The location of major quantitative trait loci (QTL) contributing to stem and leaf [Na

Rootstock HKT1 genotype affected fruit [Na(+)] and non-commercial fruit yield; QTL analysis of rootstock-mediated scion nutrition is a powerful forward genetic approach to identify wild genes for rootstock breeding. The present study approaches the QTL dissection of rootstock effects on a commercial hybrid variety grafted on a population of RILs derived from Solanum pimpinellifolium, genotyped for 4370 segregating SNPs from the SolCAP tomato panel and grown under moderate salinity. Results are compared to those previously obtained under high salinity. The most likely functional candidate genes controlling the scion [Na(+)] were rootstock HKT1;1 and HKT1;2 as it was previously reported for non-grafted genotypes. The higher fruit [Na(+)] found when rootstock genotype was homozygote for SpHKT1 supports the thesis that scion HKT1 is loading Na(+) into the phloem sap in leaves and unloading it in sink organs. A significant increment of small, mostly seedless, fruits was found associated with SlHKT1 homozygous rootstocks. Just grafting increased the incidence of blossom end rot and delayed fruit maturation but there were rootstock RILs that increased commercial fruit yield under moderate salinity. The heritability and number of QTLs involved were lower and different than those found under high salinity. Four large contributing (>17%) rootstock QTLs, controlling the leaf concentrations of B, K, Mg and Mo were detected whose 2 Mbp physical intervals contained B, K, Mg and Mo transporter-coding genes, respectively. Since a minimum of 3 QTLs (two of them coincident with leaf K and Ca QTLs) were also found governing rootstock-mediated soluble-solids content of the fruit under moderate salinity, grafting desirable crop varieties on stress-tolerant rootstocks tenders an opportunity to increase both salt tolerance and quality.

No abstract

A seedling population from hybrids between ‘Volkamer’ lemon (Citrus volkameriana) and ‘Rubidoux’ trifoliate orange (Poncirus trifoliata) was grafted with ‘Hashimoto’ Satsuma mandarin (Citrus unshiu) to study the inheritance of rootstock effects on salt tolerance in terms of fruit yield. Trees were maintained in a screenhouse, and a salt treatment (25 mm NaCl) was applied to 32 genotypes from June to September every year for 5 years. Rootstocks were genotyped for five salt tolerance candidate genes. Significant effects of rootstock genotype (G) and treatment (E) were found for most traits. Salinity decreased yield and juice volume but improved soluble solids concentration (TSS) and rind thickness. Year effects were highly significant in most cases. G × E interactions were found for fruit weight, total fruit weight, juice volume (JV), leaf water content (LWC), and leaf [Na+]. Therefore, rootstocks that induce early fruit maturation under salinity (by increasing TSS and maintaining JV) can be selected to expand the harvesting calendar of mandarin cultivars. Salt tolerance candidate genes SOS1 and NHX1 were associated with fruit yield traits under normal conditions (1.4 dS·m−1), and SOS1 and CCC were associated with LWC under salinity conditions (4 dS·m−1). Only 5% progeny induced higher accumulated yield than ‘Volkamer’ lemon under salinity. Given the low heritability of rootstock effects on fruit yield under salinity conditions (0.18 at most), marker-assisted selection might be useful.

A total of 60 traits that could be related to salt tolerance were genetically analyzed using nucellar plants as repetitions of apomictic hybrids in a reference population derived from two common citrus rootstocks, Cleopatra mandarin (salt tolerant) and trifoliate orange (salt sensitive), in two experiments differing in duration (1 versus 3 years), [NaCl] (30 versus 25mM) and environmental control (greenhouse versus screenhouse). In both experiments, the trifoliate parent always showed less aerial vegetative growth than Cleopatra and, under salinity, the trifoliate parent showed higher Na + and Clleaf concentrations than the salt tolerant parent. Salinity affected the relationships among traits, particularly those involving leaf water potential, leaf concentrations of Cl -, K + , B and Fe, and root [Na + ]. Most traits showed heritabilities below 0.6 and their QTL analyses were carried out using three mapping procedures to obtain complementary genetic information on trait inheritance. A total of 98 QTLs were detected by Interval Mapping and Multiple QTL Mapping procedures. Fresh and dried weights of the leaf, studied in both experiments, showed common QTLs, remarking their repeatability. A cluster of QTLs governing plant vigour and leaf boron concentration pointed a genomic region in linkage group 3 as the most relevant one to improve salt tolerance using the Cleopatra parent as donor. Besides, a QTL genotype in linkage group 7, associated with the smallest leaf water potential and defoliation index under salinity, corresponded to the highest leaf [Na + ] and the largest leaf area, suggesting the presence of a putative tissue salt tolerance QTL.