Effect of low pH of the root medium on proton release, growth, and nutrient uptake of field beans (Vicia faba)

Abstract

The effect of low root medium pH on growth and proton release of field beans (Vicia faba L. cv. Kristall) was studied in soil and nutrient solution experiments. Decrease of soil pH due to proton release by roots strongly depended on the proton buffer capacity of 8 different soil types tested in a pot experiment. Whereas in soils of high proton buffer capacity no pH decrease during the growth period was detectable, in soils of low buffer capacity pH in the bulk soil dropped from about pH 7.3 to 6.5, 6.3 or 5.8 during growth until maturity. This decrease in pH was closely correlated with an inhibition of plant dry weight production (Y=1.06×+3.33, r=0.94***). Growth reduction was not due to direct inhibition of nitrogen fixation. In short term experiments vegetative growth and proton release were inhibited at pH<6. At pH 5 or lower proton uptake was observed in 1 mM CaSO₄. Low pH (4.0 relative to pH 7.0) decreased uptake of all major ions except for Cl the exclusion of which was disturbed. It is concluded that the sensitivity of field beans to low pH is related to a lack of capability to release protons by ATPase activity. This sets limits to nutrient uptake and possibly cytoplasmic pH regulation.