Solute accumulation and osmotic adjustment in leaves of Brassica oilseeds in response to soil water deficit

To establish methods for improving drought tolerance in Brassica oilseeds, we examined the influence of soil water deficit on solute accumulation in expanded and expanding leaves. We assessed the contribution of individual solutes to osmotic adjustment (OA) and determined the effect of homozygosity on the variation of the expression of OA. Juvenile plants of canola (Brassica napus cvv. Karoo and Monty) and Indian mustard (B. juncea line PI-81792) were grown under glasshouse conditions. The 3 genotypes expressed similar magnitudes of OA in response to drought, compared with a 2-fold higher OA in expanding leaves than in fully expanded leaves. Drought-induced changes in OA of expanded leaves of all genotypes were largely due to the accumulation of nitrate (42−47%), soluble sugars (31−38%), and proline (11−14%). In expanding leaves, K+ accumulation was significant (23–27%) as was proline (17−22%), whereas nitrate and soluble sugars were less important than in expanded leaves. By comparison, ions of Na+, Cl–, and water-soluble Mg2+ and Ca2+ did not contribute significantly to OA. Proline was hardly detected in well watered plants, but sharply increased in leaves of droughted plants in direct proportion to the magnitude of OA (80 ± 7 mmol/MPa, r2 = 0.94). A comparison between doubled haploid (DH) and commercial seed sources of 2 B. napus cultivars showed similar mean values in OA. The DH lines had a 33% lower coefficient of variation in OA than cv. Karoo and 48% lower than cv. Monty among replicate plants. Proline may be a suitable ‘marker’ for OA in juvenile Brassica plants because of all the solutes measured, its concentration was directly proportional to the magnitude of OA across cultivars and leaf types. The use of DH lines will improve the precision in selection of genotypes that have the capacity to osmotically adjust under soil water deficit.