Extraction of nitrate reductase from members of the South African Proteaceae

The inhibition of in vitro nitrate reductase activity in extracts of Protea spp. is shown to be due to polyphenolic constituents of the roots and shoots of the plants (which can be adsorbed by insoluble polyvinylpyrrolidone) rather than to the activity of endogenous proteases. The in vitro nitrate reductase activity in shoots of Protea repens and Protea cynaroides fed 2mmoldm−3 NO3− for 24h prior to nitrate reductase extraction show a nitrate reductase activity of 2–4μmol NO2− h−1 while the roots of P. repens show an in vitro nitrate reductase activity of 0,2μmol NO2− h−1 (g fresh mass)−1. The low nitrate reductase activity of these plants possibly reflects their adaptation to growth under the low nutrient condition of the soils of the South Western Cape, South Africa

Seasonal variation in photosynthetic capacity with respect to plant water status of five species of the mediterranean climate region of South Africa

The seasonal variation in net photosynthetic rates of five plant species of the mediterranean climate region of South Africa (fynbos) was investigated. The following species, representing the major fynbos elements, were chosen for intensive investigation: Erica plukenetii (ericoid element), Thamnochortus lucens (restioid element), Protea laurifolia, Leucadendron salignum (proteoid element) and Metrosideros angustifolia, a shrub of riparian habitat. The shallow-rooted restioid and ericoid species showed the lowest net photosynthetic rates of all the species studied and they also exhibited the greatest summer declines in net photosynthesis. Photosynthetic rate generally decreased with increasing water stress. Those plant species which maintained relatively high photosynthetic rates throughout the year (the proteoid species) and those which were able to control excessive water loss (E. plukenetii) were more efficient at utilizing water as a resource than T. lucens which exhibited the lowest water-use efficiency

Differences in response between nitrate- and ammonium-fed maize to salinity stress and its amelioration by potassium

The effects of salinity (80mM NaCl) on the growth in hydroponics of ammonium- and nitrate-fed maize and the ameliorative role of potassium on salinity toxicity in these plants was assessed from measurements of growth rates and gas exchange characteristics. Ammonium-fed plants were more sensitive to salinity stress than their nitrate-fed counterparts. The photosynthetic and transpiration rates of ammonium-fed plants were significantly reduced by salinity whereas those of nitrate-fed plants were not significantly affected. Salinity caused 59 and 60% reductions in the shoot and root growth of ammonium-fed plants, respectively, whereas shoot and root growth of nitrate-fed plants was reduced by only 27 and 34%, respectively. In both nitrate- and ammonium-fed plants, raising the potassium concentration from 0.2 to 5mM in the presence of salinity produced a significant increase in maize growth.Die uitwerking van saliniteit (80mM NaCl) op die groei in waterkultuur van mielies wat met ammonium en nitraat gevoed is en die verbeterende rol van kalium op toksiese saliniteit by hierdie plante is met behulp van metings van groeitempo’s en gaswisselingskenmerke bepaal. Plante wat met ammonium gevoed was, was meer gevoelig vir saliniteitstremming as die wat met nitraat gevoed was. Die fotosintese- en transpirasie-tempo’s van plante wat met ammonium gevoed was, is betekenisvol deur saliniteit verminder, terwyl die van plante wat met nitraat gevoed was, nie noemenswaardig beïnvloed is nie. Saliniteit het verminderings van onderskeidelik 59 en 60% veroorsaak in die groei van stingels en wortels van plante wat met ammonium gevoed was, terwyl stingel- en wortelgroei van plante wat met nitraat gevoed was, slegs met onderskeidelik 27 en 34% gereduseer is. By plante wat met nitraat gevoed was sowel as by dié wat met ammonium gevoed was, het ’n verhoging van die kaliumkonsentrasie vanaf 0.2 na 5mM in die teenwoordigheid van saliniteit tot ’n betekenisvolle toename in die groei van mielies gelei