Introduction

The hormonal theory of plant growth substances states that growth substances are limiting and, therefore, regulatory factors in plant growth and development. However, renewed interest in the role of the sensitivity of plant tissues to growth substances in growth and development was sparked of by a series of articles by Trewavas and his co-workers during the early 1980s (Trewavas, 1981, 1982; Trewavas & Jones, 1981). These articles pointed to the inadequacies of the hormonal theory, and emphasised the importance of sensitivity to growth substances as the primary limiting factor in the control of growth and development by these growth substances. The presence of both the growth substance and its receptor molecule is required for a biological response to occur. Since a biological response is caused by a receptor/growth substance complex, the response is dependent on the concentration of the growth substance as advocated by the hormonal theory. It is obvious that, if receptor availibility changes during growth and development, measurements of changes in growth substance concentration on its own has limited value.

The development of sensitivity to growth substances such as auxins, cytokinins and ethylene appears to precede the developmental process which it induces (Trewavas, 1982; Whitehead & Vasiljevic, 1993). In many flowers, ethylene plays an important role in the initiation and regulation of the processes that accompany corolla senescence.