Allelopathic interference of plant-water relationships by parahydroxybenzoic acid

Soybean seedlings were used as the test species to investigate the effect of phydroxybenzoic acid (PHBA) on growth and plant-water relationships. Plants were grown in nutrient solution under greenhouse conditions and were subjected to pHBA through amendments to the growth medium. Treatments were initiated 10 days after germination and continued for either 14 or 28 days with stomatal conductance, water potential, and water use monitored periodically. At harvest, effects on growth were determined and, in the 28-day study, the carbon isotope ratio (C-13 : C-12) of leaf tissue was analyzed as an indicator of integrated effects on plant-water status. Soybean growth was significantly reduced by 0.5 mM pHBA, or higher concentrations, with the degree of inhibition being concentration dependent. Plants treated with 0.75 mM pHBA had significantly lower stomatal conductance, lower water potential, and less discrimination against C-13. Similar trends were apparent in 0.5 mM pHBA-treated plants. Interference with plant-water balance appears to be one mechanism of action of pHBA causing a reduction in plant growth.