Structural Adaptations in Overwintering Leaves of Thermonastic and Nonthermonastic Rhododendron Species

Evergreen rhododendrons (Rhododendron L.) are important woody landscape plants in many temperate zones. During winters, leaves of these plants frequently are exposed to a combination of cold temperatures, high radiation, and reduced photosynthetic activity, conditions that render them vulnerable to photooxidative damage. In addition, these plants are shallow-rooted and thus susceptible to leaf desiccation when soils are frozen. In this study, the potential adaptive significance of leaf morphology and anatomy in two contrasting Rhododendron species was investigated. R. catawbiense Michx. (native to eastern United States) exhibits thermonasty (leaf drooping and curling at subfreezing temperatures) and is more winter-hardy [leaf freezing tolerance (LT50) of containerized plants ≈–35 °C], whereas R. ponticum L. (native to central Asia) is less hardy (LT50 ≈–16 °C), and nonthermonastic. Thermonasty may function as a light and/or desiccation avoidance strategy in rhododendrons. Microscopic results revealed that R. ponticum has significantly thicker leaf blades but thinner cuticle than R. catawbiense. There is one layer of upper epidermis and three layers of palisade mesophyll in R. catawbiense compared with two distinct layers of upper epidermis and two layers of palisade mesophyll in R. ponticum. We suggest that the additional layer of upper epidermis in R. ponticum and thicker cuticle and extra palisade layer in R. catawbiense represent structural adaptations for reducing light injury in leaves and could serve a photoprotective function in winter when leaf photochemistry is generally sluggish. Results also indicate that although stomatal density of R. ponticum is higher than that of R. catawbiense leaves, the overall opening of stomatal pores per unit leaf area (an integrated value of stomatal density and pore size) is higher by approximately twofold in R. catawbiense, suggesting that R. catawbiense may be more prone to winter desiccation and that thermonasty may be a particularly beneficial trait in this species by serving as a desiccation-avoidance strategy in addition to a photoprotection role.

Germination of Stewartia pseudocamellia Seeds is Promoted by Desiccation Avoidance, Gibberellic Acid Treatment, and Warm and Cold Stratification

Japanese Stewartia, Stewartia pseudocamellia (Maxim.), seeds were extracted from immature capsules and handled under nondesiccating conditions. Moisture content of fresh seeds was 82%. After exposing seeds for 48 hrs at room conditions seed moisture content was 56% and after 24 hrs at 80C (176F) it was 49%. Both ambient and 80C (176F) drying conditions resulted in complete loss of viability based on a tetazolium test. Exposing seeds to a 24 hr aerated water soak in 1 mM gibberellic acid (GA3), and a 3 month warm (25C, 77F) stratification period increased seed germination only after 6 months of cold (4C, 40F) stratification compared to seeds given a 24 hr aerated water soak without GA3 and similar warm and cold stratification periods. Best management practices for handling and germination of fresh Japanese Stewartia seeds are: (a) harvest seed capsules when they turn from green to brown, (b) maintain seeds under nondesiccating conditions, (c) treat seeds with a 24 hr aerated water soak in 1 mM GA3 before a 3-month warm stratification, and (d) provide at least 7 months cold stratification.

The use of exposed diurnal roosts in Alberta by the little brown bat, Myotis lucifugus

The use of exposed diurnal roosts by the little brown bat,Myotis lucifugus, throughout the summer months is only known to occur among Alberta populations of this species. This study is the first to examine that behaviour. In total, 426 M. lucifugus using exposed anthropogenic surfaces as diurnal roosts were observed in the summers of 1985 and 1986. Bats were usually found alone, but groups of up to six individuals were seen. When found at these roosts, 98.8% of bats were at least 2 m from the ground and 95.1% were below an overhang. Bats were found on brick substrates in 89.0% of observations and in the corner between two surfaces in 57.0%. Bats were found on east-facing surfaces less frequently than expected under random distribution. Roost selection appears to have been based on several strategies, including thermoregulation, predator avoidance, and desiccation avoidance. After maternity roosts were vacated, the proportion of juveniles at exposed roosts was higher than their proportion in the total population. This may be the result of their inability to enter, or lack of knowledge of, many enclosed roosts used by adults.

694 PB 270 DESICCATION AVOIDANCE OF APPLE TREES WITH N-2001 ANTITRANSPIRANT

Antitranspirant N-2001 (10%), Great Lakes Chemical Corporation, was applied as a soil drench to `Fuji'/EMLA7' apple plants growing in 15 cm pots in a 25/22±3°C (D/N) greenhouse. After bringing pots to field capacity, chemical application was made and water was withheld thereafter. One hour after chemical application, stomatal conductance of treated and control plants was 0.25 and 0.70 cm/sec, respectively. Stomatal conductance of treated plants was maintained at approximately 0.25 cm/sec throughout the test period (28 days). Stomatal conductance of the control plants decreased to 0.25 cm/sec 13 days after treatment due to desiccation. The stem xylem water potential of the treated and control plants was -2.0 and -5.5 MPa, respectively, 28 days after treatment. The relative water content of leaves of treated plants was 45% greater than controls. The average loss of water via transpiration of treated plants was 32% less than the control plants.

The spatial distribution of microalgae on Antarctic fellfield soils

The horizontal and vertical distributions of cyanobacteria and algae on soil polygons on Signy Island were investigated. Soil chlorophyll concentrations increased from the centre to the edge of the polygons. Similar distributions of the non-motile genera, such as Pseudanabaena and Nostoc, were observed, whereas the motile taxa, Phormidium and Pinnularia, were evenly distributed across the polygon. Phormidium autumnale was the most widespread taxon, and other Oscillatoriaceae were also important, although large differences in community composition between polygons were observed. Most of the algal biomass was concentrated near the surface of the soil, although chlorophyll degradation products were found to depths of up to 8 cm. Examination of the soil profile by fluorescence microscopy indicated that a large proportion of the microflora occurred in the zone 0–1 mm below the surface, and scanning electron microscopy confirmed that few algae occurred on the soil surface. It is suggested that this may be a desiccation-avoidance strategy. Vertical migration of the motile microalgae to the soil surface was not observed in the field, but could be induced in the laboratory in the presence of excess water, although no diel cycle to this movement was observed.