2019 ASHS Annual Conference

Objectives were to evaluate (1) changes in root zone pH and cation/anion uptake and (2) the potential to enhance root ferric iron (Fe³⁺) reductase and phosphatase activity in response to low iron and phosphorus supply for geranium (Pelargonium × hortorum), impatiens (Impatiens wallerana), and petunia (Petunia × hybrid) grown hydroponically and in peat-based substrate. In one experiment, plants were grown hydroponically in a controlled-environment growth chamber for 21 days in three nutrient solutions containing 7.14 milli-equivalents (mEq)∙L^-1 NO₃-N with all essential plants nutrients (control), with no iron (-Fe), or with no phosphorus (-P). In the second experiment, plants were grown in a greenhouse in peat-based substrates and fertilized with the same nutrient solutions for 28 days. At the end of both experiments, plants developed visual symptoms of iron and phosphorus deficiency when supplied with -Fe and -P nutrient solutions, respectively. In hydroponics, all species raised root zone pH except for geranium, which decreased pH 3.6 units when supplied with -Fe solution and resulted in no pH change with –P solution. Cation/anion uptake ratio for geranium increased from 1.02 (control) to 1.55 and 1.79 when supplied -Fe and -P solutions, respectively, but was not affected for the other species. Iron reductase activity changed by 40%, 36%, and -14% for geranium, impatiens, and petunia, respectively, when supplied -Fe compared to the control solution. Phosphatase activity changed by 167%, 42%, and 31% for geranium, impatiens, and petunia, respectively, when supplied -P compared to the control solution. In substrate, plant species increased root zone pH with each nutrient solution type. Cation/anion uptake ratio for geranium and impatiens was greater when supplied -Fe and -P solutions compared to the control, but was not affected with petunia. It is likely that potential acidification from geranium with –Fe was highly localized in the rhizosphere and therefore not detected by measuring changes in bulk substrate-pH. In horticultural practice, geranium typically show lower susceptibility to iron deficiency at high pH compared to petunia, which may result from the ability of geranium to acidify the rhizosphere and enhance ferric iron reductase activity in response to low iron supply.