A Direct Relationship between Indoleacetic Acid Effects on Growth and Reducing Sugar in Tobacco Tissue

Numerous attempts have been made to relate the effects of auxin (indole-3-acetic acid (I.A.A.); naphthalene acetic acid; and related compounds) on growth of plants with specific changes in metabolism and cellular constituents. In the case of toxic effects of 2,4-dichlorophenoxyacetic acid (2,4-D) decreases or temporary rises and subsequent depletion of available carbohydrates have been reported (1–4). An increase in reducing capacity in tissues, measured by staining, has been obtained from treatments of bean stem cultures with 2,4-D (5).

Recently Christiansen and Thimann (6) have demonstrated a decrease in reducing sugar in Pisum stem sections and Avena coleoptile sections grown for short periods of time in solutions of various compositions with auxin concentrations which promote growth. However, they report a similar decrease in controls without I.A.A. and even greater decreases of sugar from the addition of respiratory inhibitors, which completely stop growth, such as iodoacetate, arsenite, and fluoride. In general, tests on changes in composition associated with auxin treatments which promote growth have been carried out in short time experiments under conditions leading to net decreases in dry weight of the tissues, and where their continued growth was precluded. It was of interest, therefore, to carry out analyses for changes in cell constituents in tissues supplied with I.A.A. in media adequate for continuous growth over periods of several months. The experiments reported here, with tobacco stem segments grown in vitro, show that under these conditions I.A.A. added to the medium markedly increases the tissue concentrations of reducing sugars. This effect is a function of the I.A.A. concentration and closely parallels its effect on increasing the growth of the tissues.

Materials and methods. Stem segments obtained from young internodes of tobacco (Wisconsin No. 38) grown in the green house were sterilized and cultured as described by Skoog and Tsui(7) on White's nutrient medium with 2% sucrose and 0.7% agar.