A6 Growth Alterations and Morphogenesis

Topic

Plant growth and morphogenesis, under normal conditions, depend on two sets of interacting parameters: first, the allocation of biomass among the plant's various organs, defined by its allometry and the specific leaf area of ​​its foliage; and second, the rate of cellular growth in its tissues, defined by cell elongation, meristematic division, and the thermal time accumulated by the plant, measured in growing degree-days. When environmental stress, nutritional deficiencies, hormonal imbalances, or pathogens affect the plant, both sets of parameters can deviate from their normal values; this deviation subsequently manifests as a quantifiable, anomalous phenotype.

Environmental stress, nutritional deficiencies, hormonal imbalances, or pathogens cause the allocation of biomass among plant organs to deviate from normal levels: the plant directs a different fraction of its biomass to the roots, stem, or leaves than it would under normal conditions. This shift in biomass allocation alters the plant's allometry—causing it to lose the normal proportions between its organs—and affects the specific leaf area, which deviates from the norm as the plant adjusts the thickness or density of its leaf tissue in response to the same stressor.

These same factors—environmental stress, nutritional deficiencies, hormonal imbalances, or pathogens—also cause the plant's rate of cellular growth to deviate from the norm: cell elongation slows down or accelerates relative to the usual rate, meristematic division is delayed or advanced, and the thermal time (in growing degree-days) required for the plant to reach each developmental stage no longer corresponds to the actual elapsed calendar time. Of the four causes, hormonal alterations play a specific linking role, as plant hormones simultaneously regulate biomass allocation among organs and the rate of cellular growth: hormonal alterations directly modify biomass allocation, cell elongation, and meristematic division. Consequently, when environmental stress, nutritional deficiencies, or pathogens trigger hormonal changes, a significant portion of the impact these factors have on biomass allocation and cellular growth rates is mediated precisely through those hormonal alterations.

When biomass allocation and cellular growth rates deviate sufficiently from normal values, the plant's growth and morphogenesis manifest as a quantifiable anomalous phenotype: a reduced relative growth rate; an altered root-to-shoot biomass ratio; stunting caused by severely inhibited cell elongation; lodging resulting from a stem that grows too tall or too weak to support the biomass it must bear; or disrupted phenology, where the accumulated thermal time (in growing degree-days) no longer aligns with the developmental stages the plant should have reached within the elapsed calendar time.

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