The sea urchin, Lytechinus variegatus, has been estimated to consume between 50 and 90% of annual net aboveground production in selected turtlegrass (Thalassia testudinum) meadows in the eastern Gulf of Mexico. Nevertheless, turtlegrass persists where sea urchin grazing is intense. We hypothesized that turtlegrass productivity is stimulated by grazing, as has been reported from terrestrial grassland systems, and that this best explains the persistence of heavily grazed turtlegrass in St. Joseph Bay, Fla. This hypothesis was tested by manipulating sea urchin densities (0, 10, and 20 individuals/m) in 1-m enclosures. These densities encompass the range of average densities at the study site and those reported in the literature. Changes in turtlegrass abundance (e.g., short shoot density and biomass), production by short shoots, and leaf width were monitored in these enclosures during the summer. Repeated-measures ANOVA showed that production by short shoots, leaf density/shoot, and leaf width decreased over time in all treatments. Leaf density/shoot and leaf width were not significantly impacted by grazing, nor did grazing significantly reduce seagrass biomass, but it did lead to significantly higher densities of short shoots than found in control cages. Our results indicate that turtlegrass compensates for the effects of sea urchin herbivory by increasing the recruitment of short shoots during the growing season. We estimate that this increased shoot density led to a 40% increase in net aboveground primary production (g dry weight/m) in grazing treatments, which helps to explain the lack of significant reductions of sea grass biomass during the growing season.
This paper presents the results of two cognitive load studies comparing three augmented reality display technologies: spatial augmented reality, the optical see-through Microsoft HoloLens, and the video see-through Samsung Gear VR. In particular, the two experiments focused on isolating the cognitive load cost of receiving instructions for a button-pressing procedural task. The studies employed a self-assessment cognitive load methodology, as well as an additional dual-task cognitive load methodology. The results showed that spatial augmented reality led to increased performance and reduced cognitive load. Additionally, it was discovered that a limited field of view can introduce increased cognitive load requirements. The findings suggest that some of the inherent restrictions of head-mounted displays materialize as increased user cognitive load.
-Picea mariana (Mill.) B.S.P. seedlings were hardened for overwintering under four regimes. In three regimes, seedlings were kept inside a heated greenhouse for 11 weeks during and after dormancy induction (August to mid-November), with either 1. Natural daylengths (46°31' N) and warm temperatures of 20 °C or more (NDW), 2. As 1, but fertilized (NDWF) or 3. As 1, but with shortened daylengths (SD). In the fourth regime (OD), seedlings were hardened outside at naturally declining temperatures and daylengths without fertilizer. Seedlings hardened in any warm temperature treatment had buds with more needle primordia and shoots more resistant to desiccation than OD seedlings. Initially, cold hardiness tended to be greatest in SD seedlings. As hardening progressed OD seedlings became equally cold hardy to SD. In late November when all trees were outside, NDW seedlings were usually least cold hardy. Spring root growth potential was least in OD seedlings.cold hardiness / desiccation / needle primordia / transpiration / water potential Résumé -Effets de la longueur du jour, de la température et de la fertilisation sur la résistance à la dessiccation et au froid, et au potentiel de croissance de plants de Picea mariana. On a soumis des semis de Picea mariana (Mill.) à quatre traitements pour les endurcir au froid en vue de la période hivernale. Pour trois traitements les plants ont été installés sous serre chauffée pendant 11 semaines, pendant et après l'induction de la dormance, avec les 3 modalités suivantes : (1) longueur naturelle du jour (latitude 46° 31' N) et chauffage à une température égale ou supérieure à 20 °C (NDW); (2) comme le traitement 1, mais avec fertilisation (NDWF); (3) comme le traitement 1 mais en jours courts (SD). Pour le quatrième traitement (OD) les plants ont été endurcis à l'extérieur avec la baisse de température et la diminution de la longueur des jours des conditions naturelles, sans faire appel à une fertilisation. Les plants issus des traitements comportant une phase sous serre chaude présentaient des bourgeons ayant davantage d'ébauches foliaires et des pousses plus résistantes à la dessication que les plants du traitement OD. Dans un premier stade, l'endurcissement au froid des plants SD tendait à être plus élevé. Ultérieurement celui des plants OD est devenu équivalent à celui des SD. Fin novembre, tous les plants étaient à l'extérieur, les plants NDW étaient moins résistants au froid. Le potentiel de croissance racinaire au printemps était moins élevé pour les plants OD. endurcissement pour la résistance au froid / dessication / ébauche racinaire / transpiration / potentiel hydrique
Several factors affecting the stratification process in seeds of A. saccharum were investigated. Rates of imbibition and quantitative measurements of flow rate made with an osmometer indicate that the testa offers a severe restriction to water flow and that a significant portion of the stratification period is required for adequate water uptake. Respiration rates showed a close correlation with water uptake by the embryos throughout the stratification period. Whole seed needed 40–60 days to break dormancy; however, removal of the pericarps and tearing of the testas reduced the time needed by as much as two-thirds. Presoaking whole seed at 5 °C had a similar effect. Seeds with their pericarps removed and testas pricked and treated with gibberellic acid or kinetin provided the highest percentage germination and germinated in the shortest time. Additional tests showed that oxygen was not a limiting factor, and an inhibitor present in the pericarp evidently does not play a significant role in the dormancy mechanism. The restricted movement of water through the testa apparently controls and complements the metabolic block in the embryo and accounts for the prolonged stratification requirement.
The response of black and white spruce and jack pine to slow and rapid induction of moisture stress was evaluated during soil drying and exposure to liquid cultures containing polyethylene glycol 8000. Marked changes in water, osmotic, and pressure potentials were observed in all three species, but water potentials were the most sensitive indicators of moisture stress. Osmotic potentials were sensitive to mild stress in polyethylene glycol (−400 kPa), but they held relatively steady at higher stress intensities (−800 to −1600 kPa). Large decreases in osmotic potential were recorded in white spruce and jack pine during severe drying stress late in the drought period and these changes were accompanied by large decrements in water and pressure potentials. Significant changes in osmotic potential were not observed in black spruce prior to seedling death. Losses in pressure potential were only observed at −1600 kPa of polyethylene glycol stress, although wilting in young tissue was apparent at lower stress intensities. Pressure potentials of plants in the soil-drying test fell well below those recorded in the polyethylene glycol study. The highest resistance to loss of turgor and the maximum adjustment to moisture stress were observed in white spruce. Shoot growth and transpiration declined in the three conifers at relatively low stress intensities. Total chlorophyll and the chlorophyll stability index decreased during drought, but the differences recorded among species were not clearly related to observed differences in drought tolerance. Cation concentrations did not change during stress.
The effects of drought on the free amino acid pools in 21- to 23-week-old seedlings of black spruce (Picea mariana (Mill.) Britt.), white spruce (Picea glauca (Moench.) Voss) and jack pine (Pinus banksiana Lamb.) were followed during soil drying. Although water and pressure potentials were sensitive to water deficits, large changes in osmotic potential were not recorded until after the development of severe drought. Total soluble amino nitrogen in the shoots and roots of the three species rose as turgor declined, with peak concentrations attained late in the drought period when the pressure potentials of the shoots approached zero. All white spruce seedlings were alive at zero turgor and showed large decrements in osmotic potential, but concentrations of free amino nitrogen in the roots and shoots showed only modest increases, reaching 125 to 150% of their control values. In contrast, large numbers of black spruce and jack pine were dead or severely damaged at zero turgor, and only small changes in osmotic potential were detected during soil drying. Nevertheless, concentrations of soluble amino nitrogen in both species reached 150 to 200% of control values a few days before the seedlings died. Alanine, arginine, aspartic acid/asparagine, glutamic acid/glutamine, glycine, hydroxyproline and proline were the major components of the free amino acid pools under both water-stressed and non-stressed conditions, with the largest and most consistent increases observed in the roots of all three conifers. Although proline was an important and dynamic component of the free pools, absolute concentrations were commony equalled or exceeded by other free amino acids in the roots and shoots and nearly always exceeded by the concentration of aspartic acid/asparagine in both tissues. Differences in drought resistance among the three conifers were not reflected by unique patterns of amino acid accumulation or by large differences in absolute concentrations of the free amino acid pools.
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