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The Response of Leaf Photosynthesis and Dry Matter Accumulation to Nitrogen Supply in an Older and a Newer Maize Hybrid

^a Dep. of Plant Agriculture, Crop Science Bldg., Univ. of Guelph, Guelph, ON, Canada, N1G 2W1. Current address of L. Echarte, INTA Balcarce-Universidad Nacional de Mar del Plata, CC 276, 7620 Balcarce, Argentina
^b Dep. of Molecular and Cellular Biology, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1. This study was supported, in part, by the Natural Sciences and Research Council of Canada; Ontario Ministry of Agriculture, Food, and Rural Development; and Syngenta Biotechnology, Research Triangle Park, NC

View larger version (16K): [in this window] [in a new window]   Figure 1. Leaf carbon exchange rate (CER) as a function of weeks from silking for an older (‘Pride 5’) and a newer (‘NK N25-J7’) maize (Zea mays L.) hybrid exposed to high N (HN) and low N (LN). Vertical bars represent ± SE and are not shown when smaller than the symbol size.

View larger version (18K): [in this window] [in a new window]   Figure 2. Reduction in leaf carbon exchange rate (CER) relative to leaf CER at high N as a function of weeks from silking for an older (‘Pride 5’) and a newer (‘NK N25-J7’) maize (Zea mays L.) hybrid exposed to (a) low N (LN), (b) HN switched to LN at 1 wk presilking (HLN1), and (c) HN switched to LN at 2 wk postsilking (HLN2). * Indicates when CER values were significantly lower (p < 0.05) under LN than under HN levels for each hybrid and moment of measurement.

View larger version (17K): [in this window] [in a new window]   Figure 3. Leaf chlorophyll index (SPAD units) as a function of weeks from silking for an older (‘Pride 5’) and a newer (‘NK N25-J7’) maize (Zea mays L.) hybrid exposed to high (HN) and low (LN) N levels. Vertical bars represent ± SE and are not shown when smaller than the symbol size.

View larger version (16K): [in this window] [in a new window]   Figure 4. Electron transport rate (ETR) as a function of weeks from silking for an older (‘Pride 5’) and a newer (‘NK N25-J7’) maize (Zea mays L.) hybrid exposed to high N (HN) and to low N (LN). Vertical bars represent ± SE and are not shown when smaller than the symbol size.

View larger version (21K): [in this window] [in a new window]   Figure 5. Relationship of the difference between the newer (‘NK N25-J7’) and older (‘Pride 5’) maize (Zea mays L.) hybrid in carbon exchange rate per unit absorbed photosynthetic photon flux density (CER Abs–1) vs. the difference between the newer and older hybrid in thylakoid electron transport rate (ETR) for plants grown at low and high N levels. Differences are expressed as a proportion of the value of the newer hybrid (i.e., 100% x [NK N25-J7 – Pride 5]/NK N25-J7), and the dashed line indicates the 1:1 ratio.

View larger version (25K): [in this window] [in a new window]   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.

View larger version (18K): [in this window] [in a new window]   Figure 7. Shoot N content (g N plant–1) for an older (‘Pride 5’) and a newer (‘NK N25-J7’) maize (Zea mays L.) hybrid grown at high N (HN), low N (LN), and high N switched to low N at 1 wk presilking (HLN1) at (a) silking and (b) physiological maturity. Different letters indicate significant differences between hybrids per N level (p < 0.05) by t test.

View larger version (19K): [in this window] [in a new window]   Figure 8. (a) Grain yield (kg ha–1), (b) shoot dry matter (g plant–1), (c) harvest index (HI) and (d) root:shoot ratio for an older (‘Pride 5’) and a newer (‘NK N25-J7’) maize (Zea mays L.) hybrid exposed to low N (LN), high N switched to low N at 1 wk presilking (HLN1), high N switched to low N at 2 wk postsilking (HLN2), and high N (HN). Different letters indicate significant differences between hybrids per N level (p < 0.05) by t test.