Effects of rooting medium and fertilizer rate on response of white clover to tropospheric ozone

Two white clover (Trifolium repens L.) clones with varying sensitivity to O(3) are being developed as a system to indicate effects of ambient concentrations of tropospheric O(3) on plants. One clone (NC-S) is highly sensitive to O(3) and the other (NC-R) is highly resistant. The system relies on periodic measurement of foliar injury, foliar chlorophyll, and forage production of NC-S and NC-R grown in 15-liter pots throughout a summer season. Relative amounts of foliar injury and ratios (NC-S/NC-R) for chlorophyll and forage weight can be used to estimate biologically effective ambient O(3) concentrations. The effect of variation in rooting media formulation and fertilizer rate on response of NC-S and NC-R to ambient O(3) was determined in the present study. In the rooting medium experiment, clover was grown in three mixtures of sandy loam topsoil:course washed sand:Metro Mix 220 (ratios (by volume) of 2:1:1, 2:1:5, and 6:1:1). In the fertilizer experiment, clover was grown in the 2:1:1 medium at four fertilizer rates (soluble 5-11-26 (N-P-K) at 0.0, 0.5, 1.0, or 2.0 g per pot). Ozone caused more foliar injury, more chlorosis, and a greater decrease in forage production of NC-S than of NC-R in all studies. Rooting media treatments affected both clones similarly and occasional clone x media interactions were judged to be random. Forage production by NC-S, relative to that of NC-R, was generally greater in the 0.0 fertilizer treatment, but the forage ratios were similar at all other fertilizer treatments. The relative response of NC-S and NC-R to O(3) is fairly stable under cultural conditions that support normal plant growth.