Humectants as post-plant soil amendments: effects on growth and physiological activity of drought-stressed, container-grown tree seedlings

One-year-old, container-grown seedlings of red maple (Acer rubrum L.) and yellow-poplar (Liriodendron tulipirera L.) were treated with Hydretain ES (HydES) or Ecosential (ECOS) applied as a soil drench. A progressive drought cycle was imposed after treatment and, as each seedling wilted, the leaves and roots were harvested. For both species, foliar growth was unaffected by HydES or EcoS, but root growth (surface area of roots < 1mm diameter; fine root:coarse root ratio; root area index) was less for seedlings grown in humectant-treated media. These data, along with soil moisture measurements taken during the dry down period (which showed a consistently higher water content in the humectant-treated substrate), suggest that drought-induced fine root growth in humectant-treated media was slower since there was less need for these roots to extend and proliferate to access additional soil moisture sources. In a study conducted the following year, HydES and ECOS were applied as a soil drench to one-year-old, container-grown seedlings of red maple and river birch (Betula nigra L.) prior to withholding irrigation. In these studies, each time an untreated seedling wilted, measurements of chlorophyll fluorescence, leaf gas exchange and xylem water potential were taken on the wilted seedling along with similar measurements taken on a HydES-and an EcoS-treated seedling. These data indicate that the level of physiological activity (chlorophyll fluorescence and net C02 exchange) was greater for seedlings grown in HydES-treated media than for seedlings of the same species grown in EcoS-treated media, a condition attributed, at least in part, to a lower level of water stress (lower xylem water potential) in the HydES-treated seedlings.