Stress-Induced Osmotic Adjustment in Growing Regions of Barley Leaves

Young barley seedlings were stressed using nutrient solutions containing NaCl or polyethylene glycol and measurements were made of leaf growth, water potential, osmotic potential and turgor values of both growing (basal) and nongrowing (blade) tissues. Rapid growth responses similar to those noted for corn (Plant Physiology 48: 631-636) were obtained using either NaCl or polyethylene glycol treatments by which exposure of seedlings to solutions with water potential values of -3 to -11 bars effected an immediate cessation of leaf elongation with growth resumption after several minutes or hours. Latent periods were increased and growth resumption rates were decreased as water potential values of nutrient solutions were lowered. In unstressed transpiring seedlings, water potential and osmotic potential values of leaf basal tissues were usually -6 to -8 bars, and -12 to -14 bars, respectively. These tissues began to adjust osmotically when exposed to any of the osmotic solutions, and hourly reductions of 1 to 2 bars in both water potential and osmotic potential values usually occurred for the first 2 to 4 hours, but reduction rates thereafter were lower. When seedlings were exposed to solutions with water potential values lower than those of the leaf basal tissues, growth resumed about the time water potential values of those tissues fell to that of the nutrient solution. After 1 to 3 days of seedling exposure to solutions with different water potential values, cumulative leaf elongation was reduced as the water potential values of the root medium were lowered. Reductions in water potential and osmotic potential values of tissues in leaf basal regions paralleled growth reductions, but turgor value was largely unaffected by stress. In contrast, water potential, osmotic potential, and turgor values of leaf blades were usually changed slightly regardless of the degree and duration of stress, and blade water potential values were always higher than water potential values of the basally located cells. It is hypothesized that blades have high water potential values and are generally unresponsive to stress because water in most of the mesophyll cells in this area does not exchange readily with water present in the transpiration stream.