Translocation and metabolism of glycine betaine by barley plants in relation to water stress.

The glycine betaine which accumulated in shoots of young barley plants (Hordeum vulgare L.) during an episode of water stress did not undergo net destruction upon relief of stress, but its distribution among plant organs changed. During stress, betaine accumulated primarily in mature leaves, whereas it was found mainly in young leaves after rewatering. Well-watered, stressed, and stressed-rewatered plants were supplied with [methyl-(14)C]betaine (8.5 nmol) via an abraded spot on the second leaf blade, and incubated for 3 d. In all three treatments the added (14)C migrated more or less extensively from the second leaf blade, but was recovered quantitatively from various plant organs in the form of betaine; no labeled degradation products were found in any organ. When 0.5 μmol of [methyl-(14)C]betaine was applied via an abraded spot to the second leaf blades of well-watered, mildly-stressed, and stressed-rewatered plants, (14)C was translocated out of the blades at velocities of about 0.2-0.3 cm/min which were similar to velocities found for applied [(14)C]sucrose. Heat-girdling of the sheath prevented export of [(14)C]betaine from the blade. When 0.5 μmol [(3)H]sucrose and 0.5 μmol [(14)C]betaine were suppled simultaneously to second leaf blades, the (3)H/(14)C ratio in the sheath tissue was the same as that of the supplied mixture. After supplying tracer [(14)C]betaine aldehyde (the immediate precursor of betaine) to the second leaf blade, the (14)C which was translocated into the sheath was in the form of betaine. Thus, betaine synthesized by mature leaves during stress behaves as an inert end product and upon rewatering is translocated to the expanding leaves, most probably via the phloem. Accordingly, it is suggested that the level of betaine in a barley plant might serve as a useful cumulative index of the water stress experienced during growth.