10 surfactant concentrations have been shown to compromise the biophysical integrity of PSII membranes and to modify the kinetics of chlorophyll fluorescence (Ruan et al. 2002). Thus, EcoS treatment could potentially impact chlorophyll fluorescence, assuming some absorption of the product by the roots of plants growing in EcoS-treated substrate. However, another, and perhaps more logical, explanation for the dif- ferences noted in fluorescence activity between treated and untreated seedlings might be found in the observation that HydES-treated seedlings appeared less drought-stressed (less wilted) at harvest than did either the untreated or EcoS-treated seedlings, an explanation supported in part by gas exchange and xylem water potential data reported below. No signifi- cant differences in chlorophyll fluorescence were observed between the two species (red maple and river birch, both of which are fine-rooted), or between the two production types (PPS- and BRS-grown maples) used in these trials (Table 3). Chlorophyll fluorescence has been widely used as a method for studying the mechanisms by which a range of environmental factors alter photosynthetic capacity (Bolhar- Nordenkampf el al. 1989). Using chlorophyll fluorescence, Richardson et al. (2004) found that maximum quantum ef- ficiency was slightly higher in biostimulant-treated birch seedlings than in similar seedlings receiving no biostimu- lant. Fraser and Percival (2003) reported finding a signifi- cant increase in tree vitality (assessed using a performance index based on chlorophyll fluorescence emissions) in four- year-old biostimulant-treated oak, birch, and beech seed- lings. While the effects of biostimulant treatment reported in the aforementioned studies may have been triggered by one or more of the active ingredients found in these prod- ucts (e.g., ascorbic acid, casein hydrolysate, myo-inositol, etc.), this seems less likely the case in the present study, although the extremely high surfactant concentration found in EcoS could potentially impact physiological activity. . Roberts et al.: Humectants as Post-plant Soil Amendments Leaf Gas Exchange Measurements of net Pn recorded for groups of three drought- stressed seedlings (one untreated, one HydES-treated, one EcoS-treated) at the point when each untreated seedling first wilted, showed that Pn for PPS maples grown in HydES-treated media (2.8 µmol/m2 /s) was almost three times greater than for untreated maples (0.97 µmol/m2/s) and more than five times /s) (Table 4). These observations suggest that, at greater than for similar maples grown in EcoS-treated media (0.5 µmol/m2 harvest, the level of water stress in HydES-treated maples was probably lower than it was in either EcoS-treated or untreated maples, thereby allowing the stomates of HydES-treated seed- lings to remain open for longer periods of time, and permit- ting a greater exchange of CO2 leaf and the atmosphere. Ts and rs and water vapor between the measurements, although not statistically different, lend support to this supposition (Table 4); however, time-course studies would be required to prove or disprove this assumption. No significant differences in leaf gas exchange were found between the two species (red maple and river birch, both fine-rooted) or between the two production types (PPS and BRS red maples) included in these trials (Table 3). The results of this investigation confirmed earlier results re- ported by Ferrini and Nicese (2002), who found an increase in both net photosynthesis and evaporation rate in biostimulant- treated Quercus robur L. (English oak), and by Richardson et al. (2004) who reported finding marginally higher rates of photosyn- thesis in biostimulant-treated paper birch. In contrast, Sammons and Struve (2004; 2005) found reduced short term transpirational water use in container-grown Quercus rubra L. (red oak) and field- grown Koelreuteria paniculata Laxm. (goldenrain tree) treated with a biostimulant intended to reduce stress. Dissimilarities in the above mentioned studies may have resulted from differences in the active ingredients contained in each of the various biostimu- lant products used, some of which may have an effect on hydraulic conductivity in the xylem (Berlyn and Sivaramakrishnan 1996). Table 4. Physiological activity of one-year-old, drought-stressed tree seedlings treated with Hydretain ES (HydES) and EcoSential (EcoS)z Species Red maple BRS River birch z BRS Production typey PPS Humectant 0 (control) HydES EcoS 0 (control) HydES EcoS 0 (control) HydES EcoS Tukey 0.05; ns = no significant differences. y Fv/Fmx 0.774a 0.785a 0.611b 0.675ns 0.756 0.724 0.724ns 0.756 0.752 Peat plug-grown (PPS) seedlings; bare root-grown (BRS) seedlings. x Variable/maximum fluorescence (max efficiency of PSII photochemistry). w Quantum yield of PSII. v Photosynthetic rate. u Transpiration rate t Stomatal resistance. s Xylem water potential. φPSIIw 0.077ns 0.070 0.051 0.083ns 0.128 0.070 0.104ns 0.076 0.056 Pnv (µmol/m2 0.97ab 2.80a 0.50b 0.25ns 1.27 0.95 0.50ns 0.73 1.01 /s) Tsu (mol/m2 0.20ns 0.41 0.11 0.09ns 0.24 0.19 0.05ns 0.09 0.13 /s) rs t (m2 /s/mol) 122.0ns 102.7 223.1 213.8a 81.6b 105.4b 295.1ns 147.0 168.2 XWPs (MPa) -2.57a -1.17b -2.00ab -3.74a -1.77b -2.28ab -1.53ns -1.41 -1.72 Seedlings grown in a greenhouse in 3.8 L plastic pots filled with soilless substrate (Metromix 560) and treated with 16 mL/L HydES or EcoS prior to withholding water. Each value represents the mean of six replications. For each species and production type, values in the same column differ significantly when followed by a different letter, ©2012 International Society of Arboriculture
January 2012
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