Polyribosome metabolism, growth and water status in the growing tissues of osmotically stressed plant seedlings

Relationships between growth of osmotically stressed intact seedlings and polyribosome levels and water status of growing tissues were examined. Sudden exposure of barley (Hordeum vulgare L. cv. Arivat) roots to a solution of −0.8 MPa polyethylene glycol caused leaf growth to stop almost immedately, but growth resumed at a much lower rate after 0.5–1 h. In the growing region of leaves, the polyribosome: total ribosome ratio of free (non‐membrane‐bound) ribosomes was significantly reduced after 15 min stress, but a decrease in the large polyribosome:total polyribosome ratio occurred only after 1–2 h. Membrane‐bound and free polyribosome levels both decreased to 70% of unstressed control values after 4 h stress. Recovery of total polyribosomes occurred within 1 h after relief of 4 h stress, but required 3 h after relief of 24 h stress. Stress detectably reduced the water potential and osmotic potential of growing tissue within 0.5–1.0 h, and osmotic adjustment continued for up to 10 h. Recovery of water status was incomplete after 1 h relief of a 4 h stress. In contrast, expanded blade tissues of stressed plants underwent minor changes in water status and slow decreases in polyribosomes levels. These results confirm that growing tissues of barley leaves are selectively responsive to stress, and suggest that changes in growth, water status and polyribosome levels may be initiated by the same signal. Measurements of seedling growth, polyribosome levels and water status of growing tissues of barley and wheat (Triticum aestivum L. cv. Zaragoza) leaves, etiolated pea (Pisum sativum L. cv. Alaska) epicotyl and etiolated squash (Cucurbita pepo L. cv. Elite) hypocotyl stressed with polyethylene glycol solutions of −0.3 to −0.8 MPa for 12 h or more showed that polyribosome levels were highly correlated with seedling growth rate as well as with tissue water and osmotic potentials, while turgor remained unchanged. These results suggest that long‐term growth of osmotically stressed plants may be limited by a reduced capacity for protein synthesis in growing tissues and is not dictated by turgor loss.