Issues with nut quality in walnuts may be more related to light interception within the tree's canopy rather than water stress.
Last season researchers started a 2-year project to look at the various impacts of light interception and water stress on nut quality.
Bruce Lampinen, UC Davis walnut and almond specialist updated his work on the topic at the UC walnut day in Yuba City, Calif.
Lampinen tagged 1,000 spurs to monitor their development through harvest.
“Most of the problems we've seen are in densely shaded canopies or heavily cropped orchards,” he says.
Problems in inner canopies with low-level light include shriveled kernels, oilless nuts, black Chandler and kernels with yellow pellicles.
Walnut sizing occurs during the first 14 weeks after bloom, according to Lampinen. Kernel filling begins about 8 weeks after bloom and continues until about 20 weeks after bloom.
“Interruptions in carbon availability, which could be caused by lack of water, excess water, or excessive heat during the first 14 weeks after bloom can result in smaller sized nuts, while interruptions from eight to 20 weeks after bloom can result in kernel filling and or quality problems,” Lampinen says.
Two trials were conducted in San Joaquin County last year with a 9-year-old and a 13-year-old Chandler orchard. One of the primary objectives was to look at the reason for yellow pellicles. “The canopy was like a tarp,” Lampinen says.
Mid-day stem water potential was measured during the trial and irrigation cutoff dates were evaluated — Aug. 1, Aug. 15 and Aug. 30. “By Aug. 1, the leaf water potential had dropped off quite a bit in these blocks,” he says.
However, the impact was not significant. “The irrigation cutoff date had minimal effect,” he says. “There was a little effect on leaf drop, but the effect on nut quality was minor.”
The reason for this relationship, which goes against the common wisdom that nut color is adversely impacted by water stress, is unclear, according to Lampinen. “However, we have now observed this relationship in at least three different orchards over several years,” he says.
In the other part of the study, light interception was measured by placing light sensors on individual leaves. “Usually in the range of 70 percent to 80 percent light interception, you start seeing problems with lower limbs dying back from shading,” Lampinen says.
Additionally, quality problems in nuts start emerging when sunlight is restricted due to canopy cover. The spurs in the lower canopy branches where leaves are lost tend to have more quality problems, according to Lampinen. However, some of the problems are how the nuts are perceived visually, rather than how they taste.
“The yellow pellicles haven't generally been a problem in terms of quality. They generally tasted OK and sized OK, but some of the handlers have had problems when they try to sell them because people think there is something wrong with them.”
Adequate sunlight seems to make the difference in nut quality in the inner canopy of the tree. “We're thinking that you probably need about a half hour of direct light on any given leaf at some point during the day to keep a leaf alive,” Lampinen says. “As soon as the leaf becomes a net negative, the tree will abort that leaf and that's where we'll see these kernels turning black.”
In general, the problems with trees last year weren't as significant as they have been in past years. Researchers are intent on continuing the trials this season to evaluate the situation further.
“One of the reasons we may not have seen a lot of problems last year was because it was a relatively light crop so the trees weren't pulling a lot of carbohydrates to support the crop,” Lampinen says.
Although there are numerous factors to consider and still to be studied, light interception seems to be a major contributor to walnut quality issues.
“The preliminary conclusions from the study are that the nut quality problems tend to be related to the low light conditions,” Lampinen says. “Darker nuts tended to come from lower canopy spurs with lower light levels.”
Additionally, water is a factor in nut color. However, it still remains unclear if mild to moderate water stress is a negative factor.
“Trees kept in the minus four and minus five bar range tended to produce slightly darker nuts than trees kept in the minus six to minus seven bar range,” Lampinen says.
So far, Lampinen and other researchers on the project have analyzed about 30 percent to 40 percent of the walnuts in the study. They plan to follow the same 1,000 tagged spur locations in 2008 to determine how light conditions and cropping patterns influence spur survival.