A comparative study of the distribution and density of stomata in the British flora

The distribution of stomata over both leaf surfaces may affect both the photosynthetic rate and water use efficiency of species, implying that species with different photosynthetic and water requirements may also have different stomatal distributions. A database containing data on the distribution of stomata on the leaves of 469 British plant species was used to look for relationships between stomatal distribution (including both location on the leaf and density) and both habitat and morphological variables. Statistical models were applied to the data that minimized any effects that phylogenetic constraints may have had on the data.
Hypostomaty is common in woody species, species which typically occur in shaded habitats and species with large or glabrous leaves. Amphistomaty, however, predominates in species which occur in non-shaded habitats, species with small, dissected or hairy leaves, and in annual species. Amphistomaty, therefore, tends to occur in species where CO₂ may be limiting photosynthesis (unshaded environments), or where there are structures to prevent water loss from the leaf (e.g. hairs). Hypostomaty, however, occurs in slow-growing species (e.g. trees), species with leaves which have large boundary layers (large or entire leaves) and in species where CO₂ is unlikely to limit photosynthesis (shaded habitats).     

The Role of Cotyledons in Vegetative Growth and Nitrogen Assimilation by Soya Bean

The removal of both cotyledons from soya bean seedlings 10 daysafter sowing, when the primary leaves were unfolded, reducedtheir stem height, branching, leaf production and dry weightat flowering by a similar proportion whether they were nodulatedor nitrate-dependent. Nitrogen assimilation by the shoots ofnitrate-dependent plants was increased by the removal of onecotyledon and reduced by the removal of both cotyledons althoughthese effects were not significant. Both these treatments significantlyincreased the amount of nitrogen in the shoots of nodulatedplants at flowering, mainly by more than doubling the nitrogencontent of their leaves. In contrast, the proportion of thetotal plant nitrogen in the leaves of nitrate-dependent plantswas almost constant. These results suggest that the cotyledonsmarkedly inhibit nitrogen assimilation by nodulated plants butdo not appreciably affect nitrogen assimilation by plants dependentsolely on inorganic nitrogen for their nitrogen supply. Glycine mux (L) Merr., soya bean, cotyledons, nitrogen assimilation, growth     

The photosynthetic activity of the developing hop cone

Carbon dioxide exchange of the developing hop cone was measured in field and laboratory experiments. Older cones showed a clear response of CO₂ exchange to light intensity, indicating photosynthetic activity, but this rarely exceeded respiratory loss. Cones therefore made little photosynthetic contribution to their growth. Young cones showed no response of CO₂ exchange to light intensity and therefore possessed negligible photosynthetic activity. The initiation of photosynthesis was sudden and coincided with the onset of rapid cone growth and with the response of water loss to light intensity. It is suggested that the maturation of stomata is linked with the initiation of gas exchange.     

Photosystem II and oxygen regulation in Sesbania rostrata stem nodules

The tropical wetland legume Sesbania rostrata Brem. produces nitrogen-fixing stem nodules which are green and contain chlorophyll, the chloroplasts being concentrated in a hand in the inner and mid-cortex close to the nitrogen-fixing cells. The photosystem II thylakoid membrane proteins D₁, D₂ and PsbO, which are essential for photo-synthetic O₂ evolution, were shown by immunoblotting to be present in extracts of leaves and stem nodules. Immunogold labelling confirmed their presence on stem nodule thylakoids and showed that labelling was most intense in well-developed chloroplasts in the mid-cortex and least intense in the smaller, less-abundant chloroplasts adjacent to the nitrogen-fixing cells. Concentrations of the oxygen-carrying protein leghaemoglobin (Lb) did not differ between stem and S. rostrata root nodules, and Lb was localized in bacteroid-containing cells, including those immediately adjacent to the cortex, in both nodule types. Moreover, nitrogenase component 2 was localized in bacteroids within the outermost layers of infected cells, suggesting that a low pO₂ was maintained, despite the nearby chloroplasts. Nodule extracts examined by ELISA and immunoblots, using the monoclonal antibody MAC265, showed greatly enhanced expression of a 139 kDa glycoprotein in stem compared to root nodules. Immunogold labelling showed that material containing the MAC265 antigen occluded intercellular spaces, and was present in cell walls, throughout the cortex of stem nodules (particularly in the chloroplasl-rich inner and mid-cortex), but was considerably less evident in root nodules.     

Fine Structure of Phialide and Conidiospore Development in Aspergillus giganteus 'Wehmer'

The conidiophore vesicle is composed of a peripheral regionwhich contains many nuclei and mitochondria and a central regionwhich is densely packed with glycogen granules but containsvery few organelles. The phialides, which arise as sphericalprotuberances from the surface of the vesicle, are bounded bya much thinner wall than that of the vesicle. Nuclei migrateinto the phialides at a comparatively early stage in their development.Septa are present at the proximal ends of mature phialides andalthough pores are present in these septa it is suggested thatthey are too small to permit the ingress of such organellesas nuclei and mitochondria from the vesicle. The developmentof adjacent phialides is well synchronized but the developmentof the vesicle as a whole is not precisely co-ordinated. Eachmature phialide contains a single nucleus, as do the conidiosporeswhich they produce. The first conidiospore produced by a phialidearises as a spherical protrusion from the tapered, thickenedapex of the phialide. The conidiospores are delimited from thephialide by the formation of a cross wall in a manner reminiscentof the process of septation in hyphae. When first formed theconidiospores have a cylindrical shape and they only assumetheir characteristic spherical shape at a later stage in development.     

Young Reproductive Structures Promote Nitrogen Fixation in Soya Bean

In many legumes the transition from the vegetative to the reproductivephase of development is associated with a marked increase inthe rate of symbiotic nitrogen fixation. In soya bean [Glycinemax (L.) Merr.), the removal of reproductive parts at differentstages of their development showed that the increase in nitrogenfixation rate was primarily due to the presence of flower buds.The increase in the fixation rate of intact reproductive plantswas accompanied by a rapid increase in the weight of noduleson lateral roots and it is suggested that these nodules areresponsible for much of the nitrogen fixation which occurs duringreproductive growth. Maintaining plants in the vegetative stateprovided evidence which suggests that it is the flower budsand not the flowering stimulus which are responsible for theincrease in fixation rate. The marked effects on vegetativegrowth of removing reproductive parts suggests that the mechanisminvolved in the promotion of nitrogen fixation may be hormonal. Glycine max (L.) Merr., soya bean, nitrogen fixation