Rhododendron Leaf Research Articles

Elevated ultraviolet-B radiation induces cross-protection to cold in leaves of Rhododendron under field conditions.

Previously, we have shown a cold-hardening response in Rhododendron 'English Roseum' exposed to elevated ultraviolet-B radiation (UV-B, 280-320 nm) under growth chamber conditions. We have conducted the present study under field conditions to provide for a higher ratio of photosynthetically active radiation to UV-B (PAR:UV-B) than is possible in the laboratory and to more accurately reflect natural conditions of solar irradiance. Leaf disks taken after 3 months from UV-B-exposed plants exhibited a greater tolerance to freezing temperatures than those from control plants that received no supplemental UV-B exposure during this time. Leaf disks taken from UV-B-irradiated plants survived temperatures below -8 degrees C, whereas control disks were killed at -6 degrees C. Cold hardiness did not significantly increase until September, when environmental cues such as decreasing day length and night temperatures also may have enhanced hardening. Our field findings confirm our previous laboratory study, demonstrating that elevated UV-B induces cross-protection to cold in Rhododendron leaf tissues.

PRE-CONDITIONING RHODODENDRON AND CHRYSANTHEMUM LEAF EXPLANTS AFFECTS TRANSIENT BETA-GLUCURONIDASE EXPRESSION AFTER BIOLISTIC PARTICLE BOMBARDMENT

ISHS XXVI International Horticultural Congress: Biotechnology in Horticultural Crop Improvement: Achievements, Opportunities and Limitations PRE-CONDITIONING RHODODENDRON AND CHRYSANTHEMUM LEAF EXPLANTS AFFECTS TRANSIENT BETA-GLUCURONIDASE EXPRESSION AFTER BIOLISTIC PARTICLE BOMBARDMENT

Litter breakdown and invertebrate detritivores in a resource-depleted Appalachian stream

We measured breakdown rates of leaves and small wood for the first three years in a stream in which detrital inputs were excluded for 7 years and in a reference stream located in the Appalachian Mountains of North Carolina, USA. Leaf and wood inputs were excluded using a gill-net canopy constructed over a 170-m section of stream. We hypothesized that red maple (Acer rubrum) and rhododendron (Rhodo- dendron mima) leaf breakdown rates would decline in the litter exclusion stream as shredder production decreased with each year of litter exclusion. In contrast, we expected faster wood breakdown rates in the litter exclusion stream as microbes and invertebrates shifted from leaves to wood as their primary organic matter resource. Consistent with our predictions, wood breakdown rates were significantly faster in the litter exclusion stream. We also found sigdicantly slower processing rates of maple leaves in the litter exclusion stream compared to the reference stream during years 2 and 3. Slower breakdown rates for red maple leaves in the litter exclusion stream were associated with lower shredder production and estimated consumption rates in years 2 and 3. Shredder production and consumption rate estimates were also lower in the ex- clusion stream for rhododendron leaves, but leaf breakdown rates were not affected. We also found that shredder production in litterbags was 3-4 x greater than in benthic substrates in the litter exclusion stream. In contrast, shredder production in litterbags was similar to that in benthic substrates in the reference stream. These differences were probably due to the relatively low availability of organic matter in benthic sub- strates in the litter exclusion stream. Our data show shredders track high quality or- ganic matter resources (leaves) and contribute to their loss rate, illustrating the interde- pendence of stream organisms and terrestrial organic matter input.

COMPARISON OF DIFFERENT METHODS TO DETECT PHYTOPHTHORA SPP. IN RECYCLING WATER FROM NURSERIES

Five methods for the detection of Phytophthora spp. in water recirculation systems of four commercial nurseries were compared: direct plating, three bait tests (apple, lupin seedling, Rhododendron leaf test) and DASELISA with two Phytophthora specific polyclonal antisera. Water and sediment samples were taken from different places in the recirculation systems over one year. With all methods except direct plating Phytophthora spp. could be detected in the four nurseries. In most cases the detection rates were influenced more by the method itself than by different conditions on particular nurseries. The Rhododendron leaf test trapped the widest range of Phytophthora species and was the most successful method followed by ELISA with antiserum against P. cinnamomi. Although the chemical analysis of the water samples showed a good water quality for nursery cultivation, there was a close relationship between the detection rates obtained with ELISA and the total salt content of the water. The results are compared with those of in vitro experiments and their relevance for commercial nurseries is discussed.

The importance of crayfish in the breakdown of rhododendron leaf litter

1. Rhododendron (Rhododendron maximum) is a common evergreen shrub in riparian areas of the southern Appalachians, where its leaves can comprise a large proportion of leaf litter in streams. However, they are relatively refractory and generally considered a low quality food resource for detritivores.2. Our objective was to assess whether macroconsumers [primarily crayfish (Cambarus bartonii)] influence rhododendron leaf breakdown in a forested southern Appalachian stream in both summer (when leaves other than rhododendron are relatively scarce) and autumn (when other leaves are relatively abundant). We conducted two leaf decay experiments, one in summer and one in autumn, using pre‐conditioned leaves. Macroconsumers were excluded from the benthos of a fourth‐order stream using electric ‘fences’; we predicted that excluding macroconsumers would reduce the decay rate of rhododendron leaves in both summer and autumn.3. In both experiments, breakdown rate was lower in exclusion treatments. Macroconsumers accounted for approximately 33 and 54% of rhododendron decay in summer and autumn, respectively. We attribute this effect to direct shredding of rhododendron by crayfish. Biomass of insect shredders, insect predators and fungi did not differ between control and exclusion treatments, indicating that insectivorous sculpins (Cottus bairdi) had no effect on rhododendron decay and that omnivorous crayfish did not exert an indirect effect via alteration of insect or fungal biomass.4. The influence of shredding insects varied between summer and autumn. In summer, when other, more palatable leaf types were not available, rhododendron leaf packs appeared to provide ‘resource islands’ for insect shredders. There was a significant inverse relationship between insect shredders and leaf pack mass in the summer exclusion treatment: insects were the only organisms eating leaves in this treatment and, as shredder biomass increased, remaining leaf pack mass decreased. In the control treatment, however, we did not see this relationship; here, the effect of insect shredders was presumably swamped by the impact of crayfish. In autumn, when other leaves were abundant, insect shredder biomass in rhododendron leaf packs was less than one‐third of summer values.5. Even at low density (approximately 2 m–2) crayfish were able to influence an ecosystem process such as leaf decay in both summer and autumn. Given the threatened status of many crayfish species in the United States, this finding is especially relevant. Even small alterations in crayfish assemblages, whether via loss of native species and/or introduction of exotic species, may have significant repercussions for ecosystem function.

First Report of Phytophthora Root and Collar Rot of Alder in Hungary.

In June 1999, a disease associated with mortality of Alnus glutinosa, was observed in a 12- to 18-year-old peatland plantation in northwest Hungary. The root and collar rot symptoms were similar to those caused by Phytophthora cambivora in tree species other than alders. Nine isolations were made from diseased roots and soil samples using the Rhododendron leaf baiting method. Three isolates recovered from two sites, approximately 2 km apart, exhibited similar growth and morphology in vitro and were pathogenic to 2-year-old trees of A. glutinosa following inoculation of root collars. All three isolates had amphigynous long, two-celled antheridia. The mean diameter of oogonia ranged from 39.5 to 64.6 μm. They also produced nonpapillate, ellipsoid, non-caducous sporangia 26.9 to 50.5 μm long and 19.3 to 38.5 μm wide with broad exit pores in soil filtrate. These characteristics were similar to those reported for Phytophthora on alder from elsewhere in Europe and for P. cambivora that is not a pathogen of alder (1,2). However, Hungarian isolates from alder, in contrast to P. cambivora, were homothallic like previously recorded isolates from alder, formed nonornamented oogonia and developed colonies at lower optimum (approximately 25°C) and maximum (approximately 30°C) temperatures on carrot agar. A comparison with Phytophthora from alder from other countries (courtesy of C. M. Brasier) showed that the Hungarian isolates have smooth-walled oogonia typical of Swedish isolates rather than the ornamented oogonia of U.K. isolates, but have the appressed, slightly woolly colony morphology like U.K. isolates rather than the fluffy growth found in Swedish isolates. Moreover, cellulose acetate electrophoresis of glucose-6-phosphate isomerase revealed one homodimer band in Hungarian isolates that was identical with that of the Swedish isolate from alder P876 and isolates P1010 and P1011 of P. cambivora (courtesy of C. M. Brasier). This band comigrated with the middle one of the five-banded U.K. standard isolate P772. Molecular evidence (2) indicates that the Phytophthora from alder with its unusual characteristics is not a species in the strict sense but comprises natural hybrids that may have originated in an interspecific hybridization event between a P. cambivora-like species and an unknown species similar to P. fragariae. On this basis, the Hungarian Phytophthora from alder might have evolved similarly. It remains to be determined whether the pathogen was introduced or has developed independently.

A 25-kDa dehydrin associated with genotype- and age-dependent leaf freezing-tolerance in Rhododendron: a genetic marker for cold hardiness?

Dehydrins are plant proteins that may play a critical role in stabilizing cell functions during freezing and other dehydrative stresses. This study examines whether dehydrin expression in leaves is associated with varying levels of freezing-tolerance among F2 segregants, species, and cultivars of evergreen Rhododendron. Experiments were also conducted to determine whether physiological and chronological aging affects freezing-tolerance and dehydrin accumulation in Rhododendron leaf tissues. Our results indicate that in cold-acclimated F2 populations, levels of a 25-kDa dehydrin were closely associated with differences in leaf freezing-tolerance (LFT) among segregants. Studies of wild and cultivated plants indicated that LFT increased with both chronological age and developmental phase-change (juvenile to mature plants) and that this trend was accompanied by increased accumulation of the 25-kDa dehydrin. It is suggested that presence or absence of the 25-kDa dehydrin could serve as a genetic marker to distinguish between super cold-hardy and less cold-hardy rhododendron genotypes. Similarly, the relative level of this protein within a genotype can serve as a physiological indicator of freezing-tolerance status under a range of phenological (acclimation) or developmental (age) conditions.

084 Plant Preservative Mixture (PPM) can Reduce Shoot Regeneration from Leaf Explants of Selected Plants

Plant preservative mixture (PPM) is a new broad-spectrum biocide that may be useful for plant tissue culture. The objective of this study was to determine if PPM interfered with adventitious shoot regeneration on leaf explants from several plant species. Leaf explants from Dendranthema grandiflora `Iridon', Betula pendula, Rhododendron catawbiense var. album and R.c. `America' were made from the top two apical leaves on the microshoots. In the first experiment, 0, 0.5, 1, 2, or 4 mL·L-1 PPM were added to species-appropriate regeneration media. In the second experiment, only mum leaf explants were placed on regeneration media containing 0, 0.1, 0.2, 0.3, or 0.4 mL·L-1 PPM. The percentage of explants forming shoots and the number of shoots per regenerating explant were recorded after 4, 6, and 10 weeks, for mum, birch, and rhododendron leaves, respectively. The percentages of shoot regeneration from birch and rhododendron leaf explants were unaffected by up to 4 mL·L-1 PPM, and the number of shoots formed per R.c. album explant were also unaffected by the tested concentrations of PPM. In contrast, the numbers of shoots formed on birch and `America' explants were reduced 48% and 25%, respectively, when 4 mL·L-1 PPM was used in the media. The percentages of shoot regeneration and number of shoots per explant were drastically reduced on mum explants when only 0.5 mL·L-1 PPM was used in the medium. In fact, 0.3 mL·L-1 PPM or higher reduced shoot formation by more than 5-fold. This study demonstrates that the effects of PPM on shoot regeneration from leaf explants are species specific.

Fungal Biomass of 3 Leaf Litter Species during Decay in an Appalachian Stream

Leaf decay and fungal biomass accrual was measured for maple (Acer rubrum), tulip-poplar (Liriodendron tulipifera), and rhododendron (Rhododendron maximum) leaf packs in an Appalachian stream over 8 mo. Packs were placed in 2 stream sites along a longitudinal gradient at the Coweeta Hydrologic Laboratory in North Carolina: a 1st-order headwater stream and a 4th-order stream. Decay was measured as percent mass loss over time, and fungal biomass was estimated using ergosterol. Tulip-poplar decayed faster than maple, and rhododendron decayed the slowest of the 3 in both sites. Decay was faster in the 4th-order stream for tulip-poplar and rhododendron leaves, largely because of differences in stream geomorphology. Fungal biomass showed no trend along the longitudinal gradient. Average fungal biomass (as dry mass [DM] per ash free DM [AFDM]) of aquatic hyphomycetes during the study period was 42 mg DM/g AFDM on tulip-poplar and 11 mg DM/g AFDM on maple leaves in these streams. These estimates are on the lower ...

Biological Roles and Biochemistry of the Lipoxygenase Pathway

Duxbury, C.L., R.L. Legge, G. Paliyath, R.F. Barber, and J.E. Thompson. 1991. Alterations in membrane protein conformation in response to senescencerelated changes. Phytochemistry 30:63–68. Farmer, E.E. and C.A. Ryan. 1992. Octadecanoid precursors of jasmonic acid activate the synthesis of wound-inducible proteinase inhibitors. Plant Cell 4:129–134. Fobel, M., D.V. Lynch, and J.E. Thompson. 1987. Membrane deterioration in senescing carnation flowers. Plant Physiol. 85:204–211. Gardner, H.W. 1989. How the lipoxygenase pathway affects the organoleptic properties of fresh fruit and vegetables, p. 98–112. In: D.B. Min and T.H. Smouse (eds.). Flavor chemistry of lipid foods. Amer. Oil Chem. Soc., Champaign, Ill. Hamilton-Kemp, T.R., C.T. McCracken, Jr., J.H. Loughrin, R.A. Andersen, and D.F. Hildebrand. 1992. Effects of some natural volatile compounds on the pathogenic fungi Alternaria alternata and Botrytis cinerea. J. Chem. Ecol. 18:1083–1091. Harber, R.M. and L.H. Fuchigami. 1986. The relationship of ethylene and ethane production to tissue damage in frozen rhododendron leaf disks. J. Amer. Soc. Hort. Sci. 111:434–436. Kimmerer, T.W. and T.T. Kozlowski. 1982. Ethylene, ethane, acetaldehyde, and ethanol production by plants under stress. Plant Physiol. 69:840–847. Nanaiah, G.K. and J.A. Anderson. 1992. Electrolyte leakage and evolution of ethylene and ethane from pepper leaf disks following temperature stress and fatty acid infiltration. J. Amer. Soc. Hort. Sci. 117:846–851. Pauls, K.P. and J.E. Thompson. 1980. In vitro simulation of senescence-related membrane damage by ozone-induced lipid peroxidation. Nature (London) 283:504–506. Perkins, E.G. 1989. Gas chromatography and gas chromatography–mass spectrometry of odor/flavor components in lipid foods, p. 35–56. In: D.B. Min and T.H. Smouse (eds.). Flavor chemistry of lipid foods. Amer. Oil Chem. Soc., Champaign, Ill. Purvis, A.C. and R.L. Shewfelt. 1993. Does the alternative pathway ameliorate chilling injury in sensitive plant tissues? Physiol. Plant. 88:712–718.

Exposure to ultraviolet‐B radiation increases cold hardiness in Rhododendron

The change in the cold hardiness of Rhododendron (cv. English Roseum) following chronic exposure to ultraviolet‐B (UV‐B) radiation (280–320 nm) was studied. Leaf disks removed from UV‐B‐exposed plants exbibited a greater tolerance to freezing temperatures than plants that received no UV‐B exposure. Visual browning and percent phenolic leakage indicated that UV‐B‐exposed leaf disks were killed al ‐11°C. while control disks were killed at ‐8°C. Ultraviolet‐induced production of phenolic compounds may be involved in increasing cold hardiness of Rhododendron leaf tissues.

RESPONSE OF WOODY PLANTS TO KANAMYCIN AND BIALAPHOS

Resistance to kanamycin (kan) and bialaphos is commonly used to select for transgenic plants. To determine inhibitory levels for in vitro selection, nontransformed tissues of arabica coffee, white ash, black walnut, and Rhododendron `P.J.M. Hybrids' were incubated in media containing concentrations from 0 to 400 mg/liter kan or 0 to 25 mg/liter bialaphos. Callus from coffee leaf explants was completely inhibited by 100 mg/liter kan or 1 mg/liter bialaphos. Shoot organogenesis, callus and epicotyl growth from white ash seeds were inhibited by 20 mg/liter kan or 5 mg/liter bialaphos. Secondary embryogenesis from black walnut somatic embryos was mostly inhibited by 40 mg/liter kan and totally inhibited by 1 mg/liter bialaphos. Shoot organogenesis from rhododendron leaf explants was inhibited by 10 mg/liter kan or 10 mg/liter bialaphos.

ADVENTITIOUS MICROPROPAGATION OF RHODODENDRON `P.J.M. HYBRIDS'

For genetic transformation studies, it is important that efficient adventitious regeneration systems be developed. In previous studies we had stimulated adventitious shoot formation from rhododendron leaf explants. To further determine which explants would be best to inoculate with Agrobacterium, we compared shoot organogenesis from small (2-4 mm long), medium (>4-8 mm long), and large (>8 mm long) leaves excised from in vitro shoot cultures. Across all reps (up to 66 explants), large leaves produced a mean of 23.4, medium produced 6.1, and small produced 2.2 adventitious shoots after 18 weeks. Adventitious microshoots were rooted in preformed peat plugs in high humidity flats. Prior to placement in the plugs, cut ends were treated with various combinations of IBA and NAA. After 8 weeks, control shoots rooted 60% and microshoots treated with 1.25 or 2.50 mM IBA plus 1.25 mM NAA or with a Wood's (1:20 dilution) solution rooted >95%.

The relationship between freezing tolerance and thermotropic leaf movement in five Rhododendron species.

Leaf movement kinetics in five species of Rhododendron were studied in response to leaf temperature, leaf freezing point, and leaf water deficit. There was a gradient in the degree of leaf curling among species in the following order from the greatest curling to the least curling: Rhododendron catawbiense, R. maximum, R. minus, R. macrophyllum, R. ponticum. Those species found to be tolerant of winter conditions had the most intense leaf movements (both curling and angle) while those species with minimal cold tolerance had limited or no leaf movements. Leaf curling occurred at leaf temperatures above the tissue freezing points in all species. Athough leaf angle was influenced by leaf turgor, general tissue desiccation was not the ultimate cause for thermotropic leaf curling in any species tested. Those species with the greatest leaf curling and angle movements had the highest osmotic potential, the lowest water deficit at the turgor loss point, and the lowest symplastic water fraction. These data suggest that there is a trade off in Rhododendron leaf physiology between cold tolerance (due to leaf movements) and water stress tolerance (due to turgor maintenance mechanisms).

The Relationship of Ethylene and Ethane Production to Tissue Damage in Frozen Rhododendron Leaf Disks

Abstract Ethylene and ethane production of freeze-stressed rhododendron (Rhododendron sp. ‘Sappho’) leaf disks were compared to visual rating, TTC reduction, and electrolyte leakage as possible means of measuring tissue viability. Ethane production, as caused by freezing temperatures, was highly correlated with visual rating, TTC reduction, and electrical conductivity (r = 0.96, r = −0.81, and r = 0.96, respectively). Ethylene production peaked concurrently with initial stages of visual tissue damage, then decreased as the temperature was lowered until complete death occured. Ethane production and electrolyte leakage peaked coincidentally with the decrease of ethylene. Ethylene:ethane ratios are suggested as a measurement of freeze-induced tissue damage. This study supports the view that ethylene production is related to stress and ethane production to cell death. Chemical names used: 2,3,5-triphenyltetrazolium chloride (TTC).

Rediscovery and Cladistic Placement of the Genus Cainosternum (Coleoptera: Leiodidae)

Until now, the monotypic North American genus Cainosternum Notman was known only from the unique female holotype collected in Chautauqua County, N.Y., near Westfield. A male and female specimen of C. imbricatum Notman reported here are from the southern Appalachians of western North Carolina (Macon County, near Highlands), where they were collected from rhododendron leaf litter near a stream above 900 m elevation. A generic diagnosis, redescription of C. imbricatum Notman, illustrations of diagnostic characters, and discussions of cladistic and ecological relationships are given. Cainosternum is provisionally assigned to the tribe Agathidiini (=Anisotomini) on the basis of sexually dimorphic tarsomere numbers, a putative synapomorph for the tribe.

Phosphorus Requirement of Rhododendron ‘Victor’ and Cotoneaster adpressa praecox Grown in a Perlite-Peat Medium

Rooted cuttings of Rhododendron ‘Victor’ (a PJM cultivar) and Cotoneaster adpressa praecox were grown in the greenhouse with phosphorus (P) levels ranging from 0 to 15 ppm in a perlite-peat medium. Shoot growth of Rhododendron was equal at P levels from 2.5 to 15 ppm. Maximum growth of shoots and roots of Cotoneaster was at 10 ppm. Maximum Rhododendron leaf P content occurred at 5 ppm in the medium and remained the same at higher P levels. The P content of Cotoneaster leaves increased throughout the range of P levels in the growing medium. A P level of 10 ppm in soilless media solution is suggested for most nursery crops.

Collembola Associated with Shelf Fungi (Polyporaceae, etc.) and Some Other Microcommunities in the Highlands Area of Western North Carolina1

Seventy-five species of Collembola were associated with woody shelf fungi, in particular, and also certain other microhabitats (moss, tree-hole debris, sawdust piles, and rhododendron leaf litter) in the Blue Ridge region near Highlands, North Carolina. Thirteen of these species were new state records, and one was described as a new species. These data are suggestive of certain general habitat preferences for some Collembola species in the area.

Leaf survey of flavonoids and simple phenols in the genus Rhododendron

A leaf survey of 206 Rhododendron species, subspecies and varieties showed that the genus possesses a relatively uniform flavonoid pattern. Three compounds which are generally rare in the angiosperms, gossypetin, azaleatin and caryatin, occur in 76, 34 and 10 per cent of the species respectively. Quercetin and leucoanthocyanidins are present in all taxa, while dihydroflavonols are present in 68 per cent, myricetin in 51 per cent and kaempferol in 23 per cent of the sample. Gossypetin is notably absent from the subgenus Pentanthera while caryatin characterises the single subgenus Hymenanthes. Of the three dihydroflavonols, dihydroquercetin and dihydromyricetin are reported in the genus for the first time, the former being isolated as the 3-arabinoside. The flavonols of Rhododendron leaf were found to be present as the 3-arabinosides, 3-rhamnosides, 3-galactosides and 3-glucosides. Simpler phenols were surveyed in leaves of 55 species with the following results: orcinol in 7 per cent, hydroquinone in 9 per cent, rhododendrol in 37 per cent, o-coumaric acid in 19 per cent, gentisic acid in 80 per cent and gallic acid in 84 per cent. Taxonomically, the results generally support the accepted sectional and subsectional classifications, although they suggest that on chemical grounds certain species might be misplaced. Phyletically, the data indicate that the genus still retains a wide range of primitive phenolic characters. Geographically, the separation of R. lochae, the only Australian species, from the rest of the genus in S.E. Asia is reflected in its chemistry.

Pseudoscorpions and Spiders from Moss, Fungi, Rhododendron Leaf Litter, and Other Microcommunities in the Highlands Area of Western North Carolina1

The spiders and pseudoscorpions collected from fungi, moss, and other forest floor microhabitats near Highlands, N. C, (a high-rainfall region at about 4000 feet altitude in the southern Appalachian Mountains) are listed and discussed. The evidence indicates that most species are general inhabitants of several types of humid forest microcommunities and do not show any pronounced habitat specificity.