VALUE OF SELECTION FOR OSMOTIC POTENTIAL IN TROPICAL MAIZE

Osmotic adjustment, the lowering of osmotic potential (psi-s) under drought, is considered a desirable component of drought tolerance since it has been shown to maintain turgor, growth, and photosynthesis at lower water potentials. This study attemped to determine the relative merits of selecting for osmotic adjustment and low psi-s under drought in lowland tropical maize (Zea mays L.). During 3 yr of trials in Mexico that examined 204 maize landacres, elite open pollinated populations (OPPs), experimental synthetics (ESs), and hybrids, osmotic adjustment averaged 0.10 MPa and ranged from 0.54 to -0.23 MPa. Only 7% of genotypes showed a capacity to adjust by more than 0.40 MPa, a level considered minimum for them to serve as sources for the trait. Variation for psi-s under drought for 222 to 225 S1 progeny of three maize populations ranged from -1.04 to -1.79 MPa. Divergent selection within each population for high and low psi-s under drought resulted in the formation of ESs which differed in psi-s by an average of 0.17 MPa when grown under a line source moisture gradient. Realized heritability for psi-s averaged 0.46. The ESs selected for high and low psi-s did not yield differently from the cycle bulk, suggesting little adaptive value for psi-s. Correlations between several traits indicative of performance under drought and osmotic adjustment or psi-s under drought generally were weak, inconsistent, and non-significant. These studies suggest that selecting for increased capacity to adjust osmotically or low psi-s under drought does not appear advantageous in lowland tropical maize.