Sunday, March 28, 2010

Autumn Joy Sedum

What’s blooming in the area: Tansy mustard.

What’s growing: Moss, daffodils, daylily, loco weed, white sweet clover, pigweed, dandelion, hay; village ditch meeting yesterday.

What’s in my yard: Apache plume, first tulips, more hyacinths, garlic chives, Jupiter’s beard, bouncing Bess, Autumn Joy sedum, golden spur columbine, oriental poppies, anthemis, chrysanthemum.

What’s blooming inside: Aptenia, bougainvillea.

Animal sightings: Small robins have been auditioning my trees; two people down the road are letting chickens peck over their gardens.

Weather: Smattering of rain Wednesday night; 12:22 hours of daylight today.

Weekly update: One of the mysteries of spring is what makes plants break ground or leaf out. Lilacs are so tied to temperature scientists use them to measure global warming, while the white spurge always appears around April 4.

I have no idea what influences the Autumn Joy sedum that pushed up new leaves last week. Last year it emerged March 7, while it was April 13 in 2002. It could be temperature, but I don’t know if it’s morning or afternoon, or how many hours of warmth or cold are required, or how much time must pass between the time a temperature requirement is met and emergence.

Georg Arends’ cross between a European Hylotelephium telephium and an Asian Hylotelephium spectabile is a member of the Crassula family that originated in southern Africa where members responded to global drying by altering their photosynthesis technique.

Plants normally use heat form the sun to extract carbon from atmospheric carbon dioxide and excrete the surplus oxygen. The gas exchange occurs through stomatic leaf pores which also allow water to escape when the air is dry. If the soil is too dry to replace the lost moisture, the plant dies.

The Crassulaceae adapted to drought conditions by shutting the stomata in the day, and absorbing carbon dioxide in the night. They store the carbon in malic acid molecules, which turn the leaves bitter. During the day the plants draw on those carbon reserves to support photosynthesis and the leaves return to their normal pH.

With time, the Crassulaceae moved north to the Mediterranean, then east towards Asia where the Hylotelephium diverged. Because the climate there is more temperate, this group of sedums reverted to normal photosynthesis patterns when conditions were good and converted to crassulacean acid metabolism (CAM) when drought was perceived.

The transition between the two metabolic forms is not instantaneous. A British team found telephium could change to CAM metabolism in less than 8 days when plants weren’t watered, while a Chinese group found it took spectabile 15 days to complete the transition.

This time of year, when moisture is abundant, my Autumn Joy is following a normal photosynthetic regime, but when drought appears in early summer the plants will convert to CAM chemical chains of reactions.

They can’t simply go dormant in the heat like spring blooming bulbs, because they don’t begin their reproductive phase until their nascent buds have undergone at least four days with 15 hours of daylight, an event that doesn’t occur until the first of June. Last year the flower buds were visible by the middle of July. While the spring emergence has varied widely, the earliest I’ve noticed flowers is August 12 in 2006, and they appeared around the 23rd the past three years.

Chester Werkman and Harland Wood discovered the chemical mechanics of CAM metabolism in the 1930's when they proved bacteria fixed carbon in a dicarboxylic acid as part of the process of fermentation. After the war, Meirion Thomas measured gas exchanges in Kalanchoë to establish the existence of the temporally separated photosynthesis phases through another dicarboxylic acid in the Crassula family.

Scientists now believe CAM metabolism first appeared in aquatic plants in the Paleozoic era, but was reinvented more than once by terrestrial plants in the late Mesozoic Cretaceous period when the parent group of the Crassulaceae, the Saxifragales, was first appearing.

Many have hoped to show some causal relationship between the Mesozoic atmosphere, which was characterized by high carbon dioxide levels, and CAM metabolism, but so far all they’ve been able to establish is the plants were responding to drought. Autumn Joy may reveal nothing about global warming now or in the summer, but it definitely suggests one way plants adapted to an even greater environmental challenge, the change from water to life on dry land.

Notes:
Heide, O. M. "Photoperiodic Control of Dormancy in Sedum telephium and Some Other Herbaceous Perennial Plants," Physiologia Plantarum 113:332 - 337:2001.

Keeley, Jon E. and Philip W. Rundel. "Evolution of CAM and C4 Carbon-Concentrating Mechanisms," International Journal of Plant Sciences 164:555-557:2003.

Lee, H. S. J. and H. Griffiths. "Induction and Repression of CAM in Sedum telephium L. in Response to Photoperiod and Water Stress," Journal of Experimental Botany 38. 834-841:1987.

Lin, Zhi-Fang, Chang-Lian Peng, and Gui-Zhu Lin. "Photoxidation in Leaves of Facultative CAM Plant Sedum spectibabile at C3 and CAM Mode," Acta Botanica Sinica 45:301-306:2003.

Mort, Mark E, Douglas E. Soltis, Pamela S. Soltis, Javier Francisco-Ortega and Arnoldo Santos-Guerra. "Phylogenetic Relationships and Evolution of Crassulaceae Inferred from matK Sequence Data," American Journal of Botany 88:76-91:2001.

Thomas, M. "Physiological Studies on Acid Metabolism in Green Plants. I. CO2 Fixation and CO2 Liberaton in Crassulacean Acid Metabolism," New Phytologist 48:390-420:1949

Werkman, C. H. and H. G. Wood. "Heterotrophic Assimilation of Carbon Dioxide," Advances in Enzymology 2:135-182:1942.

Photograph: Week or so old Autumn Joy sedum leaves coming up between old stems, 27 March 2010.

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