Onion plants (Allium cepa L, cv. Downing Yellow Globe) grown in pots and infected by the mycorrhizal fungusGlomus etunicatus Becker and Gerdemann were more drought tolerant than were non-mycorrhizal individials when exposed to several periods of soil water stress separated by periods of high water supply, as shown by greater fresh and dry weights and higher tissue phosphorus levels in the mycorrhizal plants. The tissues of stressed, non-mycorrhizal plants were deficient in P, despite the fact that only non-mycorrhizal plants were fertilized with high levels of P (26 mg P per 440 g soil). Differences in plant water relations (leaf water potentials or transpiration rates) and changes in soil P levels which may have affected plant growth were investigated, and discounted as factors important for the results. The P nutrition of plants has been implicated in the ability of plants to tolerate drought and it was concluded that the ability of the mycorrhizal fungus to maintain adequate P nutrition in the onions during soil water stress was a major factor in the improved drought tolerance. Infection of the root by the fungus was found not to be affected by water stress or P fertilization but fungal reproduction, as determined by spore numbers in the soil, was decreased by water stress and by P fertilization.

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.

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During controlled water stress applied at the threeleaf stage, the barley (Hordeum vulgare L.) cultivars ‘Proctor’ and ‘Excelsior’ showed different rates of free proline accumulation and leaf‐firing. Both processes were faster in Proctor (a drought‐susceptible cultivar) than in Excelsior (a drought‐resistant cultivar). The different rates of proline accumulation during stress in Proctor, in Excelsior, and in a wild barley (H. spontaneum C. Koch) could be attributed to differences in the rate of decline of leaf water potential (ψleaf); at the same ψleaf value, all three genotypes contained similar amounts of free proline.After non‐destructive screening of the F2 generation from a Proctor ✕ Excelsior cross for proline‐accumulating capacity under controlled water stress conditions, F3 seed from plants selected for high and low proline accumulation was obtained. When F3 plants were tested for proline accumulation during controlled stress, the selection for proline accumulation was found to have been effective. Reselection among F3 plants and subsequent testing of the F4 generation confirmed this result.Among individual plants of the F3 generation (which was segregating for the prollne accumulation trait) there was a positive association between the amount of free proline accumulated during stress and the severity of leaf‐firing.Massive proline accumulation is a symptom of severe internal water stress in barley and apparently has no survival value during drought. Although there is heritable variation in the amount of proline accumulated during stress, it is not of practical value in breeding for drought resistance.

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