Figure 6. Relationship between leaf chlorophyll index (SPAD units) and N content per unit of leaf area (Na) for an older (‘Pride 5’) and a newer (‘NK N25-J7’) maize (Zea mays L.) hybrid exposed to high N, low N, high N switched to low N at 1 wk presilking, and high N switched to low N at 2 wk postsilking. Data are from harvests at 1 wk presilking, silking, and 3 wk postsilking. A linear with plateau model (Jandel TBLCURVE, 1992) was fitted to the relationship between leaf chlorophyll index and Na for the two hybrids: Leaf chlorophyll index = a + b x Na if Na < c and leaf chlorophyll index = a + b x c if Na c, where a is the y-intercept or leaf chlorophyll index when Na = 0, b is the slope of the chlorophyll index–Na relationship, and c is Na at maximum leaf chlorophyll index.