Barley (cv.Prior) and radish (cv.White Icicle) plants were subjected to heat stress (39�C) for 1–5 days when growing in either a high (90–95 %) or low (50%) relative humidity environment. Although the plants were watered daily, leaf water potential (ψw) in heat-stressed plants at low relative humidity fell to a low level (– 30 bars, day 5) whereas there was no such decrease in the leaves of those maintained at a high humidity (– 4.6 bars). Growth in height was inhibited and leaf chlorophyll concentration decreased by high temperature, these effects being accentuated by accompanying water-stress in the leaves. On the other hand, proline only accumulated in the leaves when (ψw) fell, there being apparently no direct response to elevated temperature. These effects were confirmed for a range of temperatures extending to a maximum of 41�C. Proline accumulated rapidly, after an initial lag phase, in both barley and radish plants exposed to a low temperature (4�C). This accumulation was unrelated to any changes in the leaf water relationships. Neither leaf water potential, osmotic potential nor turgor demonstrated changes with low temperature sufficient to account for the accumulation of proline. The characteristics of this cold- induced accumulation of proline are compared with those of water stress-induced accumulation. * Part V, Aust. J. Biol. Sci., 1973, 26, 319–27.

The accumulation of free proline in the flrst leaves of barley, Hordeum distichum L., and wheat, Triticum aestivum L.. in response to a range of low temperatures was examined with 10-dayold plants. In barley (cv. Prior) no proline accumulated at 8°C or above, but in wheat (cv. Gabo) proline accumulated at 12°C and lower temperatures. In barley, the first leaf survived for 29 days following transfer to 5°C and continued to accumulate proline throughout this period. In contrast, the first leaves of plants maintained at 20° C survived for 13 days only and accumulated no proline. Proline accumulation at low temperature was shown to be light-dependent, both in intact plants and excised leaf sections, and the light requirement could not be replaced by supplying leaf segments with precursors of proline. Proline accumulation in response to water stress was not light-dependent at 20°C but was at 5°C.Inter-specific and intraspecific variation in the extent of accumulation in response to low temperature was also examined. Considerable variation was encountered but there was no clear relationship with geographical distribution or chilling sensitivity for the species and no correlation with accumulation in response to water stress in the cultivars of barley examined. ' Present address:

Barley plants (cv. Prior) were grown in nutrient culture and subjected to a variety of salinity stress treatments when 10-12 days old. Salinity stress was either imposed abruptly by transferring plants to a solution containing NaCl of - 5.7 or - 10.7 bar osmotic potential, or gradually by transferring plants daily to solutions of successively lower osmotic potential (- 1 bar per day). Prolonged exposure to salinity stress was achieved by holding plants in a - 10.7 bar osmotic potential solution for 6 days. Proline accumulated rapidly in plants subjected to a salinity stress greater than - 5.7 bar osmotic potential. Accumulation appeared to be controlled by tissue osmotic potential rather than by turgor, and continued for 24 h after the saline solution was removed from the root medium. Proline accumulation continued for at least 3 days in plants held at - 10.7 bar osmotic potential, during which time the plants continued to grow slowly. The accumulated proline made only a minor contribution to osmotic regulation in the plant,

Intact 12-day-old barley (cv. Prior) plants or first-leaf sections were grown on iso-osmotic solutions of polyethylene glycol or a variety of inorganic salts for 24 h. All solutions caused a similar decline in leaf water potential with external osmotic potential in the intact plant, but the decline in internal osmotic potential was least on solutions of polyethylene glycol, causing a loss of turgor in plants growing on that osmoticurn. Plants growing on solutions of NaCl, KCl, Na2SO4 or CaCl, accumulated less proline than plants on iso-osmotic solutions of polyethylene glycol, but plants on MgCl2, solutions accumulated as much or more. With leaf sections, no proline accumulated on solutions containing the monovalent cations, whereas tissue floated on salt solutions containing Mg2+ or Ca2+ accumulated as much or more proline as that on iso-osmotic solutions of polyethylene glycol. NaCl was found to inhibit the proline accumulation caused by a reduction in external osmotic potential.