Reductions in a plant’s ability to transport water, measured as hydraulic conductivity, can impact stomatal conductance and photosynthetic gas exchange. Roots are the plant organs least resistant to cavitation and, due to this, root hydraulic conductivity and vulnerability to cavitation have been proposed as an additional element for seedling quality evaluations. This trait, however, has been poorly characterized for important conifer species preventing the integration of this important physiological trait into criteria for genetic selection and nursery production. In the first part of this study, we modified the original design of the vacuum chamber method allowing for measurements of the whole root system of young conifer seedlings. This method was then compared with standard gravimetric measurements for stems of four tree species and our results confirmed that the vacuum chamber is a reliable technique to measure hydraulic conductance. We were able to perform up to 50 measurements per day of hydraulic conductance of whole root systems using a team of two people, opening the possibility for large scale assessments of root hydraulics, including genetic screening. In the second part of the study, the vacuum chamber method was used to assess the impact of nursery irrigation regime and seedling age on coastal Douglas-fir stem and whole root system hydraulic conductance. When coastal Douglas-fir seedlings grew under limited water availability, roots and stems both experienced a 65% reduction in xylem hydraulic conductance. In contrast to this, Douglas-fir seedling water transport efficiency was reduced more in roots than stems when seedlings were grown under well-watered conditions.
