| Summary: | Climate models in Japan predict that the annual mean air temperature and number of consecutive dry days will increase in the future, leading to high seedling mortality rates. Maintaining high survival rates of <i>Cryptomeria japonica</i> seedlings, a commercially important tree species, is therefore, important in terms of appropriate forest management under climate change. Although drought hardening, in which seedlings are acclimated to dry conditions in the nursery prior to planting, contributes to increased survival under drought conditions, little is known about the effective irrigation frequency of drought hardening in <i>C. japonica</i> seedlings. In this study, we therefore, examine the effectiveness of different drought-hardening treatments in <i>C. japonica</i>. We first clarify the effects on physiological and morphological traits by comparing three drought-hardening treatments [control (<i>C</i>): Irrigation once daily; mild (<i>M</i>): irrigation once every three days; and severe (<i>S</i>): irrigation once every five days] for one month. Next, to confirm the effects during consecutive dry treatment, we stopped irrigation for 13 days and once again compared the physiological traits between the three drought-hardening treatments. Drought hardening reduced whole-plant transpiration (<i>E</i><sub>whole</sub>), resulting in conserved water use, and this tendency was particularly evident under the <i>S</i> treatment. Moreover, during consecutive dry treatment, the <i>E</i><sub>whole</sub>, proportions of non-embolized xylem, and cambium status of basal stem regions were maintained for the longest duration under the <i>S</i> treatment, followed by the <i>M</i> treatment. Our findings suggest that the efficiency of drought hardening increased with drought severity. Furthermore, one month of drought hardening contributed to both water conservation and the maintenance of cell differentiation under consecutive dry treatment, likely increasing the tolerance and survival of <i>C. japonica</i> seedlings under prolonged drought.
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