Gas exchange and water relations in diploid and tetraploid Russian wildrye

Russian wildrye[Psathyrostachys juncea (Fisch,) Nevski] is a drought tolera nt, cool-season forage grass used in seeded pastures in the Northern Great Plains where water often limits production. Seedling vigor is generally poo r in diploid cultivars, but tetraploid germplasm has improved seedling vigo r. Objectives of this study were to determine the relationships between wat er-use efficiency (WUE), carbon isotope discrimination (CID), and gas excha nge rates for a diploid cultivar (Vinall) and a tetraploid entry over 3 yr, two water treatments (50 and 150% of mean monthly precipitation), and two fertilizer rates (10 and 134 kg N ha(-1)) in a rain shelter. The tetraploid entry exhibited higher carbon exchange rate (CER), stomatal conductance (g s), and transpiration (T) than the diploid entry at the 50% but not the 150 % water treatment, Leaf water potentials (LWP) were 0.6 and 0.3 MPa less ne gative for the tetraploid entry than diploid entry at the 50 and 150% water treatment, respectively. Values of CER averaged 12.4 and 14.2 mu mol m(-2) s(-1), T averaged 5.2 and 6.0 mmol m(-2) s(-1), and gs averaged 0.21 and 0 .28 mol m(-2) s(-1) for the diploid and tetraploid entry, respectively. The diploid entry had a significantly greater CID, averaging 20.37 parts per t housand compared with 19.65 parts per thousand, for the tetraploid entry an d CID was negatively associated with WUE, The more favorable plant water re lations, gas exchange, and dry matter production under limited soil water s uggests tetraploid populations of Russian wildrye should be emphasized in f uture breeding and management programs.