Photochemical reduction of iron. I. light reactions

Abstract

The role of spectral quality in the photochemical reduction of Fe³⁺ in vitro was investigated under various lamps to determine the basis for severe chlorosis observed in certain species and cultivars under low pressure sodium (LPS) lamps when used as a sole source of artificial light in plant growth chambers. The comparative efficacy of LPS and cool white fluorescent (CWF) lamps was evaluated by means of an automated spectroradiometer with capability of measuring spectral irradiance every nanometer from 250 nm to 840 nm; by visible appearance of the plants (e.g., extent of greening and morphological development); by physiological measurements (e.g., of biomass and chlorophyll content); and by use of ferrozine as a chemical actinometer. Reduction of Fe³⁺ to Fe²⁺, measured as Fe²⁺ ferrozine (at 562 nm) was more than 4 times as great under CWF lamps, which emit appreciable amounts of UV and blue irradiance, as under LPS lamps, which are deficient in these wavelengths. By use of appropriate filters to selectively remove the UV or UV + blue wavelengths from CWF lamps or other light sources, it was possible to greatly reduce or prevent the in vitro photoreduction of ferrous iron. Conversely, the addition of CWF lamps to a LPS growth room greatly enchanced Fe3+ photoreduction in solution. Citrate was found to be required for the in vitro photoreduction of 59FeC1₃ solutions. Further studies are needed, however, to relate photochemical and biochemical changes in the intact plant in order to determine the precise role of UV and blue radiation in the photochemical reduction of Fe3⁺ in vivo.

Key words:

• Spectral quality
• ultraviolet radiation
• blue radiation
• controlled environments
• low pressure sodium lamps
• cool white fluorescent lamps
• ferrozine
• chemical actinometer
• Fe chlorosis
• chelates
• spectral measurements
• organic acids