Atmospheric CO2 and mycorrhiza effects on biomass allocation and nutrient uptake of nodulated pea (Pisum sativum L.) plants

The effect of ambient and elevated atmospheric CO2 on biomass partitioning and nutrient uptake of mycorrhizal and non-mycorrhizal pea plants grown in pots in a controlled environment was studied. The hypothesis tested was tha t mycorrhizae would increase C assimilation by increasing photosynthetic ra tes and reduce below-ground biomass allocation by improving nutrient uptake . This effect was expected to be more pronounced at elevated CO2 where plan t C supply and nutrient demand would be increased. The results showed that mycorrhizae did not interact with atmospheric CO2 concentration in the vari ables measured. Mycorrhizae did not affect photosynthetic rates, had no eff ect on root weight or root length density and almost no effect on nutrient uptake, but still significantly increased shoot weight and reduced root/sho ot ratio at harvest. Elevated CO2 increased photosynthetic rates with no ev idence for downregulation, increased shoot weight and nutrient uptake, had no effect on root weight, and actually reduced root/shoot ratio at harvest. Non-mycorrhizal plants growing at both CO2 concentrations had lower shoot weight than mycorrhizal plants with similar nutritional status and photosyn thetic rates. It is suggested that the positive effect of mycorrhizal inocu lation was caused by an enhanced C supply and C use in mycorrhizal plants t han in non-mycorrhizal plants. The results indicate that plant growth was n ot limited by mineral nutrients, but partially source and sink limited for carbon. Mycorrhizal inoculation and elevated CO2 might have removed such li mitations and their effects on above-ground biomass were independent, posit ive and additive.