On the Relationship between Extracellular pH and the Growth of Excised Pea Stem Segments

Abstract

Studies with stem segments of peas (Pisum sativum L. cv. Alaska) suggest that the pH of the medium bathing elongating tissue does not always reflect intramural (cell wall) conditions or that pH is not a controlling factor in elongation. Peeled, green segments and peeled or non-peeled etiolated segments appear to regulate the pH of their bathing medium causing it to become acidified with or without the addition of auxin. The growth rates of segments are greatest during a period before acidification is evident and slow during the time in which the medium becomes acidified. The dramatic auxin-induced pH shifts reported in the literature were not reproduced because the control segments become more acid also; but there is some evidence that acifification may occur in response to auxin treatments. K+ additions mimic the acidifying tendency of auxin but are without growth-promoting effect. Emergent growth (an extremely rapid burst of growth following anaerobic treatments) is not accompanied by a drop in pH of the bathing medium. Proper aeration of the bathing medium in extracellular pH studies is crucial and may explain differences between these results and other published accounts. Possibly the techniques used for most extracellular pH studies may not very closely approximate in vivo conditions or properly reflect intramural H+ concentration fluxes.