IUNAVAILABILITY OF IRON AS A CAUSE OF FRENCHING OF TOBACCO (NICOTIANA TABACUM L.)'1 Y FIGURES)
In continuation of the work reported by JONES (4) the effort has been directed toward investigating the factors in the soil which induce frenching of tobacco (Nicotiana Tabacum L.) at a soil temperature of 350 C. and which are impotent at a soil temperature of 21° C. Previous work (4), while not discounting the soil toxin theory, did suggest that biological activity of soil flora might alter the availability of a nutrient element and thereby upset a physico-chemical balance which might disturb normal development of the plant. Since it is recognized that a soil flora is a complicated society affected by environment, particularly moisture and temperature, no attempt has yet been made to sort out the various organisms to ascertain which, if any, might be responsible for depriving a plant of its requirements. During the process of the investigation certain experiments were performed and special studies made, which, although the results had no bearing on the final conclusions, are reported in this paper as contributions pertinent to information on the subject of frenching.
SOIL TEMPERATURE AS A FACTOR IN THE FRENCHING OF TOBACCO (NICOTIANA TABACUM L.).
During a study of the effect of soil temperature on the development of a tobacco mosaic, frenching was observed on the plants at the higher soil temperatures. Since this is apparently the first time, JONES (3), that soil temperature has been recognized as a factor in inducing frenching of tobacco, Nicotiana Tabacum L., it seemed important that the technique be developed as a possible means of discovering the direct cause of this peculiar physiological malady. Tobacco plants grown at the high soil temperature, 350 C., always became frenched, characterized by chlorotic, narrow leaves with wavy leaf margins. WOLF (19) expertly describes the symptoms which were also observed in this investigation: >'. . . the internodes remain short, apical dominance is lost, and an unusually large number of leaves appear, oftentimes two to three hundred in a single plant. Such plants have the appearance of a rosette or witches' broom, and therefore the severe form of frenching may properly be called 'polyphylly'...." WOLF (19) has reviewed the reports and investigations of frenching up to 1935. Apparently the malady is of physiological origin since it has been impossible to isolate an organism or inoculate plants by using virus technique. Among the causes suggested, emphasis is placed upon field conditions, particularly a subsoil of hard-pan nature which prevents root penetration and also results in poor drainage of surface moisture. Frenching has also occurred as an aftermath of heavy rainfall. Investigations conducted with various forms of nitrogen and calcium compounds have given results so inconsistent that lack or excess of these elements forms no basis as a primary cause. Neither does soil reaction offer a good approach to an answer, although beneficial effects have been noted in soils with low pH values and when acids have been applied to soils producing frenched plants. The soil toxin theory has been investigated, but, here again, the experiments led to inconclusive results. When portions of frenched tobacco plants, containing a growing point, were grafted on to healthy tobacco plants the later growth of the graft lost its frenching habit (17, 18). Such experiments not only indicate the noninfectiousness of the causal agent, but also eliminate the possibility of a genetic character being involved. McMuRTREY (5) and SPENCER (11) have demonstrated that symptoms 1 Published as Contribution No. 655 of the Massachusetts Agricultural Experiment Station.
EFFECT OF THALLIUM ON GROWTH OF TOBACCO PLANTS.
EFFECT OF THALLIUM ON GROWTH OF TOBACCO PLANTS IN view of the recent interest in thallium poisoningl' 2 the following preliminary report of the effect of this element on the growth of tobacco is given at this time. The previous papers3 dealing with the effects of this element report the death of the plant, but no mention appears to have been made as to its effects when present in smaller amounts than that causing death. In recent years an extensive search has been conducted as to the cause and the control of the so-called frenching disease of tobacco. Valleau and Johnson4 have reported that this disease is due to nitrogen deficiency, but our results have not substantiated their findings. In connection with a study of this disease trials were made of the toxicity of several of the heavy metals as possible causes of the trouble. Thallium in the form of the nitrate was included in this study and produced very decided effects on the growth of the tobacco plant. The element was applied in pot cultures at 35 and 75 p.p.m. based on the air dry weight of the soil after the plants had become established. Three typical sandy loam tobacco soils were used, two of which were held at two moisture contents. The severity of the effects of thallium differed to some extent in the different soils and was greater at the higher moisture contents. In many instances the stem was killed at the surface of the soil. This effect may have been due to the fact that the material was applied in solution and was not leached to any considerable extent from the surface layers. Thallium, when applied as the nitrate, was not leached from an inverted bell jar