Abstract:
Physiology and genetics of drought hardiness were investigated in two-year-old coastal Douglas-fir seedlings from 39 full-sib families obtained from coastal British Columbia and grown at Oregon State University, Corvallis, Oregon. The seedlings were subjected to three drought treatments (control, moderate, and severe drought) in their second growing season (June through September 1997). Response to drought stress was assessed by measuring stomatal conductance, hydraulic conductivity, xylem cavitation in two annual growth rings, and shoot damage. Growth traits measured were seedling height and basal stem diameter. In response to severe drought, a general decrease in predawn xylem water potential, hydraulic conductivity, and stomatal conductance occurred while xylem cavitation and shoot damage increased significantly (p < 0.05). Drought stress also reduced seedling growth, with diameter being more severely affected by drought than height. Negative xylem water potential (P = -1.7 to -2.5 MPa) during the drought period reduced hydraulic conductivity of the seedlings.
Mean cavitation in the first annual growth ring was significantly (p <0.05) greater in seedlings grown under severe drought than in seedlings grown in the moderate or control treatments. Family variation for cavitation in the first growth ring, however was significant (p < 0.001) in all treatments indicating that genetic variation in cavitation in this ring is independent of drought treatment. Cavitation in the second annual growth ring showed significant (p < 0.05) differences among all the treatments. Family variation in cavitation in the second growth ring was significant (p <0.0001) only in the severe drought treatment, but not in control or moderate, suggesting that this trait was particularly sensitive to severe, current-season drought. Family variation was significant (p < 0.05) in two growth potential traits (height and diameter) and in cavitation of the second growth ring and shoot damage (damage traits). Despite relatively high ratios of specific combining ability to general combining ability variance for most traits, family mean heritabilities were moderately high (hr2 = 0.41 to 0.58), suggesting that selection based on family information will be effective for improving drought hardiness and growth potential of seedlings. Cavitation in the second growth ring and shoot damage appear to be largely under
control of the same set of genes. Although estimated family heritability was somewhat greater for cavitation (0.57) than for shoot damage (0.41), shoot damage is likely to be the better choice upon which to base selections for seedling drought hardiness because it is a much easier trait to measure. Damage traits are estimated to
have a slight, unfavorable genetic correlation with growth potential traits, thus selection for stem growth at the seedling stage is not expected to have a large influence on the drought hardiness of seedlings.
