K. Hardwick

The effects of floating mats of Azolla filiculoides Lam. and Lemna minuta Kunth on the growth of submerged macrophytes

In laboratory experiments, the growth characteristics of the submerged species Elodea nuttallii (Planch.) St. John and Potamogeton crispus L. were assessed in the presence and absence of floating mats of Azolla filiculoides Lam. and Lemna minuta Kunth. Light penetration and the development of pH and dissolved oxygen differences were monitored. The growth of P. crispus was suppressed much more than that of E. nuttallii and the effects of A. filiculoides were more severe than those of L. minuta. Findings are related to possible field responses of submerged plants under floating mats, especially their abilities to compensate for the potential suppressive effects of floating mats under natural conditions.

Intracellular Localization of Lead in a Lead Tolerant and Sensitive Clone of Anthoxanthum odoratum

Summary Tillers from lead tolerant and non-tolerant plants of Anthoxanthum odoratum were incubated in the presence of 30μM lead for 7 days then the root tips examined for the intracellular distribution of lead. Electron microscopy of sections of both chemically fixed and freeze dried sections of root tips showed an accumulation of electron dense material in the cell walls which was assumed to be lead. An X-ray probe analysis of sections of chemically fixed root tips from tolerant plants also showed lead to be present in the cell walls and excluded from the cytoplasm. Examination of the non-tolerant material by electron microscopy alone again showed electron dense material in the cell walls, and also in the cytoplasm. It was suggested that the tolerance mechanism for lead in A. odoratum was related to the capacity of the tolerant plants to restrict the lead to the cell walls.

The role of the cell wall in the mechanism of lead and zinc tolerance in Anthoxanthum odoratum L.

Abstract The respiration rate and viability of cultured cells and protoplasts isolated from two clones of Anthoxanthum odoratum tolerant to both zinc and lead were unaffected by the presence of zinc. Although intact cells were largely unaffected by the presence of lead, protoplasts isolated from cultured cells were susceptible, showing a reduced respiration rate and a high mortality. In contrast cultured cells and protoplasts of non-tolerant clones of A. odoratum were susceptible to both zinc and lead. The results provide direct evidence that in A. odoratum the cell wall is part of the mechanism of tolerance to lead, but not to zinc.

Competition between three submerged macrophytes, Elodea canadensis Michx, Elodea nuttallii (Planch.) St John and Lagarosiphon major (Ridl.) Moss

During photosynthesis, submersed aquatic plant stands create ‘envelopes’ of raised pH and dissolved O2 concentrations and depleted dissolved CO2 concentrations. These may extend beyond the plant stands, suppressing the photosynthetic performance and increasing the photorespiratory stress of adjacent species. In competition between three aquatic macrophytes, Elodea canadensis, Elodea nuttallii and Lagarosiphon major, we hypothesised that a competitive advantage is achieved by those species that have the best stress generation/toleration mechanisms. pH and free CO2 conditions were measured under laboratory conditions at two plant densities and compared with measurements of photosynthesis and respiration at various pH and free CO2 concentrations. L. major raised pH faster and photosynthesised at a greater rate under the experimental conditions and may gain competitive advantage at high pH. No clear differences in photosynthetic behaviour were observed between the Elodea spp. and it is concluded that a wider range of factors than those studied here are probably involved in competition between the three species.

Diurnal carbon restrictions on the photosynthesis of dense stands of Elodea nuttallii (Planch.) St. John

In slow-moving and static eutrophic waters, submerged macrophytes growing in dense stands produce a highly structured environment, with reduced internal water flow. An afternoon lull in the net photosynthesis of such stands has been reported from a number of previous studies. This has been attributed to increased photorespiration caused by an accumulation of photosynthetically-derived, dissolved oxygen in the surrounding water. Results here demonstrate that even in a water quite rich in dissolved inorganic carbon (2.5 mmol l−1), limitations on the supply of inorganic carbon will normally be more important in curtailing photosynthesis, with photorespiration playing only a minor role.

Metal tolerance in tissue cultures of anthoxanthum odoratum

Tissue cultures were initiated from one non-tolerant (S20) and two zinc and lead tolerant (T92 and T94) clones of Anthoxanthum odoratum. Growth of callus from the non-tolerant clone was reduced by the presence of zinc, lead, copper and nickel, whereas callus from the two tolerant clones showed no reduction of growth in the presence of zinc and lead but growth was reduced by copper and nickel. The specificity of metal tolerance shown by the parental material was maintained in the callus. Tolerant and non-tolerant callus accumulated similar amounts of zinc and lead.

Malic Acid Production in Callus Cultures of Zinc and Lead Tolerant and Non-tolerant Anthoxanthum odoratum

Summary Callus cultures derived from two zinc and lead tolerant clones and one non-tolerant clone of Anthoxanthum odoratum were grown in medium containing high concentrations of zinc (20 mg· 1 -1 ), lead (25 mg · 1 -1 ), or copper (1 mg · 1 -1 ), then the malic acid levels measured after three weeks. High concentrations of zinc caused an increase to twice the control level of malic acid in both tolerant and non-tolerant clones. Lead also caused an increase in malic acid levels in all the three clones whereas in the presence of copper there was a small increase in malic acid in the non-tolerant clone and a reduction in the two zinc and lead tolerant clones. It was concluded that the accumulation of malic acid in all clones occurred as a response to high external concentrations of metal. The accumulation of malic acid was not seen to have a primary role in the mechanism of metal tolerance.

Isolation and Properties of Protoplasts of Theobroma Cacao L.

Protoplasts provide an important technique for the study of plant: fungal interaction in disease. For cocoa (Theobroma cacao) the major disease is Witches’ Broom, caused by the fungus Crinipellis perniciosa. The primary infective stage of the fungus has a biotrophic mode of nutrition whereas the succeeding secondary stage has a necrotrophic mode of nutrition which causes tissue death (Evans, 1980). Preliminary evidence has shown the existence of phytotoxic activity in culture filtrates from the secondary stage. The mechanism of action of these phytotoxins may be through an effect on membrane permeability, which can be measured most easily using protoplasts of cocoa. The aim of this project was to:- a) Develop a method for protoplast isolation from cocoa leaves. b) Test the effect of culture filtrate from secondary stage mycelium on mesophyll protoplasts.