Leaf Growth Hormones. I. A Bio-Assay and Source for Leaf Growth Factors

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Abstract
Circular discs cut from young first foliage leaves of Raphanus sativus are floated on the solns. to be tested for leaf-growth factors. Activity is detd. by wet-weight increase, or by increase in surface area of these floated sections after ca. 30 hrs., the correspondence between the 2 methods being good. This forms the basis of a quantitative bio-assay for the factors controlling leaf-mesophyll growth. A basic medium of a 1% sucrose soln. is used in order that the growth rate may not be limited by sugar, and activities are compared to the growth of sections in sucrose soln. alone. Good sources of the hormone (giving ca. 30% better growth when tested and computed in the above manner) were found to be yeast extract, seed diffusates of pea, radish, and corn, and leaf diffusates from Nicotiana, Raphanus, and Washingtonia. The activity of yeast extract could not be explained as due to thiamin, riboflavin, ascorbic acid, or biotin. Histological studies indicate that leaves of Nicotiana, and Raphanus when grown in a crude hormone preparation show greater cell enlargement with a separation of the spongy parenchyma and formation of more intercellular air spaces. The growth of leaves is evidently controlled by a hormone that is non-species-specific and common to a considerable variety of leaves.