The 1980 Sino-American Botanical Expedition (SABE) to the Shennongjia Forest District, Hubei Province, China was the first botanical collecting trip by American scientists to that country since 1949. This collaborative venture with Chinese botanists yielded 2085 herbarium and 621 germplasm collections from the species-rich region. Our analysis tracked the fate of the SABE germplasm after its introduction to North America and represents one of the few case studies of its kind. Two hundred and fifty-eight of the original germplasm collections still survive and are in cultivation in at least one of 18 botanical institutions. Yet 115 of these (45%) are represented by a single accession growing in a single location, which suggests that the plant introduction process is more tenuous than is generally assumed. The scientific value of documented wild germplasm warrants that careful measures are in place to ensure that significant collections do not vanish. This case study outlines steps that can be taken throughout the introduction process (from propagation to distribution and follow-up) to prevent any such loss. In particular, the role of data sharing among institutions is highlighted as a means of identifying collection uniqueness, and assessing environmental adaptability and invasiveness.
The limited use of the katsura tree (Cercidiphyllum japonicum 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. Container-grown 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 drought-induced leaf abscission. Plants in the multiple- and single-drought treatments underwent a 63% and 34% reduction in leaf dry weight and a 60% and 31% reduction in leaf surface area, respectively. After leaf abscission, trees in the single-drought treatment recovered 112% of the lost leaf dry weight within 24 days. Leaf abscission and subsequent refoliation resulted in a temporary reduction in the leaf surface area: root dry weight ratio. After relief from drought, net assimilation rate and relative growth rate 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 becomes available, to maintain the greatest ornamental value in the landscape, siting of katsura should be limited to areas not prone to drought.
Sequences of the internal transcribed spacers (ITS) of nuclear ribosomal DNA were used to examine genetic divergence of the two species of katsura [Cercidiphyllum japonicum Sieb. & Zucc. and Cercidiphyllum magnificum (Nakai) Nakai] and four clones of weeping katsura (`Amazing Grace', `Tidal Wave', `Pendulum', and `Morioka Weeping'), and to characterize the affinity of these weeping katsura to both species. Our results indicate that C. japonicum and C. magnificum are genetically distinct, supporting the recognition of them as separate species. Based on our DNA sequence data and morphological evidence, all weeping selections are phylogenetically derived from C. japonicum, not C. magnificum; nor are they of a hybrid origin between C. japonicum and C. magnificum. We propose the new cultivar-group Cercidiphyllum japonicum Weeping Group to include all katsura clones of weeping or pendulous habit, and recognize the cultivar epithet `Morioka Weeping' and its application to the excurrent and upright clone obtained from Japan and distributed in North America by the Arnold Arboretum.