Proline accumulation in response to dehydration and diurnal hydration cycles varies among maize genotypes

Proline is a compatible solute that may help plants adapt to water stress by preserving enzyme activity in desiccated tissues. To learn about the genetic variation of proline accumulation that occurs with rapidly developing water stress or diurnal water stresses, 21 field-grown maize (Zea mays L.) genotypes were evaluated for free proline concentration in leaves before and after detachment and dehydration. Genotypes differed significantly for proline accumulation in the detached leaves which varied from 58 to 208% over initial concentrations. Within sampling times proline concentration was nor significantly correlated with relative water content (RWC) or solute potential normalized for RWC (Psi (s100)). In a related experiment, six maize genotypes received well-watered and water-stressed treatments. These included four topcrosses selected from the first experiment and two hybrids known to differ for response to water stress. Determinations were made of free proline concentration, RWC, and Psi (s100) on attached leaves at the beginning ("dawn") and after 9 to 10 h of the 16-h photoperiod ("midday"). Proline concentrations ar dawn and midday varied in genotype-specific patterns. Leaf proline concentration was negatively correlated with dawn (r=-0.41**, P less than or equal to0.01) and midday (r=-0.28*, P less than or equal to0.05) RWC of plants in the water-stressed treatment, but not in the well-watered treatment. Final shoot weight of plants in the water-stressed treatment was negatively correlated with dawn proline concentration (r=-0.31*) but not midday proline concentration. Comparing the two hybrids chosen for their different water stress responses, the water-stress sensitive hybrid had a significantly lower final shoot weight and a significantly higher midday proline concentration. Generally, proline concentration did not appear to be a reliable predictor of water-stress tolerance in maize.