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www.arborage.com Arbor Age / June 2014 7 to asexually propagate oaks. Asexual propagation (versus sexual propagation by acorns) was necessary in order to ensure the specifi c characteristic of high pH tolerance. (Sexual propaga- tion by acorns leads to too much species variability.) Then UHI requested pollen from colleagues around the world from oaks in the white oak group in order to make crosses (the white oak group includes white oaks, bur oaks, swamp white oaks, chinkapin oaks, and many others). "We ended up with 300 unique genotypes from the acorns that resulted from these crosses," Bassuk said. "Of those, 25 of them proved that they can stay green at a very high pH. The other ones don't like high pH but are good for exceptional form and other ornamental characteristics." Now the task of the researchers is to fast-track propagation of those 25 most promising genotypes. "Right now we only get fi ve or six new plants a year from each stock plant, and that represents a bottleneck," said Bassuk. She and her research team are exploring how to get more rooted shoots per plant in at least two ways: They are applying the plant hormone gibberellic acid to stimulate more buds to break; and they are using both greenhouse and fi eld production in order to get two fl ushes of new shoots per season, rather than one. Once the UHI team clears the propagation hurdles, new oak hybrids will become commercially available. Arborists should be on the lookout for this series, one that will mean the end of the vexing iron chlorosis that causes oak leaves to turn yellow. This will be better for tree health and appearance, but here's the thing: the demand for arborists to treat iron chlorosis (by sulfur and iron chelation treatments, for example) will go down. As plant improvement research like this solves problems, arborists will need to market themselves differently. In this case, knowledge of treatments for iron chlorosis will become less important than knowing which oak hybrids to recommend to clients from the start. The bubbles factor A second UHI research project with real industry appli- cation seeks to answer the question, "Why are some species so much more diffi cult to transplant than others?" Arborists know from their own observations that honeylocusts are a piece of cake, but that most trees in the oak family are diffi cult to transplant. Researchers have long suspected that certain tree species are more affected than others by the water stress inherent to transplanting — but why? Bassuk posited that the reason some trees became more water stressed was because they vary in the extent of the formation of air bubbles in the xylem, the water- conducting tissues of the plant. This phenomenon, known as cavitation, is analogous to a bubble in a drinking straw that makes it harder to suck up the liquid. UHI acquired a hydraulic conductivity meter that could measure precisely how much water resistance, as a function of cavitation, is present in plant tissues. They designed an experi- ment to test two species that, even though they are related, are opposite in their transplanting ease — swamp white oak (blissfully easy) and bur oak (notoriously diffi cult). They tested two-year-old seedlings, on up to 1 1/2-inch-caliper trees. "Both species take a hit when they are transplanted," Bassuk UHI found that 25 of their 300 oak hybrids exhibit high soil pH tolerance. All photos courtesy of Cornell Urban Horticulture Institute