The Relationship between Growth and Indole-3-Acetic Acid Content of Roots of Pisum sativum L.

Abstract

The indole-3-acetic acid (IAA) content of roots and shoots of light-grown pea seedlings (Pisum sativum L. `Little Marvel') growing at different rates was studied by radioimmunoassay during the first week of germination. Different growth rates were obtained by daily irrigation with either deionized water or a dilute Hoagland's mineral nutrient solution. Mineral nutrient-grown roots grew more rapidly, had more lateral roots, and initiated lateral roots at a greater distance from the apex than did water-grown roots. Growth kinetics with both treatments were biphasic. There was an initial phase of rapid cell expansion lasting about 3 days during which growth was insensitive to external mineral nutrient supply. This was followed by a slower growth phase consisting of a balance between cell division and cell expansion. Withholding nutrients resulted in the progressive inhibition of cell division during this second phase. At no time did the amount of IAA per gram fresh weight or the number of cells per gram fresh weight differ significantly in roots treated with water or mineral nutrients, whereas the total amount of IAA per organ and the total number or cells per organ were greater in roots provided with nutrients. The amount of IAA per gram fresh weight changed dramatically in roots during the first week of germination, but this was correlated with changes in the relative contributions of cell division and cell expansion to fresh weight growth rather than to growth rate. By contrast, the amount of IAA per gram fresh weight in 3-mm root tips showed a direct relationship with growth rate. The relationship between IAA content and growth in whole shoots was qualitatively and quantitatively similar to that found in whole roots.