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The limited use ofkatsura tree (Cercidiphyllumjaponicum Sieb. & Zucc.) in the landscape may be due to its reputed, but uncharacterized, intolerance of drought. We examined the responses of katsura trees subjected to episodes of drought. Containerized trees in a greenhouse were subjected to one of three irrigation treatments, each composed of four irrigation phases. Control plants were maintained under well-hydrated conditions in each phase. Plants in the multiple-drought treatment were subjected to two drought phases, each followed by a hydration phase. Plants in the single-drought treatment were exposed to an initial drought phase followed by three hydration phases. Trees avoided drought stress by the mechanism of drought-induced leaf abscission. Due to drought, plants in the multipleand single-drought treatments underwent a 63% and 34% reduction in lamina dry mass and a 60% and 31% reduction in leaf surface area, respectively. Mter abscission, trees in the singledrought treatment recovered 112% of the lost leaf dry mass within 24 days. Leaf abscission and subsequent refoliation resulted in a temporary reduction in the leaf surface area : root dry mass ratio. Mter relief from drought, net assimilation rate and relative growth rate of plants in drought treatments were maintained at least at the rates associated with plants in the control treatment. We conclude that katsura is a drought avoider that abscises leaves to reduce transpirational water loss. Although plants are capable of refoliation after water is available, to I I I I I I I I I I I I I I I I I I I 28 maintain the greatest ornamental value in the landscape, siting of katsurashould be limited to areas not prone to drought. Introduction The genus Cercidiphyllum is composed of two species native to eastern Asia. The common katsura (C. japonicum) is endemic to Japan and China, although its native range in China has diminished to the degree that it is now classified as rare (Chien, 1992). As a landscape plant, the katsura tree is valued for its brilliant autumnal leaf color, picturesque form, slightly shaggy bark, and disease and pest resistance. The species recently gained celebrity by being named the Tree of the Year for 1997 by the International Dendrology Society (Andrews, 1998). Despite these attributes, use of katsura in the landscape is uncommon. The reputation of katsura as a drought-intolerant tree may play a role in its limited usage. While experimental evidence pertaining to the drought intolerance of katsura is lacking, anecdotal references are common. Flint ( 1997) discusses the necessity for maintaining adequate soil moisture, as do others (Dirr, 1990; Wyman, 1971; Yiesla and Giles, 1992). Leaves on trees subjected to drought stress often become scorched and after prolonged periods may senesce and abscise (personal observations). While drought may induce a significant amount of leaf abscission, refoliation after water becomes available has been observed, but not quantified (Kjelgren, 1994). Soil moisture contents may vary considerably in natural and maintained landscapes as soils go through successive cycles of wet and dry conditions (Mazzoleni and Dickmann, 1988). Experiments simulating these conditions have been used to examine the response of trees subjected to cycles of drought (Liu and Dickmann, 1992; Mazzoleni and Dickmann, 1988). The objectives of our study were to examine the growth and morphological responses of katsura trees subjected to a series of drought and well-hydrated conditions. We quantified I I I I I I I I I I I I I I I I I I I 29 the degree of drought-induced leaf abscission and determined the refoliation capacity of trees after drought ended. Materials and Methods On 4 April 1997, 132, two-year-old dormant, bare-root seedlings of katsura (Lawyer Nurseries, Plains, Mont.) were planted singly into 45-dm3 Classic 816 (Nursery Supplies, Chambersburg, Pa.) nursery containers filled with Metro-Mix 700 (Scotts, Marietta, Ga.). We placed pieces of wire screen inside containers to prevent the loss of medium through drainage holes and put containers on a greenhouse bench under natural photoperiod (lat. 42°N). Containers were irrigated with tap water (pH of 8.74, EC of 0.45 mS-cm·') as needed to keep the surface of the medium moist and fertilized weekly with Nat 155 mg·L-' from Peters Excel all-purpose 21N-2.2P-16.6K fertilizer (Scotts, Marietta, Ga.) until treatments began. The study consisted of three irrigation treatments applied over four consecutive phases that simulated either drought or well-hydrated conditions. In the control treatment, all phases were well-hydrated. The multiple-drought treatment consisted of drought in phases one and three, and hydration in phases two and four. Plants in the single-drought treatment were subjected to drought in phase one, and hydrated conditions in phases two through four. For all treatments, phases one and three were complete when drought cycles in the drought treatments had ended, and lasted from 6 to 11 d and 4 to 6 d, respectively. Phases two and four were arbitrarily assigned a duration of 9 d. Destructive harvests were conducted on the first day treatments began and at the conclusion of each phase. For harvests where plants had been subjected to the same cumulative irrigation regimen, treatments were combined. On 4 June 1997, 44 plants were selected for uniformity, randomly arranged on greenhouse benches and randomly assigned to one of the three treatments. There were 16, 12, and 12 plants assigned to the control, multipleand single-drought treatments, respectively. I I I I I I I I I I I I I I I I I I I 30 The following day, phase one began and the first destructive harvest (of the remaining four plants) occurred. Stem length from the stem base to the shoot apex of each plant was measured. All plants were fertilized at the beginning of each phase with Nat 155 mg·L-1 from Peters Excel all-purpose 21N-2.2P-16.6K fertilizer (Scotts, Marietta, Ga.). Drought was imposed by withholding irrigation for the remainder of each drought cycle. A drought cycle was complete when 50% of the plants subjected to drought wilted during either midday ( 1300 to 1500 HR) or morning (0600 to 0900 HR) hours in phases one and three, respectively. Phase one consisted of two drought cycles. When the first drought cycle was complete, containers were brought to container capacity with tap water and began the second drought cycle. Phase three consisted of one drought cycle. During hydration phases, plants were irrigated daily with tap water with the amount of water lost the previous 24 h due to evapotranspiration, determined gravimetrically. Volumetric soil moisture content of the medium of each container was determined daily, before irrigation, with a Theta Probe (meter type HIIl, sensor type ML1; Delta-T Devices Ltd., Cambridge, United Kingdom) soil moisture sensor at 6 em below the soil surface. At each destructive harvest, we identified the youngest, fully expanded pair of leaves on each plant to be harvested. One of these leaves was used to determine the predawn (0300 to 0500 HR) leaf water potential ('P po) of each plant, measured with a pressure chamber (PMS Instruments, Corvallis, Ore.). Stem diameter at 2.5 em above the soil surface and stem length from the stem base to the shoot apex of each plant were measured. Internode length was determined by measuring the length between the node of the identified pair of leaves and the node immediately below it. From the remaining identified leaf, a 0.5-cm2 disc was removed from the interveinal region of the basal half of the blade and dried at 67 oc for 2 to 3 d. Specific leaf mass was calculated by dividing the dry mass of each disk by the disk area. Because the drought treatments resulted in leaf desiccation and/or abscission, leaves (including I I I I I I I I I I I I I I I I I I I 31 petioles) were removed from the plants and arbitrarily designated as viable or nonviable. Individual leaves that had not abscised and had 50% or more of their tissue undamaged were considered viable. The remaining leaves were classified as nonviable. Leaf surface area of viable leaves was measured by using an area meter (LI-31 00; LI -COR, Lincoln, Nebr.). Roots were separated from their growth medium by hand in tap water. Dry mass of viable leaves, stems, and roots was determined after the material was dried for 2 to 3 d at 67 °C. The ratio of leaf surface area : root dry mass was calculated. Net assimilation rate and relative growth rate were determined for each treatment during each phase (Harper, 1977). There were four single-plant replications (experimental units) within each harvest x treatment combination. The experiment was conducted three times, with the second and third experiments over time starting on 31 July 1997 and 6 Sept. 1997, respectively. Based upon a completely randomized design, data were analyzed by using the Statistical Analysis System (SAS Institute, Cary, N.C.). Within each harvest group, analyses of variance were performed by using the General Linear Model (PROC GLM), and differences between treatment means at P < 0.05 were considered significant. Single-plant replicates one and two, and replicates three and four were combined and the averages used for statistical analysis of net assimilation rate and relative growth rate. Results and Discussion Katsura trees avoid drought stress by the mechanism of drought-induced leaf abscission. The relationship between soil moisture content and '¥Po was described by using DeltaGraph® 4.0 (DeltaPoint, Inc., Monterey, Calif.) as the equation: MPa =(soil moisture contenr)(.09255); R = .645. At soil moisture contents of 0.16 m·m· or higher, leaves did not abscise, but as soil moisture content decreased below 0.16 m3·m·, percentage leaf abscission increased linearly [percentage leaf abscission= 0.8024.35(soil moisture content); I I I I I I I I I I I I I I I I I I I 32 R = 0.488; p = 0.0115]. As drought severity increased

An examination of drought-stress avoidance and germinability of katsura tree