Solution pH influences on growth and mineral element concentrations of ‘Waimanalo’ papaya seedlings

Abstract

Two sand culture experiments were conducted to determine the influence of substrate pH on ‘Waimanalo’ papaya seedling growth, morphology, and mineral element nutrition. The seedlings were grown in clear, plastic tubes 7 cm in diameter and 21 cm in height with the rooting substrate being silica sand. The substrate was drenched daily with a complete nutrient solution. The solution was adjusted to target the pH at 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, or 9.0. Direct seeded plants were used in the first study and plants were harvested after 9.5 weeks of growth. Transplants were used in the second study, and plants were harvested after four weeks of growth. Dry mass of plants was relatively unaffected by substrate pH within the range of 4.0 to 9.0. In both studies, however, dry mass of plants receiving a target pH of 3.0 was reduced compared to those plants receiving the other pH level nutrient solutions. This response pattern was similar for other characteristics such as canopy/root ratio, total root length, root extension rate, lateral root thickness, and root density. Various root characteristics were decreased more by pH of 3.0 than other canopy characteristics. Unit leaf area per unit root length was greatly increased for plants receiving the pH 3.0 nutrient solution, indicating increased susceptibility to drought stress. Root accumulation of manganese (Mn), zinc (Zn), and iron (Fe) was markedly greater than the canopy accumulation. The influence of pH on mineral element concentrations was similar for roots and canopy for all elements except magnesium (Mg). Canopy tissue, but not root tissue, accumulated more Mg at lower pH than higher pHs. The greatest influence of pH on mineral element concentration was for iron (Fe). The concentration of Fe at pH of 3.0 was almost three times the concentration at the other pHs. No visual symptoms of mineral deficiency and/or toxicity were evident. These results indicate that the growth and morphology of papaya seedlings provided with complete nutrient solution are relatively unaffected by pH within the range of 4.0 to 9.0.