Growth periodicity in Pinus sylvestris L., with special reference to the effect of day-length

Abstract

In recent years there has been a great revival of interest among plant physiologists throughout the world in problems associated with growth and development in plants. Most of this recent work has been carried out with annual species since it is generally considered more convenient to work with such plants. For this reason our knowledge of the physiology of growth in woody species falls short of that for herbaceous plants. There can be no doubt, of course, that many of the results obtained from the study of herbaceous plants hold also for woody species. For example, evidence will be brought forward below to support the view that the mechanism of photoperiodism is the same for both woody and herbaceous species. Similarly, it has been shown (Czaja, 1934) that auxin is present in the growing shoots of certain woody species, just as in herbaceous plants. Nevertheless, there are many features of growth peculiar to woody species and not found in typical herbaceous plants. For example, growth of the shoot in herbs is normally terminated by flower formation, whereas in most woody species the annual cycle of growth in a high proportion of the growing points is ultimately terminated not by a flower bud, but by a vegetative resting bud. Investigation of features of growth Great Britain are (1) temperature; (2) light intensity;(3) length of day. The probable importance of temperature and light intensity in affecting the annual cycle of growth in woody species has long been recognized, and in the past these, together with rainfall, have generally been considered to be the over-riding factors. The discovery, however, that such processes as the duration of extension growth and time of leaf-fall in woody species may be affected by length of day under experimental conditions (Garner ana Allard, 1923) raises the question as to how far this factor is important in affecting the annual rhythm of growth in nature. Very little definite information on this subject is available at present, and it was therefore decided to carry out an intensive study of the annual cycle of several growth processes in a single species, with special reference to the role of photoperiodic effects in the annual cycle of growth. For this purpose the species, Pinus sylvestris L, appeared to have several advantages, viz.: (1) Various other species of the genus Pinus were already known to show marked photoperiodic effects (Bogdanov, 1931, Kramer, 1936, Jester and Kramer, 1939), ana there were grounds for expecting that P. sylvestris would also prove to be 'photoperiodic' (Sylven, 1942).(2) It is a species which is indigenous to Great Britain (at least in the North, and is naturalised in southern England).Hence observations with respect to the annual cycle of growth under natural climatic conditions could be regarded as 'normal' for this species. An intensive study of the annual cycle of growth, in the light of more recent advances in the physiology of development, does not appear to have been made previously for a woody species, although it has been done for various herbaceous plants, including the onion (e.g. Heath, 1940) and certain cereals (e.g. Purvis and Gregory, 1937).Before the experimental work is described, a brief account of the normal cycle of growth in Pinus sylvestris will be given, since a knowledge of this is essential before an attempt is made to discover the factors which determine it.