Science - September 30, 2009

Symbiotic crops improve their photosynthesis

Legumes that grow symbiotically with rhizobial bacteria, produce the same amount of food as fertilized crops. PhD student Glaciela Kaschuk explains why, and improves the theory and models of photosynthesis.

Crops that live in symbiosis with rhizobial bacteria and mycorrhizal fungi, supply carbon to these organisms. In exchange they get nitrogen, derived from biological fixation, and soil nutrients. A nice deal, but providing part of their photosynthesis products to the bacteria limits plant production with approximately ten percent, according to theory.
This theory should be revisited, says PhD student Glaciela Kaschuk. She found that symbiotic crops increase their photosynthesis to compensate for the loss of carbon. Symbiotic plants recycle triose-P in the chloroplasts in the leaves at a higher rate, activating more photosynthesis enzymes. As a result, the plant has sufficient carbon available for both the bacteria and itself.
Kaschuk, connected with the Brazilian research organization Embrapa, did a couple of experiments with different soybean varieties. She compared the performance of soybean with rhizobial bacteria with soybean without these bacteria that received doses of nitrogen fertilizer. She found that soybean associated with rhizobial bacteria had higher rates of photosynthesis than fertilized plants. Furthermore, nodulated plants maintained higher chlorophyll concentrations for a longer period than fertilized plants, due to a delay of leaf senescence. She concludes that these processes compensate for the loss of carbon to the bacteria.
Most leguminous crops, like pea and soybean, live in symbiosis with rhizobial bacteria and mycorrhizal fungi. This alliance provides them with nitrogen and some phosphates. As a result, they need little to none fertilizer.
Professor Thom Kuyper, promoter of Glaciela Kashuk, says that the common theory about photosynthesis has to be readjusted. ‘According to theory plants have a maximum capacity of photosynthesis, but Glaciela found that legumes without the symbiosis only use ninety percent of that capacity. In a symbiotic relation with rhizobial bacteria they increase that capacity to a hundred percent.’
Field experiments in Brazil already proved that soy production with rhizobial bacteria was equal to soy production with fertilizer. ‘These field results couldn’t be explained by theory, which claimed that symbiotic soybean should produce ten percent less. We now have a proper theoretical explanation.’ Also the yield models for legumes have to be adjusted, says Kuyper.
Kaschuk’s article in the scientific journal Soil Biology and Biochemistry this year drew attention from a lot of collegues. ‘The article is among the ten most downloaded articles from this journal in 2009.’
Glaciela Kaschuk defends her PhD thesis on 30 September. Her supervisors are Ken Giller, professor of Plant Production Systems, and Thom Kuyper, personal chair Soil Quality.