- This study was divided into two parts. Part I was a quantitative description of some aspects of plant-water relations in Douglas-fir seedlings, and part II was tests of several techniques of screening selected families of Douglas-fir seedlings for drought resistance. Part I. During 20 mild summer days in 1976 at Corvallis, Oregon, diurnal measurements were taken on two-year-old Douglas-fir seedlings for xylem
water potential, leaf conductance of water vapor, air temperature, soil temperature, dew point temperature, and solar radiation. Leaf conductance was at a maximum early in the morning as soon as the seedlings were illuminated. It dropped rapidly in the morning and more gradually in late afternoon. There was little correlation between leaf conductance and radiation.
At low moisture stress (high pre-dawn xylem water potential) there was a good linear relationship between leaf conductance and absolute humidity deficit; leaf conductance decreased as evaporative demand increased. As plant moisture stress increased, the level of absolute humidity deficit necessary for stomatal closure decreased. The relation between leaf conductance and plant moisture stress was made clearer by separating the data into six ranges of absolute humidity deficit. Leaf conductance decreased as plant moisture stress increased, rapidly at first and more gradually later as stomata closed.
As the level of absolute humidity deficit increased, the degree of moisture stress causing stomatal closure decreased.
Transpiration was estimated by the product of leaf conductance and absolute humidity deficit, and daily estimates were calculated by integrating diurnal values over the day. Daily transpiration decreased with increasing moisture stress with the greatest decrease being between -3 and -5 bars pre-dawn zylem water potential. Daily depression in xylem water potential was greatest between -8 and -12 bars pre-dawn xylem water potential while daily transpiration, a direct cause of changes in xylem water potential, was greatest at higher pre-dawn xylem water potential. An index of plant resistance to water flux was calculated which suggested plant resistance increased with moisture stress. Plant-water relations was suggested as a possible basis for screening seedlings for drought resistance. Part II. Sixteen full-sib families of seedlings from three sites in the
Coast Range of Westcentral Washington were studied for differences in drought resistance. Pre-dawn xylem water potential of understory western hemlock indicated small differences among sites in plant moisture stress during the 1976 summer drought. Families from the driest site burst bud five days earlier than those from the wetttest site, which may indicate a drought adaptation. Neither bud burst nor origin of family were correlated with root:shoot ratio, root area:leaf area ratio or seedling productivity. Seedlings from each family were measured at four levels of moisture stress (mean pre-dawn xylem water potentials were -3.7, -6.3, -10.8 and -12.3 bars) for diurnal leaf conductance of water vapor,
xylem water potential, air temperature, dew point temperature and solar radiation. Diurnal transpiration was calculated and daily transpiration was estimated as in Part I. There were no differences among families in daily transpiration at any level of moisture stress. However, two families were always high and two were always low in transpiration. Daily transpiration was not correlated with date of bud burst or seedling productivity. Daily transpiration was correlated with root:shoot ratio at -3.7 and -12.3 bars pre-dawn xylem water potential and with root area:leaf area ratio at -3.7, -10.8, and -12.3 bars pre-dawn xylem water potential. The lack of any indications of large differences in drought
resistance among families probably resulted from the narrow range of seed resources. The good correlation between daily transpiration and root:shoot ratio suggests transpiration may be a good way to evaluate nursery seedlings for their root:shoot ratio.