Botrytis cinerea is an economically important fungal pathogen of Pelargonium species. We are currently studying this plant–pathogen interaction to identify mechanisms of host resistance. Our ultimate objective is to develop commercial Pelargonium genotypes with enhanced resistance to this pathogen. Though all stages of production may be affected by this pathogen, we are investigating foliar and floral resistance of mature plants. Through simple assays, over 200 genotypes have been evaluated for foliar resistance, and more than 100 genotypes have been evaluated for floral resistance. Resistant and susceptible control genotypes have been identified for diploid and tetraploid P. ×hortorum and P. peltatum; these genotypes are being investigated to elucidate mechanisms of resistance. The diploid ivy accession 86-23-1 and the tetraploid zonal geranium `Fox' have the greatest foliar resistance among the genotypes evaluated. The diploid P. ×hortorum `Ben Franklin' has the greatest floral resistance among the evaluated genotypes. Foliar and floral resistance appear to be inherited as separate traits. Foliar resistance is manifested as a two day delay in symptom expression when compared to susceptible genotypes. Foliar resistant accession 86-23-1 has a cuticle with 150% the mass of other Pelargonium genotypes. This difference may be responsible for the observed resistance. Cuticle mass does not appear to be important in floral resistance.
Michael S. Uchneat and Richard Craig
Kelly M. Anon and Richard Craig
Interspecific hybrids of Exacum species (Gentianaceae) endemic to Sri Lanka possess excellent qualities for domestication as a new floriculture crop. The exact mode of floral induction and development responses are unknown, impeding the introduction of this potential crop. The interspecific hybrids evaluated are the result of controlled cross pollinations of E. macranthum. Arn. ex Griseb., E. trinervium (L.) Druce ssp. trinervium, and E. trinervium ssp. ritigalensis. (Willis) Cramer. The hybrids exhibit great genetic variability for horticultural traits. In addition, two growth and flowering patterns exist within the Penn State germplasm. Continuous-flowering genotypes flower throughout the year but more profusely and rapidly under late spring and summer conditions. In contrast, periodic-flowering genotypes exhibit two distinct seasonal habits. Under winter conditions, these accessions have a rosetted habit, much secondary branching, and few or no flowers. In summer conditions, they break their apical dominance, bolt, and produce flowers. As members of the Gentianaceae, Exacum hybrids produce an elegant blue flower with a striking yellow eye and bottle-shaped anthers. We evaluated the growth and flowering responses of Exacum interspecific hybrid accessions to photoperiod and irradiance. Accessions were evaluated under greenhouse conditions for floral production, rate of floral development, and growth characteristics. For the 20 accessions evaluated, supplemental irradiance under winter conditions resulted in greater floral production and much greater shoot and root mass accumulation. Little height and branching response occurred with supplemental irradiance. Of the 15 accessions evaluated under four photoperiod regimes, flowering and growth responses to photoperiod occurred under summer conditions but not in winter. An interaction among season, accession, and photoperiod revealed the complexity of Exacum germplasm and environmental responses.
Marietta M. Loehrlein and Richard Craig
Nine cultivars of Pelargonium ×domesticum L.H. Bailey were evaluated for the effect of daily light integral on floral initiation. Plants were grown at four daily light integrals: 5, 10, 15, or 20 mol·m-2·d-1 for a 16-hour photoperiod in environmental growth chambers at constant 15.5 °C. Meristems were examined at 50 mol·m-2 intervals for morphological changes associated with floral initiation. Two phenotypes were identified, cultivars with an association between floral initiation and cumulative irradiance and those with association between floral initiation and chronological time. Genotypic variation was observed among the irradiance-associated phenotypes.
Marietta M. Loehrlein and Richard Craig
Michael S. Uchneat, Kathryn Spicer, and Richard Craig
The objective of this study was to identify geranium cultivars that exhibit differential reactions to floral inoculation with Botrytis cinerea Per. ex. Fr. Sixty-two genotypes, including both cultivars and breeding lines, were evaluated from several Pelargonium species. Resistant genotypes included the diploid Pelargonium peltatum (L.) L'Herit. cultivar King of Balcon and the diploid Pelargonium ×hortorum L.H. Bail. cultivar Ben Franklin, as well as the diploid Pelargonium peltatum accession 93-1-33 developed from an accession obtained from South Africa. Susceptible genotypes included the putative tetraploid Pelargonium peltatum cultivar Simone. Floral resistance was not correlated with foliar resistance. Diploid genotypes appeared to have greater resistance than tetraploid genotypes, and P. peltatum cultivars more resistance than P. ×hortorum cultivars. In addition, the association of petal number and resistance was investigated.
Michael S. Uchneat, Angelica Zhigilei, and Richard Craig
Foliar evaluations for Botrytis resistance of greenhouse grown plants were performed on 45 cultivars and control genotypes including diploid and tetraploid zonal (P. ×hortorum L.H. Bailey) and ivy (P. peltatum (L.) L'Hér. in Ait.) pelargoniums. Additional evaluations were performed on eight species within section Ciconium and on progeny of a susceptible by susceptible cross-pollination involving the cultivars Ben Franklin and Marilyn. Differential levels of resistance were observed. Among many genotypes that exhibited resistance, two genotypes had consistently high levels of Botrytis resistance over several experiments. These two genotypes were the diploid P. peltatum accession 86-23-1 and the tetraploid P. ×hortorum cultivar Fox. The diploid P. ×hortorum cultivar Ben Franklin was a reliable susceptible control in all experiments. Plants grown outdoors generally had higher levels of resistance than comparable greenhouse grown plants. Orthogonal contrasts indicated no trends in resistance when comparing diploid and tetraploid pelargoniums, or when comparing among ivy, zonal, and floribunda types. Genotypes patented or introduced since 1990 have greater susceptibility than older genotypes. Cross-pollinations among susceptible parents resulted in susceptible progeny, while self-pollinations of a resistant parent resulted in resistant progeny.
Andrew Riseman, Richard Craig, and Jonathan P. Lynch
Interspecific hybrids of exacum (Exacum L.) display variable responses to zinc nutrition. Our research compared two genotypes with contrasting zinc efficiency phenotypes in terms of root cation exchange capacity (CEC), whole plant 65Zn uptake, and the effects of Cu+2 and Mg+2 on 65Zn uptake and partitioning to shoot tissues. Results show that the zinc efficient and inefficient genotypes had significantly different root CEC [27.2 and 16.9 cmol(+)·kg-1 root dry weight (DW), respectively] and whole plant 65Zn uptake rates (0.048 and 0.026 μmol·h-1·g-1 DW, respectively). In equimolar concentrations to Zn+2, Cu+2 reduced Zn+2 uptake by approximately 50% in both genotypes while supplemental Mg+2 enhanced Zn+2 uptake. In addition, Mg+2 facilitated a larger proportion of absorbed 65Zn to the upper shoot of the efficient genotype. We conclude zinc is absorbed through a specific Zn+2/Cu+2 transporter and that zinc efficiency in exacum is based on a combination of apoplastic and symplastic traits. In addition, a secondary Mg+2 × Zn+2 interaction may contribute to the zinc efficiency phenotype.
Richard Craig, Richard A. Grazzini, and R.O. Mumma
Resistance to mites and small insects in geranium results from the production of a viscous exudate on tall glandular trichomes present on the plant surface. This exudate exhibits sticky-trap properties immobilizing pests and reducing feeding and fecundity. The exudate is composed of long-chain 6-alkyl salicylic acids known as anacardic acids. The exudate of resistant plants contains 86% unsaturated anacardic acids. Susceptible genotypes possess fewer tall glandular trichomes and a trichome exudate which is dry and ineffective in trapping pests. The exudate from susceptible plants contains 70% saturated anacardic acids, thus explaining the physical state of the exudate. A single dominant locus controls the production of predominantly unsaturated versus saturated anacardic acids and thus resistance versus susceptibility. Other loci condition the ratio of C22:C24 unsaturated anacardic acids and the density of tall glandular trichomes. Current research involves the elucidation of the enzymatic pathway(s) involved in anacardic acid biosynthesis, identification of the regulatory enzymes and isolation of the mRNA transcripts associated with pertinent genes.
C. Fred Deneke, Kathleen B. Evensen, and Richard Craig
The postharvest quality of regal pelargoniums [Pelargonium × domesticum L. H. Bailey] is limited by petal abscission. Cultivars that have diverse postharvest longevities were selected to study ethylene sensitivity and endogenous ethylene production. Petals of both intact and detached inflorescences abscised in response to low dosages of exogenous ethylene (0.5 μl·liter-1 for 1 hour). Ethylene sensitivity varied among cultivars and increased with floret age. Silver thiosulfate reduced ethylene sensitivity and often extended floret longevity beyond that of the controls. A climacteric-like rise in endogenous ethylene production occurred in excised gynoecia (including the receptacle) as floret age increased from 1 to 12 days postanthesis. Ethylene production increased a few days earlier and achieved a higher maximum rate in `Parisienne' than in `Virginia'; `Parisienne' also abscised petals earlier. Relatively low levels of endogenous ethylene may regulate petal abscission, since inflorescences were very sensitive to exogenous ethylene, and increased endogenous ethylene production preceded petal abscission.