Diverse Optical and Photosynthetic Properties in a Neotropical Dry Forest during the Dry Season: Implications for Remote Estimation of Photosynthesis1

Using optical and photosynthetic assays from a canopy access crane, we examined the photosynthetic performance of tropical dry forest canopies during the dry season in Parque Metropolitano, Panama City, Panama. Photosynthetic gas exchange, chlorophyll fluorescence, and three indices derived from spectral reflectance (the normalized difference vegetation index, the simple ratio, and the photochemical reflectance index) were used as indicators of structural and physiological components of photosynthetic activity. Considerable interspecific variation was evident in structural and physiological behavior in this forest stand, which included varying degrees of foliage loss, altered leaf orientation, stomatal closure, and photosystem II downregulation. The normalized difference vegetation index and the simple ratio were closely related to canopy structure and absorbed radiation for most species, but failed to capture the widely divergent photosynthetic behavior among evergreen species exhibiting various degrees of downregulation. The photochemical reflectance index and chlorophyll fluorescence were related indicators of photosynthetic downregulation, which was not detectable with the normalized difference vegetation index or simple ratio. These results suggest that remote sensing methods that ignore downregulation cannot capture within‐stand variability in actual carbon flux for this diverse forest type. Instead, these findings support a sampling approach that derives photosynthetic fluxes from a consideration of both canopy light absorption (e.g., normalized difference vegetation index) and photosynthetic light‐use efficiency (e.g., photochemical reflectance index). Such sampling should improve our understanding of controls on photosynthetic carbon uptake in diverse tropical forest stands.     

Ecological and evolutionary consequences of desiccation tolerance in tropical fern gametophytes

Ferns have radiated into the same diverse environments as spermatophytes, and have done so with an independent gametophyte that is not protected by the parent plant. The degree and extent of desiccation tolerance (DT) in the gametophytes of tropical fern species was assessed to understand mechanisms that have allowed ferns to radiate into a diversity of habitats. Species from several functional groups were subjected to a series of desiccation events, including varying degrees of intensity and multiple desiccation cycles. Measurements of chlorophyll fluorescence were used to assess recovery ability and compared with species ecology and gametophyte morphology. It is shown that vegetative DT (rare in vascular plants) is widely exhibited in fern gametophytes and the degree of tolerance is linked to species habitat preference. It is proposed that gametophyte morphology influences water-holding capacity, a novel mechanism that may help to explain how ferns have radiated into drought-prone habitats. Fern gametophytes have often been portrayed as extreme mesophytes with little tolerance for desiccation. The discovery of DT in gametophytes holds potential for improving our understanding of both the controls on fern species distribution and their evolution. It also advances a new system with which to study the evolution of DT in vascular plants.     

Hyperbaric oxygen and chemical oxidants stimulate CO2/H+-sensitive neurons in rat brain stem slices.

Hyperoxia, a model of oxidative stress, can disrupt brain stem function, presumably by an increase in O2 free radicals. Breathing hyperbaric oxygen (HBO2) initially causes hyperoxic hyperventilation, whereas extended exposure to HBO2 disrupts cardiorespiratory control. Presently, it is unknown how hyperoxia affects brain stem neurons. We have tested the hypothesis that hyperoxia increases excitability of neurons of the solitary complex neurons, which is an important region for cardiorespiratory control and central CO2/H+ chemoreception. Intracellular recordings were made in rat medullary slices during exposure to 2-3 atm of HBO2, HBO2 plus antioxidant (Trolox C), and chemical oxidants (N-chlorosuccinimide, chloramine-T). HBO2 increased input resistance and stimulated firing rate in 38% of neurons; both effects of HBO2 were blocked by antioxidant and mimicked by chemical oxidants. Hypercapnia stimulated 32 of 60 (53%) neurons. Remarkably, these CO2/H+-chemosensitive neurons were preferentially sensitive to HBO2; 90% of neurons sensitive to HBO2 and/or chemical oxidants were also CO2/H+ chemosensitive. Conversely, only 19% of HBO2-insensitive neurons were CO2/H+ chemosensitive. We conclude that hyperoxia decreases membrane conductance and stimulates firing of putative central CO2/H+-chemoreceptor neurons by an O2 free radical mechanism. These findings may explain why hyperoxia, paradoxically, stimulates ventilation.     

Pressure (< or=4 ATA) increases membrane conductance and firing rate in the rat solitary complex.

Neuronal sensitivity to pressure, barosensitivity, is illustrated by high-pressure nervous syndrome, which manifests as increased central nervous system excitability when heliox or trimix is breathed at >15 atmospheres absolute (ATA). We have tested the hypothesis that smaller levels of pressure (相似文献   

Colonization Strategies of Two Liana Species in a Tropical Dry Forest Canopy

Lianas impose intense resource competition for light in the upper forest canopy by displaying dense foliage on top of tree crowns. Using repeated access with a construction crane, we studied the patterns of canopy colonization of the lianas Combretum fruticosum and Bonamia trichantha in a Neotropical dry forest in Panama. Combretum fruticosum flushed leaves just before the rainy season, and its standing leaf area quickly reached a peak in the early rainy season (May–June). In contrast, B. trichantha built up foliage area continuously throughout the rainy season and reached a peak in the late rainy season (November). Both species displayed the majority of leaves in full sun on the canopy surface, but C. fruticosum displayed a greater proportion of leaves (26%) in more shaded microsites than B. trichantha (12%). Self-shading within patches of liana leaves within the uppermost 40–50 cm of the canopy reduced light levels measured with photodiodes placed directly on leaves to 4–9 percent of light levels received by sun leaves. Many leaves of C. fruticosum acclimated to shade within a month following the strongly synchronized leaf flushing and persisted in deep shade. In contrast, B. trichantha produced short-lived leaves opportunistically in the sunniest locations. Species differences in degree of shade acclimation were also evident in terms of structural (leaf mass per area, and leaf toughness) and physiological characters (nitrogen content, leaf life span, and light compensation point). Contrasting leaf phenologies reflect differences in light exploitation and canopy colonization strategies of these two liana species.     

Reducing Energy Inputs in the US Food System

Petroleum and natural gas are the primary fuels in the US food system. Both fuels are now in short supply and significant quantities are being imported into the USA from various nations. An investigation documented that fossil energy use in the food system could be reduced by about 50% by appropriate technology changes in food production, processing, packaging, transportation, and consumption. The results suggest that overall, farmers benefit as well as consumers.     

Motor units of the primary ankle extensor muscles of the opossum (Didelphis virginiana): functional properties and fiber types

Motor units of the medial gastrocnemius (MG) and the single lateral gastrocnemius/soleus (LG/S) muscles of the opossum (Didelphis virginiana) were found to have uniformly slow contraction times relative to homologous muscles of the cat. Though a broad range of peak tetanic tensions was found among motor units from both muscles, most of the motor units were quite large relative to tension of the whole muscle. Comparison of the relative sizes of motor units showed that those of LG/S are significantly larger and slower than the units of MG. This suggests that the motor units of the two muscles may be differentially recruited during different behaviors. All of the MG and LG/S motor units were highly or moderately resistant to fatigue. Histochemical staining for NADH-diaphorase activity indicated consistently high levels of the enzyme in all of the fibers of both muscles. Apparently, all of the fast motor units consist of fast oxidative/glycolytic (FOG)-type muscle fibers. Our data provide functional evidence that the types of myofibrillar ATPase demonstrated by Brooke and Kaiser ('70), are not necessarily correlated to physiological classification of fiber types as slow oxidative (SO), fast oxidative/glycolytic (FOG), and fast glycolytic (FG) (Peter et al., '72). Perhaps compartmentalization of muscle fiber types may be a first step in the separation of muscles into multiple heads during the evolution of specialization to diverse locomotor habits among the mammals.