Regulation of growth: the meristem network approach

Gibberellin (GA) and/or cytokinin (CK) supplied to the root medium modified the rate of growth of the shoot, the adventitious and seminal roots in young seedlings of Sorghum bicolor (L,) Moench, For each population submitted to a hormonal treatment, the link between the weight of these three organs was analysed. All these links build a network of relationships, also termed a meristem network. Treatments with 1-100 mu M CK modified the relative growth of these three organs, as well as the meristem network. Treatments with 1-100 mu M GA slightly modified the growth, but the meristem network was considerably changed. When GA and CK were applied simultaneously, the growth was similar to that of CK-treated plants, and the meristem network was similar to that of GA-treated plants. Similar experiments were performed on plants exposed to 150 mM NaCl, They showed that both growth and meristem networks are modified by salinity. Growth was unrelated to the network variations in non-salinized plants, while an optimal level of relationship for growth was observed in salinized plants. It is concluded that stress generates an imbalance between the self-stimulation of meristem activity and its regulation by the other organs. A definition of stress, independent from the rate of growth, is proposed.