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Effect of Light Quality and Temperature on Fe³⁺ Reduction, and Chlorophyll Concentration in Plants¹

Plant species differ in their response to light; some grow best in full sunlight, some in partial shade, and others in full shade. Inside, under growth chamber conditions, cool white fluorescent (CWF) lamps, in combination with supplemental incandescent (Inc) lamps, have been a satisfactory light source. Low pressure sodium (UPS) lamps have also been used. The spectral radiant power curve for the CWF lamps extends from 350 (includes UV and blue) to above 700 nm, whereas the spectral curve for LPS lamps is between 560 to 610 nm (no UV or blue) with no radiation below 550 nm. This study was conducted to determine affect of LPS and CWF lamps, alone and in combination with Inc lamps, on plant growth and reduction of Fe³⁺ to Fe²⁺. Lettuce (Lactuca saliva L. ‘Grand Rapids’), cotton (Gossypium hirsutum L. ‘M8’) and petunia (Petunia hybrida Vilm. ‘Pink Cascade’) were grown in environmental control cabinets at 22 and 28 C under LPS or CWF lamps, with and without Inc lamps. Reduction of Fe³⁺ to Fe²⁺ was determined as Fe²⁺ ferrorine [disodium salt of 3-(2-pyridyl)- 5,6-bis(4-phenylsulfonic acid-l,2,4-triazine] in solution under all light treatments to determine if chlorophyll concentration in plant tops was related to Fe³⁺ reduction in solution. We had observed that cotton was chlorotic under LPS, but not under CWF lamps. Only small amounts of Fe³⁺ were reduced to Fe²⁺ at wavelengths above 420 nm, and more Fe³⁺ was reduced to Fe²⁺ in solution under CWF than under LPS lamps. Chlorophyll concentration in lettuce and cotton leaves followed about the same pattern as Fe³⁺ reduced to Fe²⁺ in solution. Chlorophyll levels were greater in leaves grown under CWF than under LPS lamps, and generally greater in leaves grown at 28 than at 22 C. If we assume that CWF light mediates reduction of Fe³⁺ to Fe²⁺ in plant foliage as it does in ferric-ferrozine solutions then it could make Fe²⁺ available for biochemical reactions related to chlorophyll synthesis. Petunia did not respond to the light regimes and temperature as did lettuce and cotton.

Key Words: Reduction of Fe³⁺ to Fe²⁺ • Chlorosis • Ultraviolet light • Low-pressure-sodium light • Cool-white-fluorescent light • Fe²⁺ ferrozine

² Soil Scientist, Plant Stress Lab., PPHI, AR, SEA, USDA; Res. Hort. Germplasm Res. Lab., PGGI, AR, AR, SEA, USDA; Plant Physiol. Plant Stress Lab., PPHI, AR, SEA, USDA; agricultural engineer, Agric. Equip. Lab., PPHI, AR, SEA, USDA, Beltsville, MD 20705.

Received for publication February 9, 1979.