Mineral nutrition of micropropagated plants

Abstract

Aspects of the mineral nutrition of plants in in vitro cultures were studied in Delphinium 'Princess Caroline' and Hosta 'Fladspen Blue', two species with contrasting growth habits. Initially methods for accurately measuring mineral concentrations within gelled culture media using atomic absorption spectrophotometry were developed and verified.
The effects of light, EDTA, phosphate and agar concentration on mineral availability were studied. Substantial quantities of mineral impurities are introduced by the inclusion of agar and sucrose in culture media and significant precipitation was shown to occur within culture media and to be mediated by the absolute phosphate concentration of the medium. The chelating agent EDTA was most beneficial to growth when used in an equimolar ratio with iron.
Rates of mineral diffusion in agar-gelled media were measured for a range of gel strengths. It was shown that the diffusion of phosphate was inhibited more than that of other minerals and also that the restriction of diffusion was dependant on the
concentration of agar used. The common antagonistic interaction between potassium and magnesium was found not to occur in Murashige and Skoog medium, possibly because of the different structure of an agar-gelled medium compared with that of a typical soil.
Plant growth and the associated depletion of medium mineral reserves were studied over extended culture periods and showed a rapid depletion of phosphate from the culture medium, a low rate of iron uptake in both species and low tissue phosphate concentrations. Medium pH was significantly affected by the plants and a possible link between medium pH and the relative uptake of nitrate and ammonium was investigated.
The rapid initial pH changes in Delphinium cultures were shown not to be the result of differential nitrate and ammonium uptake rates though pH shifts in the culture medium were reflective of the relative supply and uptake of ammonium and nitrate in Hosta cultures. The growth of Delphinium plants was affected by the nitrogen source in the culture medium though Hosta plant growth was unaffected.
The role of the gaseous environment within the culture vessels was investigated with a view to determine the effect of the high humidity in culture vessels. Using culture vessels with filter-paper covered apertures the gaseous composition and humidity, along with plant growth and mineral uptake, were measured. Gas exchange was significantly increased by the use of the aperture vessels, however humidity was not strongly affected.
Plant growth and mineral uptake were largely unaffected by increased gas exchange though ex vitro survival was improved.
An investigation of the kinetics of phosphate uptake showed that uptake was strongly linked to the medium concentration though 28day old Delphinium plants did exhibit high affinity phosphate uptake. In addition 7day old plants of both species exhibited more rapid uptake of phosphate than either older or younger plants. This was thought to be because of higher stress at the start and end of subculture periods reducing uptake in 2 and 28day old plants.
The possibility of phosphate and iron as growth limiting factors is discussed in the context of a model detailing the major factors involved in the mineral nutrition of plants in vitro.