Aspects of Water Relations and Gas Exchange of Katsura and Tilia Seedlings Subjected to Wet-Dry Cycles: Indication of Strategies for Whole Plant Drought Tolerance

This paper reports the effects of progressive drought and wet-dry cycles on water relations, stomatal conductance and photosynthetic carbon fixation of seedlings of two tree species adapted to temperate climate Cercidiphyllum japonicum, Sieb. & Zucc. (Katsura: Cerci) and Tilia cordata (Tilia) in the climate chamber of the Institute of Ecophysiology of Plants, Technical University, Munich (TUM), Freising, Germany. The watering regimes were applied in phases to simulate either well watered /hydration conditions, dry down/single drought phase and multiple/intermittent wet-dry cycle. The control plants were maintained under well-watered conditions dry-wet cycles (drought-rehydration phases) in phases. Plants in the multiple-drought treatment were subjected to two dry-wet phases while plants in the single-drought treatment were exposed to an initial drought phase followed by one hydration phase. In both Cercidiphyllum japonicum and Tilia cordata, drought induced leaf abscission and recovery (regrowth) were observed towards the end of the experiment. However, against expectation, although Cerci exhibited drought induced leaf abscission, but its water potential was kept below values obtained for Tilia. After relief from drought (post drought rewatering), leaf water potential and stomatal gas exchange recovered in magnitudes close to those of well watered control plants. As a result of increasing dehydration, water potential in Tilia declined sharply and one of the plants died off. It appeared that the magnitude of soil drought imposed was sufficient to initiate hydraulic failure in Tilia, an observation that can be attributed to a vulnerability to embolism. The water consumption measured via evaporative flux/weighing method and sap flow sensors), was higher for Katsura while its leaf water potential declined far more in Cerci relative to Tilia without causing noticeable damage. The respective pre-dawn and midday leaf water potentials were: Tilia (-0.88; -0.1250 MPa) and Cerci (-0.85; -0.104 MPa). It is concluded that both Katsura and Tilia are drought avoiders that abscise leaves to reduce transpirational water loss. The species are capable of regrowth and recovery of physiological functions after relief from drought, In order to maintain the greatest ornamental value in the landscape. Katsura and Tilia should be planted in areas that are not prone to drought.