Interrelationships between Light Intensity, Temperature, and the Physiological Effects of 2: 4-Dichlorophenoxyacetic Acid on the Growth ofLemna minor

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Abstract
The effects of light intensity and temperature on the changes in growth and development induced by sodium 2 4-dichlorophenoxyacetate have been studied in multifactorial experiments. Random samples from a clonal population of Lemna minor were subjected to three intensities (700, 275, and 180 foot-candles) and three temperatures (30°, 25°, and 20° C.), while varying concentrations of the growth regulator were added to a phosphate-buffered culture solution (pH 5:1 There were up to five sampling occasions when the dry weights and, for some purposes, the total frond areas were determined. The replication was at least twofold, and the largest experiment involved 720 dry-weight determinations. At each combination of light and temperature, sublethal concentrations of the growth regulator induced cumulative reductions in the relative growth rate and the order of the depression increased progressively with concentration-3, 9, 27, and 45 p.p.m. Because of the cumulative effects and the great differences in the growth rates of the controls under the nine combinations of light and temperature, the statistical analysis of the data was involved. Employing two different approaches it was concluded that the relative depression in the growth rate increased with temperature while the growth of plants receiving the highest intensity was slightly more inhibited. At a concentration (27 p.p.m.) which reduced both the relative growth and frond area, over all combinations of light intensity and temperature the net assimilation rate during the periods o–2 and 2–4 days was depressed by 2 and 12 per cent. respectively. There was some indication that the depression was larger at the highest temperature in the second period. At greatly increased concentrations the fronds exhibited chiorotic symptoms, and the concentrations required to produce a standard degree of chiorosis at the end of 3 days were determined under twelve combinations of light intensity and temperature. It was found that varying the intensity during the time of treatment was without effect, but that there was a highly significant influence of the light level prior to treatment: the lower the intensity the smaller was the concentration subsequently required to induce chlorosis. Likewise, with a rise in temperature the magnitude of the equi-effective concentration was reduced. These interactions are discussed in relation to changes in the ratio of frond area to frond weight, the mechanism of uptake, and possible physiological effects at cell level.