Physiological responses of selected taxa of Salvia, Taxus, Cephalotaxus, and Syringa to heat and/or flooding

by Lasseigne, Francis Todd.

LASSEIGNE, FRANCIS TODD. Physiological Responses of Selected Taxa of Salvia, Taxus, Cephalotaxus, and Syringa to Heat and/or Flooding. (Under the direction of Drs. Frank A. Blazich and Stuart L. Warren.) High-temperature stress impacts growth of plants in warm-temperate, continental climates. Landscape plants, in particular, are subjected to extreme conditions in urban environments, due to reflected light and retained heat from brick, concrete, or asphalt. For most ornamental plants, tolerance to high temperatures remains largely unstudied. Two experiments were conducted to ascertain degrees of heat tolerance across a diverse taxonomic range of cultivated Salvia L. (salvia, sage) taxa. In the first study, eight taxa – Salvia chamaedryoides Cav. (germander sage), S. greggii A. Gray ‘Furman’s Red’ (autumn sage), S. guaranitica St.-Hil. ex Benth. (blue anise sage), S. leucantha Cav. (Mexican bush sage), S. nemorosa L. ‘Ostfriesland’ (East Friesland European wood sage), S. pratensis L. (meadow sage), S. splendens Sell. ex Roem. & Schult. (scarlet sage), and S. ×sylvestris L. ‘Mainacht’ (May Night European wood sage) – were grown under long day conditions with 15-hour days of 20, 25, 30, 35, or 40 °C and 9-hour nights of 15 or 25 °C. All taxa originating from North or South America performed poorly – S. chamaedryoides, S. greggii ‘Furman’s Red’, S. guaranitica, S. leucantha, and S. splendens – exhibiting severe chlorosis and growth distortions at high temperatures (35 and 40 °C). European-derived taxa – S. nemorosa ‘Ostfriesland’, S. pratensis, S. ×sylvestris ‘Mainacht’ maintained or increased shoot and root dry weights under high temperature treatments, and exhibited lesser or no physical symptoms of heat stress. The second study demonstrated that S. ×sylvestris ‘Mainacht’ and S. nemorosa ‘Pusztaflamme’ were better able to maintain steady rates of net photosynthesis (Pn) at high temperatures (35 and 40 °C) than S. guaranitica and S. leucantha. In regions with hot, humid climates, a second critical limiting factor to plant growth resides in tolerance of root systems to hot, wet conditions in poorly drained, clayey soils. Two genera that are difficult to grow under these conditions are Taxus L. (yews) and Syringa L. (lilacs). A third study was conducted to determine flood and heat tolerance across a range of cultivated yews – Taxus canadensis Marsh. (Canadian yew), T. ×media Rehd. ‘Brownii’ (‘Brownii’ Anglojapanese yew), T. ×media ‘Densiformis’ (‘Densiformis’ Anglojapanese yew), T. ×media ‘Taunton’ (‘Taunton’ Anglojapanese yew), T. wallichiana Zucc. var. chinensis (Pilg.) Florin (Chinese yew) – and the related Japanese plum-yew, Cephalotaxus harringtonia (Knight ex Forbes) K. Koch ‘Prostrata’. Despite anecdotal evidence that Taxus are intolerant both of heat and poor soil drainage, cultivars of T. ×media, especially ‘Densiformis’, survived a 30-day flood and grew better than the other taxa under 16-hour days/8-hour nights of 30/26 °C. In a fourth study, three taxa of Syringa were tested for growth responses to flooding at 16-hour days/8-hour nights of 25/20, 30/25, or 35/30 °C: Syringa ×hyacinthiflora (Lemoine) Rehd. ‘California Rose’ (early flowering lilac), S. ×persica L. (Persian lilac), and S. vulgaris L. (common lilac). No plants survived a 10-day flood. Both S. ×persica and S. ×hyacinthiflora ‘California Rose’ exhibited greater growth and higher Pn and dark respiration rates in response to increasing day temperature (up to 35 °C) compared to S. vulgaris.