植物の低温・乾燥耐性における水分生理学的研究

抄録

Plants cannot escape adverse environmental conditions. Abiotic stresses associated with water (such as drought, continual rain and temperature extremes) reduce plant growth and thereby decrease yield and grain quality. Such stresses negatively affect natural distribution ranges and cultivation areas. Plant stress responses have been characterized using physiological and biochemical approaches, but few studies have applied biophysical approaches to investigate plant water relations. First, this review introduces the use of differential thermal analysis of tissue water to examine freezing tolerance in woody plants, especially various Rhododendron species that have different ecological habitats. The supercooling ability of flower buds appears to be sufficient to avoid freezing stress at the northern limit of natural distribution of each species. Second, the dynamic states of water determined from 1H-NMR relaxation times (T1, T2) in various plant organs such as flower buds, petals, seeds, hypocotyls, leaves and roots closely reflect senescence, maturation and stress injury. Therefore, analysis of water status in plant organs provides a sensitive and non-invasive way for evaluating both primary and lethal responses to stresses at various growth stages. Plants protect cell metabolism in response not only to drought as abiotic stress but also to seed desiccation. The production of highly vigorous seeds is important to a stable yield. Intracellular ‘glass transition’ is indispensable for the long-term seed survival in a dry state. The change in water compartments and the loss of water mobility in ripening seeds reflect cellular heterogeneity with accumulation of nutrients such as carbohydrates, lipids and proteins. This article also describes serious agricultural problems that result in yield losses and low grain quality in crops exposed to environmental stresses, with a particular focus on the remobilization of nutrients and dynamic states of water during seed filling. Because water is an essential factor that facilitates seed development and maturation, seeds must have mechanisms that allow them to withstand water loss.