Studies on the physiology of germination of Cirsium arvense Scop and the response of the plant to regulators

Abstract

The importance of the seed as a potential source of spread of Californian thistle (Cirsium arvense Scop) has been emphasised. By means of a series of experiments involving periodic variations in temperature and light, it has been established that (1) there is little possibility of any inhibitor of germination present in the seed, (2) the low germination generally obtained in laboratory and pot tests is due to lack of permeability of the pericarp to moisture and air and (3) when this impermeability is overcome by suitable periods of alternating temperatures, and light and darkness, a very high percentage of germination can be obtained. Thus the so called dormancy is governed by the photoperiodic and thermoperiodic responses of the pericarp. The rate of germination is affected by the lack of physiological uniformity in seeds and the phenomenon of periodicity in germination.

The response of the C. arvense plant to various regulators - particularly 2,4-dichlorophenoxyacetic acid (2,4-D) 2-methyl,4-chlorophenoxyacetic acid (MCPA) and 2,-methyl, 4-chlorophenoxy butyric acid (MCPB) - has been studied in detail both in the glasshouse and in the field. The differences in the mode of action of these three regulators have been studied and discussed in terms of morphological reactions, translocation and metabolism (respiration). MCPA proves to be superior both to 2,4-D and MOPB, since it affects the underground parts and their sprouting capacity to a great degree, while 2,4-D has its main effects on the aerial shoots. MOPB is inferior and slow in its reaction, possibly because of need for the operation of the Beta-oxidation phenomenon. The superiority of salt formulations to esters in general in affecting the underground parts has been demonstrated both in pot and field trials.

The influence of environmental factors such as nutritional status of plants, light intensity and day length has been studied and it has been found that high nutrition (nitrogen), low light intensity and short day increase the sensitivity of the plant to the herbicidal action of the regulator.

In the study of translocation of 2,4-D by the use of labelled material, C¹⁴ was found to be distributed in various parts of the plant indicating multidirectional movement of the radioactive carbon. The site of accumulation of C¹⁴ in treated plants is the apical portion and not the hypocotyl. The extent of the injury caused to the weed is not mainly dependent on the extent of translocation but on the total amount of the regulator absorbed and accumulated via the translocation stream. Factors such as high nutrition, low light intensity and short day enhance the effect of the regulator by increasing the amount absorbed and also by increasing the susceptibility of the plant.

Addition of sucrose or boron has not been found to aid in the translocation of C¹⁴ labelled 2,4-D, in plants grown in the light, although this may be so during darkness as reported in the literature.

The extent of the kill of underground parts is dependent on some physiological reactions set up in these organs by the regulator absorbed within the plant body through leaves. The growth stage of the plant and the quantity of regulator absorbed by the leaves are, thus, important factors. Repetition of spraying - thrice at weekly intervals or twice with a fortnightly interval - on plants at the pre-bud stage of growth have been shown to be effective. The practical suggestion of complete coverage of the plants during spraying is indicated through the translocation studies, and he nitrogen status of the plant has been shown to be the most important factor in the plant's susceptibility to regulators.

In these investigations it has been established that movement of 2,4-D is independent of the assimilate (carbohydrate) stream and it appears more likely that the control obtained at the pre-bud stage of growth depends upon the higher nitrogen status of the underground parts which were depleted of carbohydrates during the earlier phases of the life cycle.