,” Gensler explains. “You dip the stick into the plant and it has wetness over a certain area; then you use that measurement to make an evaluation of how much water the plant needs in order to maintain the desired amount of water on the sensor surface.”

The sensors measure water levels in the plant every half hour and transfer the information to a computer which averages the minimum water value over a 24-hour period. The information is then posted on the Internet, helping the grower determine when the plants need to be irrigated.

Gensler has had success using the water sensors in other crops such as wine grapes. Only recently did he begin experimenting with tomatoes and cotton. Tomatoes are a bit more challenging than grapes since the plants grow rapidly throughout the season. Because the sensors are installed shortly after transplant, they usually have to be readjusted at least once during the season to compensate for the tomato plant’s rapid growth.

“We have to move the probe into the upper part of the tomato plant because the plants grow so quickly,” Gensler notes. “The goal is to optimize the location so that we can get the maximum amount of continuous information from the plant for the longest period of time. Generally, sensors work best when they’re attached no higher than six or seven inches below the upper parts of the plant.”

John Bennett, owner of Bennett Ranch in Firebaugh, Calif., has been working with Gensler over the past two years on the experiments.
“We don’t know what all the numbers mean yet, but it makes sense to measure water in the plant and not just in the soil,” Bennett says.
For Bennett, conservation is important because water is scarcer than it used to be and the price of water has more than tripled over the past few years.

Sensing Provides Additional Information
In addition to using the sensors to measure water in the petioles, Gensler also inserts the probes directly into the tomato fruit itself to obtain even more detailed information.
“We do that with wine grapes, and now we’re doing it with tomatoes,” he explains. “We want to measure the relationship between the source of the tomato’s energy from the leaves and the energy sink in the actual tomato. Our goal is to try to figure out how much sugar goes from the leaf to the fruit and when it’s going.”

By quantifying the correlation between sucrose transfer from the leaves to the fruit, Gensler says he wants to help growers both maximize Brix in their tomatoes and tonnage. In addition, he hopes to learn which leaves, in particular, contribute the most to the sugar content of the crop.

“We know where the sugar comes from, so far as the leaf is concerned,” he says. “What we hope to learn is which leaves contribute the most sugar—whether, for instance, it’s the leaves above or below the tomato cluster.”

Gensler also hopes to find out which tomatoes in a cluster receive sugar first and how they receive that sugar.

“If we put a probe into two or three tomatoes, we can find out how and when they receive the sugar,” he explains. “It could be like chicks in a nest, where one tomato grabs the sugar from another one in the cluster.”

By studying the behavior of fruit in a tomato cluster, Gensler believes scientists may also be able to get a better handle on nitrogen activity in the plants and a feel for the best times to apply fertilizer. Although he is still working out the kinks in his system, he hopes it will eventually enable growers to average 55 ton-per-acre and see Brix levels of 5.5 or higher.

Drip and Furrow Trials
Since California tomato farmers use both drip and furrow irrigation, Gensler has been conducting his sensor trials in both types of fields over the past two years. In furrow-irrigated fields, most growers will generally dry the tomatoes out near harvest to increate the sugar to water ratio.

“With drip-irrigated tomatoes, we found you can’t use the same strategy so much because they’re dry in the first place,” he says. “The plants in drip fields aren’t as succulent as in furrow, so you can lose tonnage and not make that much up in sugar content.”

Trying to compensate for the differences in drip irrigation, Gensler has been experimenting in drip-irrigated fields by avoiding putting on too much water at the beginning of the season.

“You don’t want to put so much water into the plants that they put too much energy into making leaves and stems. You want to to keep the water content to a minimum, so the plant puts its energy into making fruit first,” Gensler stresses.

Scientists know that there’s a correlation between vegetative and reproductive growth of plants, but, up until now, it’s been a relatively unquantified subject, the researcher explains.

“There are no figures on the tonnage of leaves compared to sugar in the fruit or yield from the fields,” Gensler says. “We know that the plant’s energy either goes into the stems or into the fruit. What we want to do is maximize both tonnage and sugar content.”

Bennett hopes the sensors will help him in both directions.

“There are a lot more canners concerned about Brix these days because the Brix levels affect their bottom line,” he points out, adding that if growers can produce a tomato crop with a higher sugar level with less water, that would be ideal.

© 2006 Columbia Publishing

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