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For many pear growers, irrigation seems like just a routine chore, but irrigation may be the single most important factor determining the size and quality of the fruit, says Dr. Ken Shackel, ­pomologist at the University of California, Davis.

Trees need water because, in order to make sugar to support the tree and fruit growth, the leaves need to stay open and allow carbon dioxide to enter. Carbon dioxide is needed for photosynthesis, and the problem is that when carbon dioxide enters the leaves, it allows water to diffuse out, Shackel explained during the annual conference of the International Fruit Tree Association in California.

The environment determines how much water is diffused out. On a hot day, a lot of water comes out of the leaves, but even on a cool, humid day, many hundreds of molecules of water can be lost for every molecule of carbon dioxide that enters. As a result, tree roots are always having to take up water to replace the water lost by transpiration, but the amount of transpiration is driven by the weather, he said. The water lost is evapotranspiration.

When using the standard water-budget method of irrigating, growers tend to think of the soil as a reservoir or a gas tank, Shackel said. As the tree draws on the gas tank, the level goes down, and at some point, growers decide that the tree needs to be irrigated. Evapotranspiration (ET) is not the only thing to pay attention to, Shackel stressed, though it’ a good place to start. There are numerous Web sites where growers can look up the ET rate for their crop and location at a particular time of year.

Shackel discussed research he conducted from 1992 to 1994 to determine the influence of irrigation and nitrogen fertilizer on fruit size, fruit quality, fruit storability, and tree vigor of pear trees in the Ukiah area of California.

The research was done on mature Bartlett trees planted 12 by 20 feet apart. The site had deep soil and was next to the Russian River.

For the study, the trees received one of three different irrigation regimes: 100 percent, 80 percent, or 65 percent of ET. In addition, there were two fertilizer treatments: 400 pounds of nitrogen per acre per year versus no nitrogen added during the years of the study.

Results in 1993 (which were typical of all the years of the study) showed that average fruit weight differed by only 3 grams between the 100 percent ET treatment and the 65 percent ET treatment, from 162 grams to 159 grams, respectively, which was not statistically significant. Shackel said it was surprising that such a large difference in the amount of irrigation applied would result in such a small fruit size difference.

However, there was great variability in fruit size within the same treatment. For example, fruit size for the 100 percent ET treatment ranged from 137 to 197 grams, and for the 65 percent ET, fruit size ranged from 116 to 196 grams. The mid treatment of 85 percent ET ranged from 125 to 181 grams.

Shackel said some trees in the experiment were just poor-sizing trees that would always produce relatively small fruit, regardless of the irrigation treatment.

In the experiment, Shackel and his colleagues harvested every fruit from some of the trees to see what the fruit size distribution looked like and found that on a good-sizing tree, even though there was quite a spread in fruit size, the fruit was much bigger than the fruit on a poor-­sizing tree. The difference was not related to the irrigation, but it was related to the water deficit that the scientists measured in the trees, using a pressure bomb.

Pressure bomb

A pressure bomb is a device for applying air pressure to a leaf. Most of the leaf is inside the chamber, but a small part of the leaf stem (the petiole) is outside. Pressure is applied until water is squeezed out of the petiole. The more pressure it takes to squeeze the water out, the greater the water stress of the tree.

Measurements are usually done between noon and 3 p.m. Because tension is measured, negative values are typically reported. The more the stress, the more the plant is experiencing a deficit of water. The scientific name given to this deficit is the  water potential of the plant.

Shackel and his colleagues measured the water potential of all the trees in the experiment and found that the more stress the trees were under, the smaller the fruit. Trees in the 100 ET treatment covered the range from very wet trees with big fruit to dry trees with small fruit. A surprising result was that some trees in the low-irrigation treatment produced good-sized fruit.

The water stress was also related to the soluble solids of the fruit, with stressed trees producing fruit with more solids.

Fruit color was also affected, with the wetter trees having greener fruit and fruit on the drier trees being more yellow.

Shackel wondered if these were direct effects of the water stress experienced by the tree or if there was another factor determining the wide size range. He looked at the effect of crop load on fruit size, but found that fruit size was not determined by the crop load.

The loads, at least for the kinds of loads and fruit size we had, were not influencing fruit size in a negative way, he said, concluding that the fruit size effects were caused by the water stress. Just because you irrigate, it doesn’t mean that the tree is going to be wet he said.

For some reason, those hundred-percent ET trees were very dry. Just basing ­irrigation on a water budget is a fine way to start, but you need to check the tree.