THE EFFECT OF DEHYDRATION ON MICROSPOROGENESIS IN TRADESCANTIA

NUMEROUS STUDIES have been made of the effect of various agencies such as chemicals, abnormal temperature, radiation, and genetic factors in producing abnormal nuclear and cell division. Experimental observations have been made on both living and fixed material. In general it is found that many distinctly different agents produce similar effects, of which the majority seem to result from a disturbance of the normal synchronization of the cell's activities during the division cycle. Reviews of the experimental study of nuclear and cell division include those of Politzer (1934) and Kiister (1935). Recently the importance of an adequate cellular water content for the normal course of somatic nuclear and cell division has been demonstrated by the work of Wada (1936a, 1936b) on stamen hairs and Shigenaga (1937) on petal cells of Tradescantia L., and by Milovidov (1938) working on root tips of Vicia faba L. Oehlker and his students, in a series of papers dealing with studies in the physiology of meiosis, have investigated the effect of dehydration, occasionally in combination with high and low temperatures, as indicated by variations in chiasma frequency at diakinesis in V7icia faba, Campanula perifolia L. (Straub, 1937), and various species of *Oenothera L. (Kisch, 1937). These studies indicate that the degree of hydration of the cell is a controlling factor in the normal synchronization of chromosome behavior and nuclear and cell division. The present investigations deal with the effect of dehydration upon cellular phenomena during microsporogenesis in Tradescantia. MATERIALS AND METHODS.-During the course of an investigation of the effect of certain chemicals such as saponin and colchicine on meiosis in Tradescantia, it was noted that the control stems with paraffin sealed ends showed the same abnormalities as the chemically treated stems. This observation led to a more detailed study of the effects of water deficiency. Plants of a clonal line of a hybrid derived from Tradescantia canaliculata Raf. and a clonal line of T. paludosa Anders. & Woods. were used. Flowering stalks were cut and the ends sealed and coated with warm paraffin to a distance of about one cm. up the stalk. These stalks were then placed in water maintained at a level of about three cm. in containers kept in the greenhouse. In some cases the paraffin was removed from the stalks after a certain time and the free ends left in the water. In others the paraffin was left intact for the duration of the experiment. In the latter cases evidently the stems were often able to obtain some water, for some stalks 1 Received for publication February 14, 1939. The writer wishes to acknowledge his sincere appreciation to Prof. Karl Sax for suggestions and criticisms during the course of this work. d veloped roots which broke through the paraffin. Obviously the methods used did not give any definite or uniform indication of the amount of water received by the stem. However, this was not an essential factor, since the aim was simply to study the effects of a general dehydration on meiosis and related phenomena. A few observations were alsomade on entire potted plants which were not watered for several days. As controls, cut stalks with unsealed ends were placed in containers of water and kept in the greenhouse. In no cases were abnormalities observed in these. Acetocarmine smear preparations of the anthers were made at intervals in order to study the effect of the restricted water supply upon meiosis and the microspore division. OBSERVATIONS.-Nuclear division.-Apparently the effects of dehydration on nuclear division in Tradescantia are similar to those produced by temperature (Sax, 1937) and colchicine treatment (Walker, 1938). The restriction of the water supply results in the suppression of the spindle mechanism at the first or second meiotic division, thus preventing the normally paired chromosomes from passing to the poles. Apparently the chromosomes then reassemble into a single nucleus and proceed to divide equationally to produce dyads with the diploid number of 12 instead of the haploid number of 6. In some cases both nuclear divisions are inhibited, the 12 chromosomes resulting from the first equational division then dividing again equationally, thus producing 24 univalents organized into a tetraploid. The suppression of either the first or the second meiotic division results in the production of two diploid microspores which undergo normal mitosis at the microspore division (fig. 3, 4). The tetraploid monads also divide normally at the microspore mitosis (fig. 6, 7). The cytological effects of sealing off the stems were first noted after 11 days. The cells were then in the young microspore stage just after the second meiotic division. The experiments were carried out in February and March, at which season the duration of the meiotic cycle is increased from the normal summer period of about a week (Sax and Edmonds, 1933) to one of about two weeks' duration (from early prophase to the stage when the young microspores begin development). These time relations seem to indicate that the interruption of the water uptake was not immediately effective in upsetting nuclear divisions already in progress at the beginning of the treatment. Apparently a certain degree of dehydration of the cells is necessary before spindle inhibition and other abnormalities result. The indications are that only those -cells that were in the early stages of meiotic prophase or ear-