Hypericum L. H2003-004-016 is a complex hybrid among Hypericum frondosum Michx., Hypericum galioides Lam., and Hypericum kalmianum L. and exhibits valuable ornamental characteristics, including compact habit, bluish green foliage, and showy flowers. Inducing polyploidy may further enhance the ornamental traits of this hybrid and provide new opportunities for hybridizing with other naturally occurring polyploid Hypericum sp. In this study, in vitro shoot regeneration and treatment of regenerative callus with the dinitroaniline herbicide oryzalin (3,5-dinitro-N4,N4-dipropylsufanilamide) were investigated as a means of inducing allopolyploidy. First, in vitro regeneration was optimized for callus and shoot induction by culture of leaf explants on medium supplemented with benzylamino purine (BA) or meta-topolin (mT) at 5, 10, or 15 μM in combination with indoleacetic acid (IAA) at 0, 1.25, 2.5, or 5 μM. Both BA and mT treatments successfully induced regenerative callus and shoots. Multiple regression analysis estimated maximum regenerative callus (94%) and shoot induction (18 shoots per explant) in medium supplemented with 5 μM BA and 3.75 μM IAA. In the second part of the study, exposure of regenerative callus to oryzalin at 0, 7.5, 15, 30, 60, or 90 μM for durations of 3, 6, or 9 d was investigated for polyploid induction. There was no survival for any of the calli in the 60- or 90-μM oryzalin treatments, but calli subjected to the other treatments exhibited some survival and polyploid induction. Duration had no effect on callus survival or ploidy level, but oryzalin concentration was a significant factor in both. The greatest percentage (44%) of polyploids was induced with 30 μM oryzalin. Spontaneous chromosome doubling was observed in 8% of control explants receiving no oryzalin treatment.
Trees in the Theaceae tribe Gordonieae are valuable nursery crops, but some of these taxa are known to be highly susceptible to root rot caused by Phytophthora cinnamomi Rands. The objective of this study was to evaluate a collection of Gordonieae taxa for resistance to this pathogen. These taxa included Franklinia alatamaha Bart. Ex Marshall, Gordonia lasianthus (L.) Ellis, Schima wallichii Choisy, S. khasiana Dyer, ×Schimlinia floribunda Ranney & Fantz, and ×Gordlinia grandiflora Ranney & Fantz. Abies fraseri (Pursh) Poir. was also included in the study as a positive control. Container-grown trees were inoculated with three isolates of P. cinnamomi and symptoms were rated over an 84-day period during the summer of 2008. Disease symptom ratings from 1 (healthy) to 4 (dead) were collected twice weekly and area under the disease progress curve (AUDPC) values were calculated. None of the S. khasiana or S. wallichii exhibited any root rot symptoms or mortality, whereas the remaining species showed symptoms of infection at varying levels over time. Symptoms in F. alatamaha and A. fraseri were apparent before other taxa, and mortality for both species reached 100% by the end of the experiment. Comparison of AUDPC values indicated that F. alatamaha was the most susceptible followed by A. fraseri. There was no significant difference in AUDPC among the more resistant taxa, including G. lasianthus, both Schima species, and the intergeneric hybrids. Values for AUDPC in the hybrid taxa were similar to their more resistant parental genus, indicating that resistance to P. cinnamomi is a partially dominant trait in these plants. These results further suggest the potential to breed improved hybrids of Gordonieae trees with substantial resistance to P. cinnamomi.