The effect of high CO₂ and low O₂ concentrations in simulated landfill gas on the growth and nodule activity of leucaena leucocephala

When roots of Leucaena leucocephala seedlings (White Popinac, a tropical legume tree belongs to the Family Mimosaceae) were fumigated with simulated landfill gas (CO₂ above 10% and O₂ from 10% to atmospheric level), the stem elongation rate and stomatal conductance were inhibited at the absence of any apparent leaf water deficit. When compared with a treatment where root system was flooded, the effect of gas fumigation on the shoot physiology was relatively mild and appeared later. On the other hand, nodule activity (measured as rate of acetylene reduction activity, ARA) was much more severely inhibited by gas fumigation. Although nodule dry weight and carbohydrate storage in nodules were reduced, the inhibition was not likely a result of the shortage of carbohydrate reserve in the nodules. This was because the ARA of untreated fresh nodules was also inhibited immediately following exposure to the simulated landfill gas. In further experiments where CO₂ and O₂ were manipulated separately, although a reduction of O₂ concentration to half of the atmospheric level might account for up to 30% loss of ARA with considerable variation, the high CO₂ alone showed a much more severe inhibition. This CO₂-induced inhibition was not reversible one hour after the high CO₂ gas was removed. There was some recovery of activity 5 day after plants were fumigated, suggesting that the legume plant can maintain some nitrogen-fixation activity under the influence of landfill gas.