In plant physiology, data analysis is based on the comparison of mean values. In this perspective, variability around the mean value has no significance per se , but only for estimating statistical significance of the difference between two mean values. Another approach to variability is proposed here, derived from the difference between redundant and deterministic patterns of regulation in their capacity to buffer noise. From this point of view, analysis of variability enables the investigation of the level of redundancy of a regulation pattern, and even allows us to study its modifications. As an example, this method is used to investigate the effect of brassinosteroids (BSs) during vegetative growth in Sorghum bicolor . It is shown that, at physiological concentrations, BSs modulate the network of regulation without affecting the mean value. Thus, it is concluded that the physiological effect of BSs cannot be revealed by comparison of mean values. This example illustrates how a part of the reality (in this case, the most relevant one) is hidden by the classical methods of comparison between mean values. The proposed tools of analysis open new perspectives in understanding plant development and the nonlinear processes involved in its regulation. They also ask for a redefinition of fundamental concepts in physiology, such as growth regulator, optimality, stress and adaptation.

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Nuclear DNA amounts were measured by Feulgen cytophotometry in Sorghum bicolor cv. 610 plants early exposed to 150 mM NaCl, a treatment known to induce an increased tolerance to salinity in plants carrying this genotype. In salt-treated plants, the percentages of 8C, 16C, and 32C nuclei in roots in the primary state of growth were 21.9%, 13.3%, and 4.3%, respectively. By contrast, in nonsalinized plants, only 3.5% of the nuclei had an 8C content and no higher DNA contents were observed. The salt treatment induced chromosome endoreduplication during the differentiation of cells in the root cortex, where 41.2% of the cells displayed a DNA content higher than 4C (versus 1.3% in control plants). No enhancement of endopolyploidy was observed in cells of the root vascular cylinder or the leaves of the salt-treated plants. In another S. bicolor genotype (DK 34-Alabama), noncompetent for salt adaptation, the same NaCl treatment did not induce chromosome endoreduplication in root cortex cells. Endopolyploidy may be considered as a part of the adaptive response of S. bicolor competent genotypes to salinity.

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In Sorghum bicolor, salt-adaptation was defined as the capacity of plants to grow at 300 mM NaCl (a lethal concentration) following a 3-week pre-treatment with 150 mM NaCl (a sublethal concentration). Large populations induced for salt-adaptation were analysed. Two modes of osmotic regulation (Na-includer and Na-excluder) were differentiated during induction of salt-adaptation, in spite of the genetic and environmental homogeneity. This variability was not related to any incapacity of the Na-includers to control accumulation of Na + ions in the shoot, because most of the pre-treated plants displayed a similar capacity of growth and control of Na + uptake after transfer to 300 mM NaCl. This unsuspected complexity of initially homogeneous populations points on the inadequacy of physiological studies focused on the 'average individual'. Further analyses showed that variability was not directly related to micro-environmental variations. It is concluded that a process of individuation is caused by a third source of variability, which is the expression of a self-organizing process normally occurring during the transition phases in development. In constraining environments, this phenomenon of individuation includes adaptive modifications.

Gibberellin (GA) and cytokinin (CK) were exogenously supplied at different periods of the vegetative development in Sorghum bicolor. Growth response to these hormonal treatments differed according to the developmental stage. This reveals the existence of discrete phenophases, each one characterized by a specific sensitivity to plant growth regulations (PGRs). Developmental changes in sensitivity were less accentuated in plants grown in optimal conditions than in plants exposed to 150 mM NaCl. Variations in organ connectance (the level of coordination in growth of the shoot, adventitious roots and seminal root) were analysed during vegetative growth of salt‐treated plants. This analysis shows a temporary decrease in connectance during the transition period between phenophases. From the effect of hormonal treatments on connectance, it was concluded that (i) the transition period coincides with a partial dismantling of the initial regulatory network followed by the emergence of a new network coordinating growth of the different organs; (ii) GA is involved in the process of emergence of a transition period; and (iii) duration of the transition period is considerably enlarged for plants exposed to NaCl stress. This dynamics of alternance of phenophase and transition periods enables the integration of the two modes of action of the PGRs (dose–response and change in sensitivity) within a unified framework.

An increase in tolerance to salinity is induced in Sorghum bicolor by exposure to a sublethal concentration of NaCl during early vegetative development. The phase of competence for induction of this response, termed salt adaptation, is well defined in time and it coincides with the emergence of the first adventitious roots. The link between these events was investigated. Competence for salt adaptation varies among genotypes. It is shown that competence is especially high for genotypes in which the link between the seminal root and the shoot is reduced during emergence of the adventitious root. These data relate the capacity for salt adaptation with development in the absence of NaCl, suggesting that: (i) functional integration of the adventitious roots within the whole plant has an adaptive nature in normal development; (ii) salt adaptation results from an integration of the environmental constraint (NaCl) during this developmental readjustment. It is concluded that perturbations generated by emergence of a new organ are the cause of rapid variations in sensitivity required to open a competence window.