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ARS Home » Pacific West Area » Davis, California » Nat'l Clonal Germplasm Rep - Tree Fruit & Nut Crops & Grapes » Research » Research Project #442946

Research Project: Characterization of Variation in Phenology and Chill Requirement in the USDA-NCGR Pistacia Collection

Location: Nat'l Clonal Germplasm Rep - Tree Fruit & Nut Crops & Grapes

Project Number: 2032-30100-001-001-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Sep 1, 2022
End Date: Aug 31, 2024

The annual farm-gate value of pistachio to U.S. agriculture is almost three billion dollars and is increasing rapidly. Pistachio is more tolerant of drought and saline soils than most other tree crops and is being widely planted in the Central Valley of California where water quality and quantity are often limiting. However, most bearing acreage of pistachio is planted to a single female and a single male cultivar (“Kerman” and “Peters” respectively). Peters has a higher chill requirement than Kerman, and in years with inadequate chilling, Peters pollen production can be delayed to an extent that causes catastrophic crop losses. Low-chill winters are becomingly increasingly frequent in pistachio production areas. However, trees with very low chill requirements are at risk of ending dormancy too soon, before winter is over, and suffering frost damage. Phenology (leafing date and flowering date) measurements are generally well correlated with chill requirement. However, only a single leafing date measurement can be taken from a single tree in a single year. We have developed a chill requirement assay that involves sampling short branches (“sticks”) periodically throughout the winter, forcing them to break bud in permissive conditions (heat, light, and moisture: a tub of water in a growth chamber), and measuring the heat units required to break bud after various degrees of chill (ie: different sampling dates through the winter). Chill requirement phenotyping, although more laborious than phenology phenotyping, provides more data—a data point for each sampling date, rather than a single data point per year. To safeguard the US pistachio industry, protect US pistachio farmers, and take full advantage of this drought-tolerant, salt-tolerant, high-value tree crop, variation in phenology and chilling requirement in the USDA-NCGR pistachio collection should be better characterized, and this information should be integrated with pistachio breeding activities to allow the development of additional female and male cultivars with lower chill requirements.

Phenotyping: All 640 living Pistacia trees in the USDA-NCGR collection, representing 186 total accessions, will be phenotyped for phenology. Phenology phenotyping will be performed by walking the four Pistacia blocks (A-D) weekly from February through April 2023 and noting the date on which 50% of vegetative buds have opened (leafing date) and the date on which 50% of floral buds have opened (flowering date). Weekly phenology scoring necessitates pre-dating and post-dating by up to 3 days to obtain a continuous distribution. All 327 living P. vera trees in the USDA-NCGR collection, representing 96 total accessions, will be phenotyped for chilling requirement. In addition, 5 trees representing at least 3 accessions of each of 6 additional species (P. integerrima, P. chinensis, P. atlantica, P. terebinthus, P. palestina, and P. khinjuk; 30 trees total) will also be included for chill requirement phenotyping. This process will involve harvesting 1-3 dormant “sticks” from each tree on 3-4 sampling dates 2-3 weeks apart during the winter of 2022-2023, from approximately mid-December to mid-February. On each sampling date, sticks approximately 9” long will be cut in the field, tagged, and transported back to the laboratory, where a fresh cut will be made immediately before submerging the cut end of each stick in tube racks in a tub of water (containing ~1 g NADCC per 12 L of water to prevent microbial growth). Tubs will be kept in 25C growth chambers with a 16/8 day/night cycle, and will be monitored twice weekly for signs of bud-break. Bud-break is defined as the presence of any green or red color in cracks between expanding bud scales. Obtaining repeatable data from this assay requires vigorous vegetative growth during the growing season, which results in a supply of straight, uniform, 1-year old sticks for dormant season sampling. This vigorous vegetative growth is promoted by winter hedging, which creates a more open, accessible canopy and prevents more vigorous trees from shading out their neighbors. For each individual tree, the growing degree hours required for budbreak will be calculated for each each stick and plotted against the chill hours received on each sampling date. Linear regression will be applied to the resulting “heat versus chill” plots for each taxon (eg: see Aslamar et al. 2009, Figure 1), and the resulting slope and intercept terms for each taxon will be used for a genome-wide association study, along with phenology data, using genotype data previously collected on these trees. All phenotypes will be deposited in the Germplasm Resources Information Network (GRIN) database.