| Summary: | As climate change progresses, the breeding of drought-tolerant forest trees is necessary. Breeding drought-tolerant trees requires screening for drought stress using a large number of individuals and a high-throughput phenotyping method. The aim of this study was therefore to establish high-throughput methods for evaluating the clonal stress responses to drought stress using infrared thermography and chlorophyll fluorescence methods in <i>Cryptomeria japonica</i>. The stomatal conductance index (<i>Ig</i>), maximum photochemical quantum yield of photosystem II (<i>Fv</i>/<i>Fm</i>), and axial growth of four plus-tree clones of <i>C. japonica</i> planted in pots were measured weekly for 85 days after irrigation was stopped. The phenotypic trait responsivity to drought stress was estimated by a nonlinear mixed model and by introducing the cumulative water index, which considers the past history of the soil water environment. These methods and procedures enabled us to evaluate the clonal stress responses in <i>C. japonica</i> and could be applied to large-scale clone materials to promote the breeding program for drought tolerance.
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