Effects of Temperature on the Growth and Development of Lemna minor, under Conditions of Natural Daylight

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Abstract
The effects of temperature on the growth and development of Lemna minor in the open have been studied in the east of Scotland by means of four water baths constructed to maintain constant water temperatures of 12.5, 17.5, 22.5, and 27.5 °C when subjected to natural insolation. Experiments were conducted at weekly intervals between August and November in 1958 and May and July in 1959. At the beginning of every experiment, for all temperature treatments, 134 fronds were placed in each of six containers. From the initial and final samples, the weights of roots and fronds together with frond (leaf) area were measured, so that weekly values for net assimilation rate, leaf-area ratio, and relative growth-rate could be calculated. Daily solar radiation was recorded by means of bimetallic radiation recorder. In 1958 linear regressions of a satisfactory fit were obtained when the data for net assimilation rate, leaf-area ratio, and relative growth-rate were calculated on the logarithms of the radiation for each temperature. Since radiation remained relatively constant in 1959 it was not possible to evaluate very reliably the effects of radiation on the growth parameters but only occasionally, notably for the final leaf-area ratio (12.5 °C) were the lines for 1958 and 1959 significantly different. Single lines were fitted to the points for both years. In all the regressions, apart from that for final leaf-area ratio (12.5°C) the proportion of the variation accounted for ranged from 87 to 97 per cent. The results showed that the net assimilation rate was positively linked with radiation and was optimal at 17.5 °C, though the rise from 12.5 to 17.5 °C was not significant. At the higher temperatures (22.5 and 27.5 °C) there was a significant negative effect of temperature on the net assimilation rate. The leaf-area ratio and relative growth-rate were positively dependent on radiation and reached the highest values at the highest temperatures. The maximum growth-rate recorded amounted to no less than 0.39 g.g−1 day−1. The results are discussed in relation to those for other aquatic and terrestrial plants.

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