Changes in the ability of plants to alter the pH of the flooding medium as an early symptom of flooding stress in Helianthus annuus

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Abstract
Roots of sunflower plants (Helianthus annuus L.) were flooded in nutrient medium and nonflooded controls were continuously bathed with nutrient medium in a hydroponic system. The pH of the nutrient medium was measured over a period of days. Within 24 h the pH of the nutrient medium in the flooded treatments was 1 pH unit higher than that of the nonflooded treatments and remained higher over several days. Nonflooded plants were able to acidify the medium to a pH value of 4 but when flooded at this pH plants underwent a rapid "catastrophic death," different from normal senescence, in which the shoot dehydrated before chlorosis set in. This pH phenomenon is consistent with both an active acidification of the nutrient medium under nonstressed conditions and a passive drift to a pH dictated by the materials of the hydroponic system, under flooded conditions. It is postulated that flooding rapidly shuts down an acidification mechanism and that this may be symptomatic of early biophysical changes brought about by flooding.