Phenotypic segregation of seedling UCB-1 hybrid pistachio rootstock

Authors: JACYGRAD, EWELINA - University Of California, Davis; Preece, John; PALMER, WILLIAM - University Of California, Davis; MICHELMORE, RICHARD - University Of California, Davis; GOLINO, DEBORAH - University Of California, Davis

Submitted to: Trees
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2019
Publication Date: N/A
Citation: N/A

Interpretive Summary: The UCB-1 hybrid is the main pistachio rootstock used in the USA. It is produced from a controlled cross between a specific clone each of Pistacia atlantica (female) and P. integerrima (male). Variation has been observed in orchards planted with seedling UCB-1 rootstocks for over 20 years. Reduced vigor and stunting of some trees are of particular concern to growers. The stunting results in decreased nut yield and therefore is a significant economic problem. This study was conducted to better understand the growth of non-grafted UCB-1, as well as caliper growth between UCB-1 rootstock and Pistacia vera scions in commercial pistachio orchards. Phenotypic traits, including tree height, trunk caliper, canopy volume, branching, and suckering, were evaluated in the non-grafted orchard. Grafted tree data were collected for trunk and scion calipers for both P. vera female scions and their UCB-1 seedling rootstocks in commercial orchards. The uniformity of tree height, trunk caliper, and canopy volume decreased annually during the first five years of growth. Individual tree growth was not linear because the trees grew at uneven rates during different years and growth was poorly synchronized among siblings. This uneven growth caused the population to become increasingly less uniform as it aged. Consequently, growth in the earliest years is a poor predictor of subsequent growth. The strongest correlation was between growth parameters during the later years. There was a significant correlation between rootstock and scion caliper of the grafted trees in commercial orchards, with the least vigorous rootstocks producing the least vigorous scions. These data confirm grower observations and the need to reliably rogue out seedlings that later will not be vigorous; however, our data show that rogueing based on the first years of growth will not be effective. Our data provide the foundation for the development of predictive molecular markers for rootstock vigor. The genus Pistacia (Anacardiaceae) consists of at least 11 species of trees and shrubs, of which, P. vera L. (2n = 2x = 30) is the most important due to its edible nuts (Zohary, 1952; Zohary, 1972). Pistachios are one of the top nut crops in California, which produces approximately 98% of the pistachios in the USA (USDA NASS, 2017). All Pistacia species are dioecious, wind pollinated, and obligate outcrossers (Crane and Iwakiri, 1981); consequently, they are highly heterozygous. Most pistachio trees in the USA are produced by grafting clonal scions of P. vera onto interspecific hybrid rootstocks of P. atlantica Desf. x P. integerrima J. L. Stewart ex Brandis. (UCB-1) or P. integerrima x P. atlantica (PGII). The UCB-1 rootstock was developed by Dr. Lee Ashworth at the University of California, Berkeley (Morgan et al., 1992) and was selected for commercial use in the 1980s because of its moderate resistance to Verticillium wilt and frost tolerance (Epstein et al., 2004). Beginning in the 1980s, UCB-1 seeds have been produced from a single female P. atlantica tree pollinated by a single male P. integerrima. Both were located at the University of California Kearney Agricultural Research Center in Parlier, CA. In 1996, production and distribution shifted to the Foundation Plant Services (FPS) at UC Davis after ten clonal copies of the female P. atlantica and four clonal copies of the male P. integerrima parents were propagated. In 2002, FPS began distributing the UCB-1 seed produced at Davis to nurseries who germinated, grew, and sold the rootstock. Because UCB-1 seedlings are the products of an interspecific cross between two outbreeding, highly heterozygous species, they segregate for vegetative vigor. Commercial cultivars grafted onto UCB-1 seedlings grow unevenly, with nut yield varying greatly from tree to tree. Clonal UCB-1 rootstocks are available, but some of these suffered from delet

Technical Abstract: The UCB-1 hybrid is the main pistachio rootstock used in the USA. It is produced from a controlled cross between a specific clone each of Pistacia atlantica (female) and P. integerrima (male). Variation has been observed in orchards planted with seedling UCB-1 rootstocks for over 20 years. Reduced vigor and stunting of some trees are of particular concern to growers. The stunting results in decreased nut yield and therefore is a significant economic problem. This study was conducted to better understand the growth of non-grafted UCB-1, as well as caliper growth between UCB-1 rootstock and Pistacia vera scions in commercial pistachio orchards. Phenotypic traits, including tree height, trunk caliper, canopy volume, branching, and suckering, were evaluated in the non-grafted orchard. Grafted tree data were collected for trunk and scion calipers for both P. vera female scions and their UCB-1 seedling rootstocks in commercial orchards. The uniformity of tree height, trunk caliper, and canopy volume decreased annually during the first five years of growth. Individual tree growth was not linear because the trees grew at uneven rates during different years and growth was poorly synchronized among siblings. This uneven growth caused the population to become increasingly less uniform as it aged. Consequently, growth in the earliest years is a poor predictor of subsequent growth. The strongest correlation was between growth parameters during the later years. There was a significant correlation between rootstock and scion caliper of the grafted trees in commercial orchards, with the least vigorous rootstocks producing the least vigorous scions. These data confirm grower observations and the need to reliably rogue out seedlings that later will not be vigorous; however, our data show that rogueing based on the first years of growth will not be effective. Our data provide the foundation for the development of predictive molecular markers for rootstock vigor. The genus Pistacia (Anacardiaceae) consists of at least 11 species of trees and shrubs, of which, P. vera L. (2n = 2x = 30) is the most important due to its edible nuts (Zohary, 1952; Zohary, 1972). Pistachios are one of the top nut crops in California, which produces approximately 98% of the pistachios in the USA (USDA NASS, 2017). All Pistacia species are dioecious, wind pollinated, and obligate outcrossers (Crane and Iwakiri, 1981); consequently, they are highly heterozygous. Most pistachio trees in the USA are produced by grafting clonal scions of P. vera onto interspecific hybrid rootstocks of P. atlantica Desf. x P. integerrima J. L. Stewart ex Brandis. (UCB-1) or P. integerrima x P. atlantica (PGII). The UCB-1 rootstock was developed by Dr. Lee Ashworth at the University of California, Berkeley (Morgan et al., 1992) and was selected for commercial use in the 1980s because of its moderate resistance to Verticillium wilt and frost tolerance (Epstein et al., 2004). Beginning in the 1980s, UCB-1 seeds have been produced from a single female P. atlantica tree pollinated by a single male P. integerrima. Both were located at the University of California Kearney Agricultural Research Center in Parlier, CA. In 1996, production and distribution shifted to the Foundation Plant Services (FPS) at UC Davis after ten clonal copies of the female P. atlantica and four clonal copies of the male P. integerrima parents were propagated. In 2002, FPS began distributing the UCB-1 seed produced at Davis to nurseries who germinated, grew, and sold the rootstock. Because UCB-1 seedlings are the products of an interspecific cross between two outbreeding, highly heterozygous species, they segregate for vegetative vigor. Commercial cultivars grafted onto UCB-1 seedlings grow unevenly, with nut yield varying greatly from tree to tree. Clonal UCB-1 rootstocks are available, but some of these suffered from delet