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The influence of N forms and Cl^(-) on solution pH and ion uptake in the hydroponic culture of Ageratum houstonianum (NH₄^(+)-tolerant) and Salvia splendens (NH₄^(+)-sensitive) for a time course of 216 h was investigated. The pH of the hydroponic solution (initially 6.50) supplied with either NH₄^(+), or NH₄^(+) + NO₃^(-) was drastically lowered to 3.08, whereas that of the same solution supplied with NO₃^(-) was raised up to 7.74. Solution pH changed more by ageratum than by salvia. The solution Cl^(-) concentration did not influence solution pH significantly. However, the addition of Cl^(-) in the solution lowered transpiration rate in both NH₄^(+) and NO₃^(-) treatments. Total N uptake was greatest in the NH₄^(+) + NO₃^(-) treatment and lowest in the NO₃^(-) treatment. In the NH₄^(+) + NO₃^(-) treatment, NO₃^(-) uptake was suppressed by NH₄^(+) (to about 50%), while NH₄^(+) uptake was not affected by NO₃^(-). The rate of Cl^(-) uptake was lowest in the NH₄^(+) treatment but was similar in the NH₄^(+) + NO₃^(-) and NO₃^(-) treatments. The uptake of K^(+), H₂PO₄^(-), SO₄^(2-), Mn^(2+), and Zn^(2+) was significantly enhanced in the NH₄^(+) treatment. The uptake rate of Ca^(2+) and Mg^(2+) was highest in the NO₃^(-) treatment. Absorption on Cu^(2+) and B was not affected by N source. Ion uptake was more stable in the solution containing both NH₄^(+) and NO₃^(-) than in the solution containing either NH₄^(+) or NO₃^(-). The uptake rates of total N, NH₄^(+), NO₃^(-), Mn^(2+), Cu^(2+), and Zn^(2+) were significantly higher in ageratum than in salvia, whereas those of Cl^(-) and Mo were significantly lower in ageratum than in salvia. The amounts of total anion (TA) absorbed, but not total cation (TC), the difference (TC-TA), and sum (TC+TA) were all affected by N source, Cl^(-) level and their interaction. The total anion and cation uptake was significantly depressed, due to reduced uptake of both TA and TC, but TC-TA was significantly increased in the NO₃^(-) treatment compared to the NH₄^(+) or the NH₄^(+) and NO₃^(-) treatments, especially in the absence of Cl^(-), Plant tissue ion contents were also affected by N source and Cl^(-) level.
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