Differences in physiological and biochemical responses to short-term flooding among the three avocado (Persea americana Mill.) races.

Avocado (P. americana Mill.) trees are classified into three botanical races, Mexican (M), Guatemalan (G), and West Indian (WI), each distinguished by their geographical centers of origin. While avocados are considered highly sensitive to flooding stress, comparative responses of the different races to short-term flooding are not known. This study assessed the differences in physiological and biochemical responses among clonal, non-grafted avocado cultivars of each race to short-term (2-3 days) flooding. In two separate experiments, each with different cultivars of each race, container-grown trees were divided into two treatments: 1) flooded and 2) non-flooded. Net CO2 assimilation (A), stomatal conductance (gs), and transpiration (Tr) were measured periodically over time beginning the day before treatments were imposed, through the flooding period, and during a recovery period (after unflooding). At the end of the experiments, concentrations of sugars in leaves, stems, and roots, and reactive oxygen species (ROS), antioxidants, and osmolytes in leaves and roots were determined. Guatemalan trees were more sensitive to short-term flooding than M or WI trees based on decreased A, gs, and Tr and survival of flooded trees. Guatemalan trees generally had less partitioning of sugars, particularly mannoheptulose, to the roots of flooded compared to non-flooded trees. Principal component analysis showed distinct clustering of flooded trees by race based on ROS and antioxidant profiles. Thus, differential partitioning of sugars and ROS and antioxidant responses to flooding among races may explain the greater flooding sensitivity of G trees compared to M and WI trees.

A Rosaceae Family-Level Approach To Identify Loci Influencing Soluble Solids Content in Blackberry for DNA-Informed Breeding.

A Rosaceae family-level candidate gene approach was used to identify genes associated with sugar content in blackberry (Rubus subgenus Rubus). Three regions conserved among apple (Malus × domestica), peach (Prunus persica), and alpine strawberry (Fragaria vesca) were identified that contained previously detected sweetness-related quantitative trait loci (QTL) in at least two of the crops. Sugar related genes from these conserved regions and 789 sugar-associated apple genes were used to identify 279 Rubus candidate transcripts. A Hyb-Seq approach was used in conjunction with PacBio sequencing to generate haplotype level sequence information of sugar-related genes for 40 cultivars with high and low soluble solids content from the University of Arkansas and USDA blackberry breeding programs. Polymorphisms were identified relative to the 'Hillquist' blackberry (R. argutus) and ORUS 4115-3 black raspberry (R. occidentalis) genomes and tested for their association with soluble solids content (SSC). A total of 173 alleles were identified that were significantly (α = 0.05) associated with SSC. KASP genotyping was conducted for 92 of these alleles on a validation set of blackberries from each breeding program and 48 markers were identified that were significantly associated with SSC. One QTL, qSSC-Ruh-ch1.1, identified in both breeding programs accounted for an increase of 1.5 °Brix and the polymorphisms were detected in the intron space of a sucrose synthase gene. This discovery represents the first environmentally stable sweetness QTL identified in blackberry. The approach demonstrated in this study can be used to develop breeding tools for other crops that have not yet benefited directly from the genomics revolution.