Factors concerned in the Rooting Responses of Isolated Leaves

Abstract

The rooting responses of isolated leaves of the dwarf French bean and the ivy were studied by applying solutions of growth substances to the petiole: β-indolyl-butyric acid (I.B.A.) for the former and α-naphthyl-acetic acid (N.A.A.) for the latter. The following factors were investigated: The most responsive leaf in the bean was the 2nd from the apex, in the ivy the 9th leaf. Very young leaves were killed. With petioles immersed for 24 hours the optimal concentrations were I.B.A. at 2·5 parts per million (p.p.m.) for the bean, and N.A.A. 100 p.p.m. for the ivy. It was shown that the active substance entered mainly through the cut end in the transpiration stream; entry through the cuticle was much slower. Starvation of the cuttings whether by low light intensity, darkness, or separation of the lamina from the petiole reduced root formation. Feeding with sucrose and asparagine increased the response. Analysis of the leaves was carried out at regular intervals for total sugar, total and soluble nitrogen. In the bean soluble nitrogen increased slightly in the petiole during the first 5 days after treatment, after which insoluble nitrogen continued to accumulate until the 7 day, when the roots first emerged. After this the total nitrogen decreased. Sugar content of the petiole at first increased but rapidly fell before the roots emerged and then again increased. In the ivy petiole the increase in soluble and total nitrogen was very slow, the former being greatest on the 15th day and the latter on the 20th day after treatment. The total sugar increased to the 10th day, but then decreased until the emergence of roots, after which it again increased. Treated leaves of the bean and ivy of different ages and the leaves starved for varying periods were also analysed for total sugars and total nitrogen. Treated bean leaves receiving varying periods of daily illumination were also analysed to elucidate their rooting response. Histological examination showed that in the bean the roots arose in the rays between the vascular bundles and in the ivy external to the vascular bundles opposite the phloem. The growth of the rooted leaves was followed for more than a month in the bean and over one year in the ivy. Some growth was found in the immature bean leaves which survived treatment, but the final size was much below that of the control leaves attached to the plant. In the ivy very little further growth occurred although the leaves produced a large root system and after 2 years are still alive: the original differences in size of the successive leaves at the time of removal from the plant still remain.