Variation in stomatal characteristics over the lower surface of Commelina communis leaves

Abstract. The silicone rubber impression technique was used to measure stomatal apertures in 9 mm² sampling areas covering the entire lower surface of leaves of Commelina communis L. The data were analysed using a computer program which produced 'iso-aperture' contours illustrating local differences in mean stomatal aperture. Little consistency was seen in the iso-aperture patterns among sampling times, although the stomata were always relatively closed at the leaf tip and base. When stomata in the middle of the lamina were open, those near to the leaf margin tended to be relatively closed. In places, gradients of mean stomatal aperture were as high as 1 μm mm⁻¹. Measurements along a transect across the lower epidermis revealed no correlation of stomatal aperture with the presence of major veins in the mesophyll tissue. Variation in guard-cell size and stomatal frequency on the lower leaf surface was also analysed. The guard cells were smallest and the stomata more frequent near to the leaf margins. The significance of the results is discussed in relation to measurements of leaf conductance and models of stomatal function.     

Soil‐nutrient availability under a global‐change scenario in a Mediterranean mountain ecosystem

Changes in rainfall availability will alter soil‐nutrient availability under a climate‐change scenario. However, studies have usually analyzed the effect of either drier or wetter soil conditions, despite the fact that both possibilities will coexist in many climatic regions of the world. Furthermore, its effect may vary across the different habitats of the ecosystem. We experimentally investigated the effect of three contrasting climatic scenarios on different carbon (C), nitrogen (N), and phosphorus (P) fractions in soil and microbial compartments among three characteristic habitats in a Mediterranean‐type ecosystem: forest, shrubland, and open areas. The climatic scenarios were dry summers, according to the 30% summer rainfall reduction projected in the Mediterranean; wet summer, simulating summer storms to reach the maximum historical records in the study area; and current climatic conditions (control). Sampling was replicated during two seasons (spring and summer) and 2 years. The climatic scenario did not affect the nutrient content in the litter layer. However, soil and microbial nutrients varied among seasons, habitats, and climatic scenarios. Soil‐nutrient fractions increased with lower soil‐moisture conditions (dry scenario and summer), whereas microbial nutrients increased under the wet summer scenario and spring. This pattern was consistent both studied years, although it was modulated by habitat, differences being lower with denser plant cover. Holm oak seedlings, used as live control of the experiment, tended to increase their N and P content (although not significantly) with water availability. Thus, the results support the idea that higher rainfall boosts microbial and plant‐nutrient uptake, and hence nutrient cycling. By contrast, a rainfall reduction leads to an accumulation of nutrients in the soil, increasing the risk of nutrient loss by leaching or erosion. These results show that the projected climate change will have significant effects on nutrient cycles, and therefore will have important implications on the ecosystem functioning.     

Pterosaur tracks and the terrestrial ability of pterosaurs

Unusual tracks of a quadrupedal animal with a three-digit (occasionally four-digit) manus print and four-digit pes print were first interpreted as those of pterosaurs in the 1950s. In the 1980s these tracks were reinterpreted as crocodilian, but new material shows that the original identification was correct. Two features: evidence for elongate penultimate phalanges in digits two to four of the pes, and manus trackways up to three times the width of pes trackways, can only be attributed to pterosaurs. Recent improvements in understanding of pterosaur anatomy and functional morphology explain remaining difficulties regarding the interpretation of ich-nites such as the orientation of the manus digits and the absence of some expected ichnological features. Pteraichnus and Pteraichnus-like tracks show that, when grounded, some, perhaps all, pterosaurs were plantigrade, quadrupedal, and had a semi-erect stance and gait. This is consistent with some functional interpretations of pterosaur anatomy and resolves a long-running debate regarding the terrestrial ability of this group.     

Photoreversible association of phytochrome with membranes. I. Distinguishing between two light-induced binding responses

Abstract Two types of association between phytochrome and crude membrane fractions from oat (Avena sativa L.) are distinguished and compared, and that which comprises only a small fraction of the total phytochrome in extracts prepared in the absence of added divalent cations (Watson & Smith. 1982b) has been studied in detail. Extraction in the presence of phenylmethylsulphonyl fluoride shows that proteolysis of P_r (the red-light absorbing form) probably does not account for the lower levels of membrane-associated phytochrome measured after far-red light than after red light. Difference spectra of soluble and membrane-associated phytochrome indicate that the latter is much less susceptible to spectral degradation in vitro than is the soluble pool. The stoichiometry of association with the membranes is such that for each phytochrome molecule associated after far-red light there are three associated after red light and it is argued that this stoichiometry is maintained independent of the extraction pH. The characteristics of this photo-reversible association of phytochrome with membranes are compared to the characteristics of the widely studied light-induced enhancement of phytochrome pelletabilily that is dependent on electrostatic interaction of phytochrome and membranes.     

WHEAT POWDERY MILDEW INVESTIGATIONS

The development of Erysiphe graminis f. tritici Em. Marchal, from spore germination to spore formation, occupied 6 days and ascospore discharge in the field covered a period of i month. On immune hosts development of the fungus generally ceased at the point of invasion of cell cytoplasm. The features of host plant resistance have been discussed.
Lithium chloride soil applications were made as o-8% solution in glasshouse seedling plant experiments and at 56 lb./acre rate in a factorial field trial. In the former, mildew was reduced from 11% leaf area infected to 1 %, but under field conditions only from 1–29 to 1–04% over two sampling periods. The limiting effects of lithium on mildew began to decline 18 weeks after soil treatment.
Race 4 of E. graminis f. tritici (Canadian typing) was present in all, and Race 3 in about half the New Zealand wheat localities during 1948-9. The infection types of Races 3 and 4 on 180 wheat varieties and breeding selections have been recorded, and the basis for a mildew resistance plant-breeding programme is discussed, including consideration of adult plant resistance.     

THE EFFECT OF SALT CONCENTRATION ON THE GROWTH OF BACTERIA IN DILUTE NUTRIENT SOLUTIONS

SUMMARY: In dilute nutrient solutions Bacterium coli and Streptococcus faecalis were able to grow only over a narrow range of salt concentrations.