EFFECTS OF SUBSTRATE RELIEF ON THE DISTRIBUTION OF PERIPHYTON IN LABORATORY STREAMS. II. INTERACTIONS WITH IRRADIANCE1

Laboratory streams were used in a 42-day experiment designed to investigate how the spatial and temporal distribution of lotic periphyton created by current flow over cobble-size substrates is a affected by irradiance. The streams contained 22.5 × 22.5 × 4 cm substrate blocks and were exposed to either 385, 90 or 20 μE·m^?2·s^?1. We monitored periphyton succession in fast current regimes on top of blocks and in slower current regimes on surfaces recessed between blocks. The absolute differences in AFDW algal biomass between top and recessed substrates were significantly affected by irradiance and time. At the end of the experiment, biomass in streams exposed to 385 μE·m^?2·s^?1. was approximately 2 and 8 times greater than in streams exposed to 90 and 20 μE·m^?2·s^?1, respectively. Differences in biomass were greater between irradiance levels than between top and recessed substrates within an irradiance level. Irradiance also had a greater effect than current regime on the taxonomic composition of assemblages. Oscillatoria agardhii Gomont and Navicula minima Grun. characterized assemblages at 20 μE·m^?2·s^?1, whereas Fragilaria vaucheriae (Kütz.), Nitzschia oregona Sov., Navicula arvensis Hust. and Stigeoclonium tenue (Ag.) Kütz. were more abundant at the two higher irradiances. Detrended correspondence analysis indicated that the rate of succession was relatively high for assemblages at high irradiance and in the slow current regimes between blocks. The results suggested that in natural streams, periphyton patches produced by large differences in irradiance should have a greater effect on periphyton heterogeneity than substrate-induced patches. Moreover, the heterogeneity of algal patches produced by hydrologic differences over a substrate is constrained by irradiance level.     

PH-DEPENDENCE OF CHLORTETRACYCLINE(CTC)-INDUCED PLUG FORMATION IN NITELLA FLEXILIS (CHARACEAE)1

The formation of chlortetracycline(CTC)-induced wall appositions (callose plugs) in Nitella flexilis (L.)Ag. was pH-dependent in the range between 4.3-8.3. Plug number and plug diameter increased with the pH of the CTC solution. At pH 4.3 plug formation was light-dependent and occurred below the alkaline regions of the cell surface which form during photo synthetic assimilation of HCO₃^?. Inhibition of photosynthesis by 3–(3′,4′-dichlorophenyl)-1, 1-dimethylurea prevented plug formation in the light. Dark-treated cells could be induced to form plugs by raising the pH of the CTC solution. The formation of large but incomplete plugs in the presence of cytochalasin B is explained by the formation of numerous weak alkaline sites. I suggest that CTC enhances locally the Ca²⁺content at the cytoplasm near the plasmamembrane. The ionophoric character of CTC is probably more pronounced at high pH mainly because of a weaker binding with cations and a closer contact with the membrane.     

A New Method for Finding Small Vertebrate Fossils: Ultraviolet Light At Night

Vertebrate fossils from many different formations fluoresce when exposed to ultraviolet (UV) light. In this paper field observations and controlled experiments in the Chadron Formation (White River Group, late Eocene) of Wyoming are used to assess the utility of searching for fossils at night using ultraviolet light. The results indicate that, especially for very small teeth and egg-shell fragments, searching with ultraviolet light at night can result in significantly more specimens than searching during daylight hours. This method has the potential to increase sample sizes for small vertebrate specimens that are often overlooked when using standard collecting techniques.     

Feeding by the pine weevil Hylobius abietis in relation to sun exposure and distance to forest edges

Abstract 1 The intensity of feeding by adult pine weevils Hylobius abietis (L.) on the stem bark of Norway spruce Picea abies (L.) Karst. seedlings planted in rows with a north–south orientation across a clear‐cutting, was measured throughout a growth season. The feeding was then correlated to light interception, soil temperature and distance to the nearest forest edge. 2 Feeding was at least twice as intense on seedlings in the central part of the clear‐cutting compared to those at the edges. The decline began approximatety 15 m from the edge and was of similar proportions on both the sun‐exposed and shaded sides. 3 Measures of global radiation and soil temperature correlated well with consumption on the shaded side. However, on the sun‐exposed side, there were no apparent correlations with global radiation or soil temperature that could explain the decline in consumed bark area. 4 We conclude that the decline in feeding towards the forest edges was mainly due to factors other than the microclimate variables we monitored. We suggest that the presence of roots of living trees along the forest edge may reduce damage to seedlings, since they provide an alternative source of food for the weevils. This alternative‐food hypothesis may also explain why seedlings in shelterwoods usually suffer less damage from pine weevils than seedlings in clear‐cuttings.     

Impact of Atta colombica Colonies on Understory Vegetation and Light Availability in a Neotropical Forest1

We quantified patterns of vegetation removal and light availability above Atta colombica nests on Barro Colorado Island, Panama. Ants cleared vegetation less than 1 cm in diameter from an area of 77 m², and up to 3 m above ground level. Overall light availability 1.5 m above ground level was 49 percent greater at ant nest sites than at sites in undisturbed understory. These higher light levels fell within the range known to enhance growth of both shade tolerant and pioneer species.     

Foraging strategies in orb-spinning spiders: Ambient light and silk decorations in Argiope aetherea Walckenaer (Araneae: Araneoidea)

Abstract Many species of orb-spinning spiders construct silk decorations within the structure of the orb-web. The evolutionary significance of these decorations is poorly understood, but the silk decorations of many species reflect UV light, suggesting that they may function to attract insects. In these species, relatively more silk decoration may be required under dimmer light conditions in order to maintain a constant UV-reflecting signal, and hence level of insect attraction. We investigated experimentally whether the orb-spinning spider Argiope aetherea adjusts the amount of silk decoration added to the web according to light conditions. Consistent with the prey-attracting function, we found that spiders adjusted the quantity of silk decoration to their webs, adding more silk decoration when the web was located in dim light rather than bright light.     

Internal rhythms in humans

Human physiology and behavior are characterized by a daily internal temporal dimension. This so-called circadian rhythmicity is present for almost all variables studied to date, persists in the absence of external cycles, and is synchronized to the external 24-h world by an internally generated circadian rhythm of light sensitivity. The light-sensitive circadian pacemaker, presumably also in humans located in the suprachiasmatic nucleus of the hypothalamus, drives the endogenous circadian component of rhythmicity for a number of variables including plasma melatonin, alertness, sleep propensity and sleep structure. Overt rhythmicity and the consolidation of vigilance states are generated by a fine-tuned interaction of this circadian process with other regulatory processes such as sleep homeostasis.     

Does photorespiration protect the photosynthetic apparatus in french bean leaves from photoinhibition during drought stress?

Dithiothreitol (DTT), an inhibitor of violaxanthin de-epoxidation and zeaxanthin formation in chloroplasts, inhibited blue-light-stimulated stomatal opening in epidermal peels of Vicia faba L. in a concentration-dependent fashion. Complete inhibition was observed at 3 mM DTT. The DTT effect was specific for the stomatal response to blue light, and the red-light-stimulated opening, which depends on photosynthetic reactions in the guard cells, was unaffected. Preirradiation of stomata in epidermal peels with increasing photon fluence rates of red light, prior to an incubation in 10 mol·m^-2·s^-1 of blue light and 100 mol·m^-2·s^-1 red light, resulted in a DTT-sensitive, blue-light-stimulated opening that was proportional to the fluence rate of the red light pre-treatment. Guard cells in epidermal peels and guard-cell protoplasts irradiated with red light showed increases in their zeaxanthin content that depended on the fluence rate of red light, or on the incubation time. The increases in zeaxanthin concentration were inhibited by DTT. The obtained results indicate that zeaxanthin could function as a photoreceptor mediating the stomatal responses to blue light.Abbreviation DTT dithiothreitol This work was supported by grants from the National Science Foundation and the US Department of Energy to E.Z.     

Enhancement of the light-triggered electrical response in plant cells following their de-energization with uncouplers

Light-triggered membrane potential changes in cells of a liverwort Anthoceros are greatly enhanced by the ionophorous uncouplers nigericin and monesin. Stimulation of the light-triggered electrical response (LTER) by nigericin occurred concomitantly with inhibition of a slow decline in the chlorophyll fluorescence, which suggests that the transmembrane pH gradient in thylakoids is not essential for generation of LTER at the plasma membrane. The extent of monensin-stimulated LTER remained high under a diminished driving force for the ionophore-induced proton-cation exchange across the plasma membrane (elevation of the external Na⁺ concentration from 1 to 50 m M ), which indicates that energy uncoupling in chloroplasts is more related to the electric response enhancement than the induction of the H⁺/K⁺(Na⁺) exchange at the plasma membrane. Enhancement of LTER by ionophores occurs in parallel with stimulation of light-triggered pH changes (alkalinization) in the vicinity of the cell surface, which suggests an association of trans-membrane H⁺ fluxes with LTER. The results are consistent with the hypothesis that illumination produces a temporary inhibition of the plasma membrane H⁺ pump with a subsequent activation of gated channels and transient rapid depolarization of the cell.     

Expression of ferredoxin-dependent glutamate synthase in dark-grown pine seedlings

Pine seedlings are able to accumulate chlorophylls and develop green plastids in a light-independent manner. In this work, we have characterized ferredoxin-dependent glutamate synthase (EC 1.4.7.1; Fd-GOGAT), a key enzyme in nitrogen interconversion during this process. Fd-GOGAT has been purified about 170-fold from cotyledons of maritime pine (Pinus pinaster). As occurs in angiosperms, the native enzyme is a single polypeptide with an apparent molecular mass of 163–168 kDa that is confined to the chloroplast stroma. Polyclonal antibodies generated against the purified enzyme were used to immunoscreen a gt11 expression library from Scots pine (Pinus sylvestris) seedlings and partial cDNA clones were isolated and characterized. The clone with the longest cDNA insert (pGOP44) contained the codification for the C-terminal (550 amino acids) of the pine Fd-GOGAT polypeptide. Immunological cross-reactivity and comparative amino sequence analysis revealed that Fd-GOGAT is a well conserved protein in higher plants. Western blot analyses showed that protein was expressed in chloroplast-containing pine tissues and this expression pattern was not affected by exogenously supplied nitrogen. Fd-GOGAT mRNA, polypeptide and enzyme activity accumulated in substantial amounts in dark-grown pine seedlings. The presence of a functional Fd-GOGAT may be important to provide the required glutamate for the biosynthesis of nitrogen compounds during chloroplast biogenesis in the dark.