Evaluation of Free Proline Accumulation as an Index of Drought Resistance Using Two Contrasting Barley Cultivars

The two barley [Hordeum vulgare L.] cultivars, ‘Proctor’ and ‘Excelsior’, differ in their stability of grain yield under dryland conditions; Proctor produces a low yield in drought seasons and Excelsior a relatively high yield. This varietal difference in drought resistance was clearly expressed at the three to four-leaf stage, both in soil-grown plants in greenhouse tests and in perlite-grown plants in growth chamber experiments. Water deficits were imposed either by withholding water from the soil, or by flooding the perlite rooting medium with polyethylene glycol 6000 (PEG) of osmotic potential –19 bars. Under both conditions the drought-susceptible Proctor sustained more severe leaf kill than the resistant Excelsior. Leaf water potentials (ψleaf) and free proline concentrations in various zones of the second leaf blade were determined during the development of PEG-imposed stress. In both cultivars, a gradient of decreasing ψleaf was established from the base to the tip of the blade; a ψleaf value lower than –30 to –40 bars in any leaf zone indicated leaf kill, i.e. failure of that zone to recover on relief of stress. The ψleaf at the mid-blade zone always fell more rapidly in Proctor than in Excelsior, and consequently reached the critical –30 to –40 bar value earlier. In both varieties, free proline accumulated in leaf tissue as ψleaf fell, and reached the highest concentration as leaf kill became severe; at this stage much of the free proline was localized in the non-viable leaf zone. Under stress, Proctor leaves always accumulated free proline more rapidly than Excelsior leaves. On relief of water stress, free proline levels declined in viable leaf tissue, but remained very high in the drought-killed, desiccated leaf zone. These data demonstrate that reports of a simple positive correlation between proline-accumulating potential and drought resistance in barley may be in error. Therefore, proline-accumulating potential should not be utilized as a positive index of drought resistance in screening methods for cereal breeding programs.