Search

Yucca glauca is a C(sub 3) evergreen rosette species locally common in the C(sub 4)-dominated grasslands of the central Great Plains. Most congeners of Y. glauca are found in deserts, and Y. glauca's morphological similarities to desert species (steeply angled leaves, evergreen habit) may be critical to its success in grasslands. We hypothesized that the evergreen habit of Y. glauca, coupled with its ability to remain physiologically active at cool temperatures, would allow this species to gain a substantial portion of its annual carbon budget when the C(sub 4) grasses are dormant. Leaf-level gas exchange was measured over an 18-mo period at Konza Prairie in northeast Kansas to assess the annual pattern of potential C gain. Two short-term experiments also were conducted in which nighttime temperatures were manipulated to assess the cold tolerance of this species. The annual pattern of C gain in Y. glauca was bimodal, with a spring productive period (maximum monthly photosynthetic rate = 21.1 +/- 1.97 p, (mu)mol*m(super -2)*s(super -1)) in March through June, a period of midseason photosynthetic depression, and a fall productive period in October (15.6 +/- 1.25 (mu)mol*m(super -2)*s(super -1)). The steeply angled leaves resulted in interception of photon flux density at levels above photosynthetic saturation throughout the year. Reduced photosynthetic rates in the summer may have been caused by low soil moisture, but temperature was strongly related (r(super 2) = 0.37) to annual variations in photosynthesis, with nocturnal air temperatures below -5 degrees C in the late fall and early spring, and high air temperatures (> 32 degrees C) in the summer, limiting gas exchange. Overall, 31% of the potential annual carbon gain in Y. glauca occurred outside the 'frost-free' period (April-October) at Konza Prairie and 43% occurred when the dominant C(sub 4) grasses were dormant. Future climates that include warmer minimum temperatures in the spring and fall may enhance the success of Y. glauca relative to the C(sub 4) dominants in these grasslands. Key words: climate change; cold tolerance; evergreen; grasslands; Liliaceae; photosynthesis; tallgrass prairie; water relations; Yucca.

In gynodioecious species. which contain females and hermaphrodites, the outcrossed seeds of females have been found to outperform the outcrossed seeds of hermaphrodites, in spite of the fact that their seeds are not larger in mass. Females do not make pollen. Hence the nutrients that hermaphrodites allocate to pollen, such as nitrogen, might be allocated to seeds by the females, such that individual seeds from females are better provisioned than those from hermaphrodites. Alternatively, females might make more seeds, rather than better provisioned seeds. We tested the hypothesis that seeds from females would be better provisioned for the gynodioecious species Silene acaulis, by comparing seed mass, embryo/endosperm mass, nitrogen and phosphorus content, and energy content for outcrossed seeds from females and hermaphrodites produced in a natural population. We also measured the proportion of flowers that set fruit in both morphs. Seeds from the two sexual morphs were not found to differ significantly for any of the measures of seed provisioning, with seeds from females containing either nonsignificantly less or equivalent amounts of each of the measures as compared to hermaphrodites. However, females set a significantly higher proportion of their flowers to fruit, as compared to hermaphrodites. These results indicate that females do not provision individual seeds more than hermaphrodites in S. acaulis, and alternative hypotheses will need to be examined to explain the difference in the performance of the seeds from the two sexual morphs. Key words: Caryophyllaceae; embryo mass; fruit set; gynodioecy; nutrient content; seed mass; Silene acaulis.

Arctic tundra plant species exhibit striking variation in leaf character and growth form. Both are likely related to differences in vessel anatomy, and all may affect responses to climate changes in the Arctic. To investigate the relationships among leaf character, growth form, vessel anatomy, and susceptibility to freeze-thaw-induced xylem cavitation, xylem vessel characteristics were compared among six deciduous and six evergreen arctic dicot species of erect and prostrate growth forms. We hypothesized that deciduous and erect species would have larger and longer vessels than evergreen and cushion/mat-forming species. Vessel lengths, diameters, and densities were measured for each species. Theoretical vessel flow rates were calculated using Poiseuille's law for ideal capillaries. Flow rates were used to determine the susceptibility of vessels to cavitation induced by freeze-thaw events that may become more frequent with global warming. Vessel diameters were larger in deciduous species compared to evergreens, and in shrubs/trees vs. cushion/mat-forming plants. Vessel length distributions, however, did not differ for growth form or leaf character. Vessel density was greater in cushion/mat-forming species than in shrub/tree species. Deciduous plants showed a greater contribution to total conductivity by relatively larger vessels than evergreens. One of the deciduous species, Vaccinium uliginosum, is predicted to be susceptible to freeze-thaw-induced cavitation. These results have important implications for future arctic species composition and plant community structure. Key words: cavitation; cushion plant; tundra; vessel density; vessel length.

Buxaceae are a small eudicot family of mostly monoecious evergreen shrubs and comprise up to five genera (Buxus, Notobuxus, Sarcococca, Pachysandra, Styloceras) and 120 species. Delimination of the family and inter- and intrageneric relationships within the family have been controversial. Sequences of nuclear ITS and plastid ndhF from 25 representatives of Buxaceae were analyzed separately and in combination to resolve phylogenetic relationships among and within genera. Two major clades were identified: the clade of Pachysandra, Sarcococca, and Styloceras and the clade of Buxus and Notobuxus. Pachysandra and Sarcococca were two strongly supported monophyletic groups. The sister relationship of Sarcococca to the clade of Pachysandra and Styloceras is also strongly supported. American and Eurasian Buxus each form a strongly supported clade. The controversial genus Notobuxus is embedded among African members of Buxus. Ongoing morphological studies will improve our understanding of the evolution of morphological characters and phylogenetic relationships of Buxaceae.