Effects of overwatering and fertilizer formulation on growth, nutrient content, and substrate redox status of Petunia ×hybrida during greenhouse production

Abstract

Overwatered spring crops such as Petunia x hybrida are subject to a range of biotic and abiotic disorders including hypoxia, nutrient deficiencies, and increased susceptibility to root rot diseases. Overwatering often results in symptoms of interveinal chlorosis of youngest foliage leading to the assumption that it causes iron (Fe) deficiency. Five greenhouse experiments were conducted to evaluate effects of substrate water status and fertilizer formulation on four petunia cultivars produced in a glass-glazed greenhouse during spring seasons of 2024, 2025, and 2026. Petunia ‘Cascadias Indian Summer’ (IS) and ‘Headliner Strawberry Sky’ (SS) plants were grown under three different watering regimes of overwatered (Over; rewatered when weight of sentinel pots dropped to 90 to 95+5% of container capacity, CC), optimally watered (Opt; 60+5% of CC), and underwatered (Under; 35 to 45+5% of CC) and two different fertilizer sources of Jack’s Professional General Purpose (Gen) and Jack’s Professional FeED Plus Mg (Pet). The following experiment had only Over and Opt as well as cultivars SS, ‘SlingShot Black Magic’ (BM), and ‘SlingShot Phantom’ (PH). The optimal watering regime yielded larger or similar plant growth based on width, fresh and dry weights. SPAD readings of youngest foliage were different based on fertilizer source with Pet resulting in greener young foliage. While IS, BM, and PH petunia did not develop yellowing of youngest foliage in any treatment, SS petunia developed distinctive symptoms of interveinal chlorosis in youngest foliage of overwatered plants fertilized with Gen fertilizer, suggesting a genetic dimension to the disorder. However, tissue analysis of SS petunia revealed no difference in Fe between watering regimes in the first study and greater Fe in Over in the second study, suggesting that Fe deficiency was not likely the cause of the disorder. Other tissue differences occurred across watering treatments for P, Ca, Mg, S, and Mn and between fertilizer formulations in P, Mg, and S. Notably, Mn was reduced in young tissue of Over plants in both studies. Symptomology is effectively mitigated by using Pet fertilizer. Tissue Mn content was reduced by overwatering, but Fe was not deficient. Waterlogged or hypoxic substrate conditions cause microbially mediated reduction of Mn and Fe that changes their plant bioavailability and leaching potential. Iron-inefficient crops such as petunia may be more vulnerable to waterlogged conditions as the substrate availability of these micronutrients change. In a third experiment with Petunia ‘Headliner Strawberry Sky’ (SS), plants were grown with two different watering regimes of Over and Opt and four different fertilizer sources: Jack’s Professional General Purpose (JPGP, Gen), JPGP with additional Mn EDTA (Gen+Mn), JPGP with additional Fe EDTA (Gen+Fe), and Jack’s Classic Petunia Feed (Pet), all at 150 ppm N. The following spring, two experiments were performed to assess the effect of plant maturity on overwatering stress using SS and the additional petunia cultivar ‘SureShot Dark Blue’ (DB). This fourth experiment used young plants that were newly transplanted when experimental watering treatments of Over and Opt started compared to the fifth experiment where plants were allowed to become well established before the same watering treatments of Over and Opt began. Water status was crossed with three different fertilizers: Gen, Gen+Mn, and Control (Con; no micronutrients). Both Fe and Mn Indicator of Reduction In Soils (IRIS) films were used to quantify the amount of Fe and Mn reduction in each experimental treatment via microbial activity. Plant weights, leaf greenness, substrate solution nutrient levels from pour though extraction, and tissue nutrient content of youngest leaves were measured. Tissue Fe levels were greater in Over compared to Opt in all three experiments, and tissue Mn was greater in Opt compared to Over in the third experiment, but the same in the fourth and fifth experiments. Across all three of these experiments, total Mn removal from IRIS films was greater in Over compared to Opt, ranging between 10.9% and 61.4%. Total Fe removal was greater in Over treatments as well; however, the percentages of removal across treatments were much less than for Mn, between 0.4% and 1.6%. While SPAD differences occurred based on fertilizer formulation and water status, very little overwatered chlorotic foliage occurred, suggesting that more mature plants are less likely to develop symptomology in waterlogged conditions. Changes in leaf greenness in response to Over varied by plant maturity: Young plants in the fourth experiment had greener leaves as the experiment progressed and older plants in the fifth experiment had less green leaves.