PLANT GROWTH AND THE AERIAL ENVIRONMENT

I. EFFECT OF ARTIFICIAL SHADING ON IMPATIENS PARVIFLORA

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Summary

This is the first of a series of papers describing quantitative studies on the ecology and morphogenesis of the woodland annual, Impatiens parviflora. The relationships between the plant and the aerial environment are best understood by combining the results of observations under natural and semi-natural conditions with those made under controlled artificial conditions in the laboratory. If the latter are to have meaning the plants grown under artificial conditions must have essentially the same structure and functioning as those grown under natural conditions. Here are presented the results of growing plants under semi-artificial conditions in the field using shades of five known transmissions with the aim of establishing a quantitative framework to which observations made under artificial conditions can be referred. The data include observations on mean daily total radiation, temperature and humidity, together with the fresh and dry weights of leaves, stems and roots, and the leaf areas of the plants grown under the five screens, with observations on plants transferred from one screen to another.

Methods of analysing the data are discussed and a distinction is made between indices relating to the morphogenetic condition of the plants at any time and those relating to growth processes. The complications involved in computing these latter are considered, with particular reference to fluctuating environments, and to plants transferred from one environment to another. Furthermore, the importance of ontogenetic drift in both the morphogenetic conditions of the plant and its growth processes is demonstrated.

Several aspects of the light regime prove to have important influences on growth, (a) The rate of increase of dry weight per unit leaf area is practically proportional to the mean total daily radiation up to about 100 cal./cm2/day, and almost independent above 300 cal./cm2/day, in which region it is, however, affected by daylength. (b) The ratio of leaf area to leaf dry weight shows a marked ontogenetic drift, the form of which is much affected by the mean daily total radiation, (c) The ratio of leaf dry weight to total plant dry weight is practically independent of the mean daily total radiation, but is slightly affected by daylength.

Temperature has little effect on the rate of increase of dry weight per unit leaf area at low light, but a marked effect at high light. Low temperatures also lower the ratio of leaf area to leaf dry weight.

Consideration of the performance of plants transferred from one environment to another emphasizes the importance of the contribution made by the ratio of leaf area to leaf dry weight to the overall relative growth rate for dry weight of the plant.

Finally a comparison of data for the growth of young plants of I. parviflora and Heltanthus anmnts shows that the former responds to shading by a marked increase in the ratio of leaf area to total plant dry weight, while the rate of increase of dry weight per unit leaf area falls much less for a given degree of shading than in the case of H. anmnis. The combination of these two responses leads to a much higher relative growth rate of Impatiens parviflora under shaded conditions.

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