Detection of Plant Water Stress Using Leaf Spectral Responses in Three Poplar Hybrids Prior to the Onset of Physiological Effects

ABSTRACT The negative impact of water stress on forest and tree plantation productivity has been the focus of many investigations. However, moderate water stress that can decrease productivity and is difficult to detect, has received less attention. Therefore, we designed a greenhouse experiment where the main objective was to test the efficacy of published biochemical and water stress indices along with physiological traits in detecting moderate water stress at leaf scale. Potted saplings of three hybrid poplars ((Populus × canadensis) × P. maximowiczii, Populus × canadensis and Populus × generosa ‘Boelare’) were subjected to moderate water stress. During the experiment we recorded (i) the biomass accumulation and allocation to leaves, stem, and roots; (ii) Net CO2 assimilation rate (A) and stomatal conductance (g s) and (iii) Spectral biochemical and water indices. Results indicate that moderate water deficit had a negative impact on biomass accumulation, but no change was detected in physiological traits. Nitrogen and chlorophyll contents also remained unaffected. Spectral biochemical indices failed to detect differences between treatments, whereas water stress indices succeeded. We also have identified unique wavelengths in the shortwave infrared region of the spectrum that proved sufficiently sensitive on their own to detect moderate water deficit. The results were not influenced by a genotype effect, suggesting that these unique wavelengths could be responding to a general rather than a species-specific leaf feature, thus widening opportunities for the early detection of plant stress, particularly moderate water deficit.