Abstract
Summary Two varieties of Euphorbia pulcherrima Willd. with different chilling tolerance were compared in several physiological parameters (dry weight, osmotic potential, proline content, net photosynthesis, transpiration) and levels of the hormones zeatin (Z), zeatin riboside (ZR) and abscisic acid (ABA) before and during exposure to chilling temperature (8°C for 24 days). The higher chilling tolerance of the variety « Preduza » (Zimmer, 1980, 1981) in comparison to « Diamond » was confirmed by this treatment. « Preduza » showed less visible chilling stress symptoms than « Diamond » and was able to recover even after 24 days, whereas « Diamond » plants were irreversibly damaged after this time. « Preduza » adapted faster to the low temperature conditions than « Diamond » by changing its water status and ABA level, and showed only a slight reduction in the level of the two cytokinins, whereas in « Diamond » the cytokinin level was drastically reduced. Unchilled « Preduza » plants had a much lower cytokinin level than unchilled « Diamond » plants. A low diffusion resistance and the ability to form more side branches in the variety « Diamond » are also related to its much higher cytokinin level. The results are discussed in view of defining parameters that characterize «low temperature tolerant» plants.
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