Impact of phosphorus supply on root exudation, aerenchyma formation and methane emission of rice plants

This study evaluated the impact of P supply on rice plant development and the methane budget of rice fields by 2 different approaches: (1) root growth, exudation and aerenchyma formation were recorded in an experiment with hydroponic solution; (2) dissolved CH4 concentration and CH4 emission were investigated in a pot experiment. In both approaches, we used three different cultivars and three levels of P supply. In the experiment with solution culture (0.5 ppm, 5 ppm, and 10 ppm P), root exudation ranged between 0.5 to 36.7 mu mol C plant(-1) h(-1) and increased steadily with plant growth at given P level. Low P supply resulted in depressed shoot growth but increased root growth in culture solution, increments in the root/shoot ratio by factors of 1.4 to 1.9 at flowering stage, enhanced the development of root aerenchyma, and stimulation of root exudation per plant by factors of 1.3-1.8 as compared to medium P supply and by factors of 2.1-2.4 as compared to high P supply. However, root exudation did not differ among treatments when related to the dry weight of roots. Thus, high exudation rates were caused by larger root biomass and not by higher activity of the root tissue. The pot experiment was conducted with a P-deficient soil that was either left without amendment or fertilized by 25 and 50 mg P kg(soil)(-1), respectively. Low P supply resulted in higher CH4 concentrations in soil solution; i.e., at flowering stage the soil solution concentrations were 34-50 mu M under P deficiency and 10-22 mu M under ample P supply and significant increases of CH4 emission rates during the later stages of plant growth. These findings reflect a chain of response mechanisms to P stress, that ultimately lead to higher methane emission rates.