Drought stress and paper birch (Betula Papyrifera) seedlings: Effects of an organic biostimulant on plant health and stress tolerance, and detection of stress effects with instrument-based, noninvasive methods

We conducted a progressive-drought greenhouse experiment, using potted paper birch (Betula papyrifera) seedlings in their third year of growth, to investigate whether a commercially available organic biostimulant improved plant health and stress tolerance, and to compare four noninvasive, instrument-based methods for monitoring plant stress. In the well-watered (no drought) plants, the biostimulant application significantly increased foliar nitrogen concentrations (P = 0.01) and led to marginally higher rates of photosynthesis (P = 0.10) and slightly higher F_v/F_m fluorescence ratios (P = 0.14). Reflectance indices further indicated that the biostimulant application resulted in increased chlorophyll content (Chl NDI, P = 0.07) and either (depending on interpretation) a significantly higher Chl:carotenoid ratio or a lower proportion of xanthophyll cycle pigments in the de-epoxidated state (PRI, P = 0.02). The PRI results suggest less oxidative stress in the treated plants, which may be related to the fact that the biostimulant used (Roots 3) contained ascorbate, an antioxidant. In the plants exposed to progressive drought, the biostimulant application had similar effects but did not appear to dramatically improve the drought stress tolerance of seedlings, in that impaired physiology occurred at about the same level of soil moisture in both treated and untreated seedlings. Photosynthesis responded to the drought treatment at about 12% to 15% soil moisture content (SMC), whereas PRI did not respond until about 9% to 10% SMC, and F_v/F_m did not respond until about 4% to 5% SMC. Chl NDI did not show a significant response to SMC.