Effects of metals on enzyme activity in plants

Abstract. Uptake of phytotoxic amounts of metal by higher plants or algae can result in inhibition of several enzymes, and in increase in activity (= induction) of others. Two mechanisms of enzyme inhibition predominate: (1) binding of the metal to sulphydryl groups, involved in the catalytic actionor structural integrity of enzymes, and (2) deficiency of an essential metal in metalloproteins or metal-protein complexes, eventually combined with substitution of the toxic metal for the deficient element. Metal accumulation in the cellular compartment of the enzyme is a prerequisite for enzyme inhibition in vivo. The induction of some enzymes is considered to play a significant role in the stress metabolism, induced by metal phytotoxicity. Peroxidase induction is likely to be related to oxidative reactions at the biomembrane; several enzymes of the intermediary metabolism might be stimulated to compensate for metal-sensitive photosynthetic reactions. The induction of enzymes and metal-specific changes in isoperoxidase pattern can be used as diagnostic criteria to evaluate the phytotoxicity of soils, contaminated by several metals. Lines for future research on metal phytotoxicity are proposed, involving the study of inhibition and induction of enzymes at the different cell membranes (especially the plasmamembrane) in vivo.     

The influence of the rate of temperature change on the activation of dormant seeds of Rumex obtusifolius L.

1. One temperature shift from 20 to 30°C in darkness induces 30–40% germination in Rumex obtusifolius seeds. The same germination percentages are found with heat treatment varying between 1 and 6h duration, indicating that the total heat sum of the temperature shift is not important.
2. Germination is greatly enhanced by three consecutive heat shifts of 1h at 30°C separated by 1h periods at 20°C.
3. The seeds are activated to a small extent after a slow warming (+2°Ch^–1) from 20 to 30°C, followed by incubation for 1h at 30°C. Germination is much higher after rapid heating (+10°Ch^–1) to 30°C, followed by 1h incubation at this temperature. Repeated fast heating treatments on four consecutive days enhances germination. Moderately rapid heatings (+3·3°Ch^–1) give intermediate results.
4. The rate of cooling does not influence the germination percentage. Cooling alone cannot induce germination.
5. Heating alone from 15 to 25°C without cooling also activates germination. In this temperature range the seeds are more activated by rapid warming than by slow warming.
6. The ecological relevance of the response to different warming rate is discussed. The insensitivity of seeds to a slow warming might keep deeply buried seeds in a dormant stage.     

The use of inositol hexaphosphate as a phosphorus source by mycorrhizal and non-mycorrhizal Scots Pine (Pinus sylvestris)

1. The external mycelia of the ectomycorrhizal fungi Thelephora terrestris and Suillus luteus , associated with Pinus sylvestris roots, exhibited a substantial extracellular acid phosphatase activity. The activity was positively correlated with the ergosterol concentration in the growth substratum and decreased with an increasing P nutrition.
2. The pioneer species T. terrestris grew best at a high P_i nutrition level whereas S. luteus , a 'late-stage' mycobiont, produced more active biomass at a low P_i nutrition level.
3. The phytase activity of the external mycelia could not be detected; at the root surface a phytase activity was observed. Mycorrhizas had significantly higher activities than uninfected roots.
4. The addition of a relatively high concentration of a soluble phytate to the growth substratum resulted in an increased relative growth rate (RGR) in both mycorrhizal and non-mycorrhizal plants. The influence of the mycorrhizal fungi on the use of the phytate-P was small, despite the phytase activity of the mycorrhizal feeder roots.
5. The addition of phytate fixed on a HPLC resin did not result in an increase of the RGR and P uptake neither in the non-mycorrhizal nor in the mycorrhizal Pines. The experiment did not support the hypothesis that phytate, which has a low solubility in soils, is a useful P source for ectomycorrhizal plants.