Carbon Dioxide Affecting Photosynthesis

In the atmosphere, the concentration of carbon dioxide ranges from 0.3% to 0.4 %.But a difference of 0.1% may be very important in increasing the rate of photosynthesis.
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Gas exchange is generally regarded to occur between the leaf interior and ambient air, i.e. in vertical (anticlinal) directions of leaf blades. However, inside homobaric leaves, gas movement occurs also in lateraldirections. The aim of the present study was to ascertain whether lateral CO2 diffusion affects leaf photosynthesis when illuminated leaves are partially shaded.
Measurements using gas exchange and chlorophyll fluorescence imaging techniques were performed on homobaric leaves of Vicia faba and Nicotiana tabacum or on heterobaric leaves of Glycine max and Phaseolus vulgaris. 
For homobaric leaves,gas exchange inside a clamp-on leaf chamber was affected by shading the leaf outside the chamber. The quantum yield of photosystem II (ΦPSII) was highest directly adjacent to a light/shade border (LSB). ΦPSII decreased in the illuminated leaf parts with distance from the LSB, while the opposite was observed for nonphotochemical quenching. These effects became most pronounced at low stomatalconductance. They were not observed in heterobaric leaves.

The results suggest that plants with homobaric leaves can benefit from lateral CO2 flux, in particular when stomata are closed (e.g. under drought stress). This may enhance photosynthetic, instead of nonphotochemical, processes near LSBs in such leaves and reduce the photoinhibitory effects of excess light.
This change is achieved in thegreenhouses which are enclosed chambers where plants are grown under controlled conditions. The concentration is increased by installing gas burners which liberate carbon dioxide as the gas burns. Crops like tomatoes, lettuce are successfully grown in the greenhouses. These greenhouse crops are found to be bigger and better-yielding than their counterparts growing in natural conditions.
The study,from researchers at the University of Illinois and the U.S. Dept. of Agriculture, appears the week of February 9 in the Proceedings of the National Academy of Sciences.
Plants draw CO2 from the atmosphere and make sugars through the process of photosynthesis. But they also release some CO2 during respiration as they use the sugars to generate energy for self-maintenance and growth. How elevatedCO2 affects plant respiration will therefore influence future food supplies and the extent to which plants can capture CO2 from the air and store it as carbon in their tissues.
While there is broad agreement that higher atmospheric CO2 levels stimulate photosynthesis in C3 plants, such as soybean, no such consensus exists on how rising CO2 levels will affect plant respiration.
"There's been agreat deal of controversy about how plant respiration responds to elevated CO2," said U. of I. plant biology professor Andrew Leakey, who led the study. "Some summary studies suggest it will go down by 18 percent, some suggest it won't change, and some suggest it will increase as much as 11 percent."
Understanding how the respiratory pathway responds when plants are grown at elevated CO2 is key toreducing this uncertainty, Leakey said. His team used microarrays, a genomic tool that can detect changes in the activity of thousands of genes at a time, to learn which genes in the high CO2 plants were being switched on at higher or lower levels than those of the soybeans grown at current CO2 levels.
Rather than assessing plants grown in chambers in a greenhouse, as most studies have done,Leakey's team made use of the Soybean Free Air Concentration Enrichment (Soy FACE) facility at Illinois. This open-air research lab can expose a soybean field to a variety of atmospheric CO2 levels – without isolating the plants from other environmental influences, such as rainfall, sunlight and insects.
Some of the plants were exposed to atmospheric CO2 levels of 550 parts per million (ppm), the...