8 using a single leaf positioned either directly opposite (maple), or opposite and immediately below (birch) the leaf used to mea- sure fluorescence. Gas exchange measurements (five consecu- tive five-minute readings) were initiated after CO2 levels in the enclosed leaf chamber reached 400–425 ppm. After measuring leaf gas exchange, each seedling was destructively harvested, and xylem water potential determinations were made on the termi- nal 15 cm shoot section using a Scholander pressure chamber (SoilMoisture Equipment Corp., Santa Barbara, California, U.S.). Data Analyses Data for both the growth (2008) and physiological (2009) stud- ies were analyzed as randomized complete block designs using an analysis of variance (ANOVA). Since there was only one production type each for yellow poplar (2008 study) and river Production typey PPS BRS Yellow poplar Seedlings grown in a greenhouse in 3.8 L plastic pots filled with soilless substrate (Metromix 560) and treated with 12 or 16 mL/L of HydES or EcoS prior to withholding water. Since no significant differences in growth were noted between the two concentrations used in these studies, only data from the 16 mL/L treatment are included here. Each value represents the mean of seven replications. For each species and production type, values in the same column differ significantly when followed by a different z letter, Tukey 0.05; ns = no significant differences. y x Peat plug-grown (PPS) seedlings; bare root-grown (BRS) seedlings. Growth index (height + two-dimensional crown width / 3). w Length of roots < 1 mm diameter. v Surface area of roots < 1 mm diameter. u Length of roots > 5 mm diameter. Fine root:coarse root ratio. t s Root area index (root surface area/substrate surface area). BRS Humectant maple 0 (control) HydES EcoS 0 (control) HydES EcoS 0 (control) HydES EcoS GIx RL < 1w (cm) 1831a 944b 29.7ns 29.0 29.1 26.3ns 25.0 24.6 1134b 2074ns 1590 1793 632ns 695 393 symptoms, it was sampled along with a droughted, HydES- treated and a droughted, EcoS-treated seedling (all three with the same rep number). At sampling, in situ chlorophyll fluores- cence and leaf gas exchange measurements were taken on one young, fully expanded leaf from each seedling using the leaf plastochron index (Larson and Isebrands 1971) as the crite- rion for selecting leaves with comparable morphological traits. Chlorophyll fluorescence was measured at room tempera- ture (22°C) on the adaxial leaf surface using a pulse-modulated fluorometer (FMS-2; Hansatech Instruments Ltd., King’s Lynn, UK). Each leaf was dark-adapted prior to measuring initial, maximum, and variable fluorescence. Following the dark period, light-adapted fluorescence readings (steady-state and maximum fluorescence) were taken on the same leaf. From these data, the maximum efficiency and quantum yield of photosystem II were calculated (Maxwell and Johnson 2000). After measuring chloro- phyll fluorescence, leaf gas exchange measurements were made on the same seedling using a portable leaf chamber analyzer (LCA-4; Analytical Development Co., Hertfordshire, UK). Net photosynthesis (Pn), transpiration (Ts), and stomatal resistance (rs ) were measured at a light intensity of 650 µmoles/m2 /s PAR Roberts et al.: Humectants as Post-plant Soil Amendments birch (2009 study), the designs were unbalanced, meaning that statistical analyses involving more complex interactions were not possible. Models for both designs were fit and analyzed using sta- tistical software (Minitab). Differences in treatment means were compared using Tukey’s pairwise comparison test at significance levels of 0.05 and 0.01. Because there were no significant differ- ences between the two humectant levels used in 2008 (12 and 16 mL/L), the growth data reported here reflect only results obtained with the 16 mL/L concentration since this allowed comparisons to be made with the physiological data collected in 2009, in which only one concentration (16 mL/L) of each humectant was used. RESULTS AND DISCUSSION Foliar Growth There was no significant effect of humectant treatment on fo- liar growth of droughted red maple or yellow poplar seed- lings in the 2008 experiments (only GI data shown), nor was there any significant difference between the two hu- mectant products tested (HydES and EcoS) (Table 1). Root Growth For PPS red maples, fine root growth [root length (RL < 1) and root surface area (RSA < 1) of roots < 1 mm diameter] was signifi- cantly greater for seedlings grown in untreated (control) substrate than for similar seedlings grown in either HydES- or EcoS-treated substrate (Table 1). These differences are reflected in the calcula- tion of fine root:coarse root ratio (F:C) and root area index (RAI), both of which were significantly greater for PPS maples grown in the untreated substrate (Table 1). While the pattern of fine root growth for BRS maples was similar to that for PPS maples (i.e., greater fine root growth in untreated substrate), the differences were not statistically significant (Table 1). No significant differences in root growth were found for yellow poplar seedlings grown in ei- ther humectant-treated or untreated (control) substrate (Table 1). Table 1. Growth of one-year-old, drought-stressed tree seedlings treated with Hydretain ES (HydES) and EcoSential (EcoS)z Species RSA < 1v (cm2 ) Red 21.6ns 18.3 19.9 161a 83b 104b 227ns 188 212 87ns 84 39 RL > 5u (cm) 30ns 27 27 38ns 39 35 42ns 40 40 F:Ct 60.3a 35.1b 42.4b 55.6ns 41.1 51.6 15.1ns 16.9 9.5 RAIs 2.06a 1.32b 1.55b 3.20ns 2.84 2.96 2.11ns 1.88 1.57 . ©2012 International Society of Arboriculture
January 2012
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