John Tjepkema

John Tjepkema

Professor of Plant Physiology

Degree: Ph.D. 1971 University of Michigan
Research Topic:
Whole-plant physiology; physiology and ecology of nitrogen fixation

 

Research Program:
I am especially interested in the physiology and ecology of nitrogen fixation, and other aspects of the nitrogen cycle. I am currently studying actinorhizal nodules such as those of alders, where the nitrogen-fixing symbiont is an actinomycete (Frankia). There are 23 genera of dicots, in 8 families, that form actinorhizal nodules. These plants are important in forestry, land reclamation, and the global nitrogen budget.My work with actinorhizal nodules concerns the regulation of gas exchange and the function of nodule hemoglobins. These hemoglobins are similar to the myoglobins of animal muscle tissue, but their function is unknown. I have shown that hemoglobins are present in high concentrations in the nodules of some actinorhizal species, and in low concentrations in other species. My working hypothesis is that hemoglobins are needed for the extremely high rates of respiration and gas exchange that occur in root nodules. We have recently shown that Frankia produces hemoglobin when grown in pure culture. Current research focuses on the establishing the function of this hemoglobin, both in the free-living and symbiotic states of Frankia.