Comparative anatomy and morphology of fertile complexes of Prumnopitys and Afrocarpus species (Podocarpaceae) as revealed by histology and NMR imaging, and their relevance to systematics

Fertile complexes (individual reproductive units of ovulate cones) of three Prumnopitys species and Afrocarpus falcatus (Podocarpaceae) were subjected to histological examination and non-destructive NMR imaging. The latter technique allowed the display, frame-by-frame analysis and electronic 'dissection' of internal structures such as the number and courses of vascular traces and resin canals and their morphology. Characters of these internal structures distinguished all three Prumnopitys species from each other and thus were shown to be taxonomically diagnostic. Fertile complexes of Prumnopitys andina and P. taxifolia were most similar, possessing simple vascular traces and few unbranched resin canals. Those of P. ferruginea were very different and possessed an interconnected network of resin ducts within the sarcotesta. These findings are congruent with relationships inferred from molecular phylogenetic studies, in which two subclades were recovered within Prumnopitys . The anatomy of the female fertile complexes of Afrocarpus falcatus was very distinct from all Prumnopitys species analysed. Its most distinctive feature was the existence of a complex network of radial vascular strands originating from within the outer layers of the sarcotesta and penetrating the inner layers of the fertile complex. The surface texture and morphology of the sclerotesta of the seed was also unique to each species.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 145 , 295–316.     

C4 characteristics of photosynthesis in the C3 alga Hydrodictyon africanum

Abstract Results obtained with Hydrodictyon africanum, and data from the literature, show that most green algae of the chlorophyte type (e.g. Chlorella, Chlamydomonas, Hydrodictyon) differ in their photosynthetic C fixation characteristics from most green algae of the charophyte type (e.g. Spirogyra, Chara) and from C₃ higher plants. The chlorophyte algae fix inorganic carbon by the photosynthetic carbon reduction cycle pathway, but have a low CO₂ compensation point in 250 μM O₂, a low inhibition of CO₂ fixation from 10 μM CO₂/250 μM O₂ when compared with 10 μM CO₂/zero O₂, and a low half-saturation constant for CO₂. These three characteristics are different from those of charophytes and C₃ higher plants, and resemble those of C₄ higher plants. It is suggested that these characteristics of chlorophyte algae are the result of a ‘CO₂ concentrating mechanism’ which increases the CO₂/O₂ ratio at the site of ribulose bisphosphate carboxylase-oxygenase action in a similar way to that achieved by the C₄?C₃ acid cycle in C₄ plants. In the chlorophyte algae, however, CO₂ concentration probably involves active HCO₃^? transport at the inner membrane of the chloroplast envelope. Active HCO₃^? transport can occur at the plasmalemma of charophyte algae and submerged aquatic higher plants as well as chlorophyte algae, so it is unlikely to explain the differences between the two groups of aquatic green plants. Differences in the properties of ribulose bisphosphate carboxylase-oxygenase, and differences in CO₂ production in the light, also seem inadequate to account for the different photosynthetic characteristics. The chlorophyte type of ‘C0₂ concentrating mechanism’ appears to be common in other classes of eukaryotic algae, and in cyanophytes. Some of the ‘advanced’ members of these eukaryotic algal classes (including the chlorophytes) may lack the mechanism, while some ‘primitive’ charophytes may retain the mechanism which their ancestors presumably possessed.     

Photorespiration: Ribulose Diphosphate Oxygenase or Hydrogen Peroxide?

Oxygen uptake and evolution in illuminated cells of Hydrodictyonqfricanum have been measured using ¹⁸O₂ mass spectrometry. Earlierwork showed that the light-stimulated O₂ uptake under conditionsof light and CO₂ saturation was insensitive to cyanide and toCCCP. This was interpreted as due to a pseudocyclic electronflow, possibly associated with glycolate synthesis by the pathwayproposed by Coombs and Whittingham. The work reported here wascarried out at the CO₂ compensation point, where light-dependentO₂ uptake is higher than at CO₂ saturation. The component ofO₂ uptake which is stimulated by lowering the CO₂ concentrationis inhibited by either cyanide or CCCP. This component is attributedto the activity of ribulose diphosphate (RuDP) oxygenase, whichis completely inhibited by CO₂ at high CO₂ levels. The rateof pseudocyclic electron flow seems to be insensitive to changesin CO₂ concentration between saturation and the compensationpoint     

Measurement of Simultaneous Oxygen Evolution and Uptake in Hydrodictyon africanum

Oxygen uptake and evolution in illuminated and darkened cellsof Hydrodictyon africanum have been measured using ¹⁸O₂ massspeetrometry. Under conditions of light and CO₂ saturation forphotosynthesis, light stimulates oxygen uptake more than two-fold.This stimulation is prevented by DCMU but is not affected bycyanide or the uncoupler CCCP. The data are consistent withthe occurrence of a pseudocyclic electron flow and photophosphorylationin vivo in H. africanum; this agrees with data on light-dependentactive phosphate influx in this alga. Part of the light-stimulatedoxygen uptake might be involved in glycolate synthesis by thepathway proposed by Coombs and Whittingham.