Light stimulation of proline synthesis in water-stressed barley leaves

The effect of light on ¹⁴C]glutamate conversion to free proline during water stress was studied in attached barley (Hordeum vulgare L.) leaves which had been trimmed to 10 cm in length. Plants at the three-leaf stage were stressed by flooding the rooting medium with polyethylene glycol 6000 (osmotic potential-19 bars) for up to 3 d. During this time the free proline content of 10-cm second leaves rose from about 0.02 to 2 mol/leaf while free glutamate content remained steady at about 0.6 mol/leaf. In stressed leaves, the amount of ¹⁴C]glutamate converted to proline in a 3-h period of light or darkness was taken to reflect the in-vivo rate of proline biosynthesis because the following conditions were met: (a) free-glutamate levels were not significantly different in light and darkness; (b) both tracer ¹⁴C]-glutamate and ¹⁴C]proline were rapidly absorbed; (c) rates of ¹⁴C]proline oxidation and incorporation into protein were very slow. As leaf water potential fell, more ¹⁴C]glutamate was converted to proline in both light and darkness, but at any given water potential in the range-12 to-20 bars, illuminated leaves converted twice as much ¹⁴C]glutamate to proline.