Students of California State University, Fresno pomology professor Dr. Gurreet Brar are actively involved in research to better understand the physiology of pistachio trees, including how it impacts fruit yield.

Brar’s research, using a hands-on approach funded by the California Pistachio Research Board (CPRB), focuses on a better understanding of how chill hour accumulation effects pistachio tree performance. He and his students are also evaluating mechanical and chemical strategies to enhance winter chill accumulations.

“They are learning a lot of basic principles of pomology, plant physiology at the whole tree level, and how to apply this knowledge in the field,” Brar said.

Fresno State’s 25 acres of mature pistachio trees provide students with this opportunity, including onsite studies during this fall’s nut harvest.

Brar, who joined CSU, Fresno’s faculty last year, holds the Rodger B. Jensen Endowed Professorship in Pistachio Physiology and Pomology. The faculty position was established in 2013 with a pledge of $1.5 million from the CPRB.

His focus is applied research in pistachio physiology, teaching courses in pomology (fruit production), and solving industry challenges. California is a major pistachio producer in the world, due to ideal growing conditions, plus industry technology and research support.

Along with teaching, Brar is also hiring and mentoring graduate students to work on pistachio research projects. Besides the chill accumulation work, students are studying the physiology behind the pistachio being in most cases an alternate bearing tree. Brar is studying bud retention and the carbohydrate budget through the growing season and dormancy to find ways to avoid the alternate bearing trait.

Daniel Syverson, a graduate student researcher, is focusing on the mystery behind pistachio alternate bearing cycles. His goal is to increase production in the ‘off year’ without reducing yields in the ‘on year.’

Syverson says the main symptom of alternate bearing is flowering bud (the fruit for the next year) abscises during an on year. The buds can be sprayed with plant hormones to prevent loss. But instead of falling off the tree, the buds die but remain on the tree. He has not yet determined why.

According to Syverson, most researchers believe the cause is likely nutritional or hormonal. Some blame bud death on the developing crop using all of the tree’s nutrients. The other theory is the current crop may secrete hormones that impact the buds.

He believes it could be both scenarios since choosing one or the other approach to bud retention still causes bud failure. Finding the right synergies between nutritional and hormonal treatment is the likely way forward.

The graduate student wants to explore more of an applied approach that could benefit pistachio growers. He wants to find out if nutrient demands or secreted hormones alone can cause flower bud death.

Other work by Syverson at Fresno State includes developing procedures for monitoring tree nutrition status using a photosynthesis measurement machine to monitor pistachio tree leaf nutritional status. He’s also working with Brar on yield and grade data analyses from this year’s harvest for chill accumulation studies.