The nucleotide sequence of the translation initiation regions of 96

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1983), Sinoquet (1989), and Sinoquet and Bonhomme (1992), among others. However, to date, measured two-The amount and distribution of leaf area and leaf angles in a crop dimensional leaf area and leaf angle data have not been canopy determine how photosynthetically active radiation (PAR) is intercepted and consequently influences canopy photosynthesis and incorporated into a light interception-canopy photosynyield. Factors such as plant shape, plant populations, and row width thesis model. will affect these leaf distributions and can occur in an almost infinite Two-dimensional aspects of light interception are parnumber of different combinations. To supplement experimentation, ticularly important in mid-to short-season production a mathematical model was developed to use measurements of leaf areas where leaf area index is rarely in excess of that area and leaf angles in two dimensions (with height and across the required to maximize canopy light interception. Develrow) to calculate PAR interception and canopy photosynthesis. Maize opment of new phenotypes for these production areas, (Zea mays L.) hybrids with phenotypic differences were planted at including leafy and leafy-reduced stature (Dwyer et al., several plant populations to produce a wide range of two-dimensional 1995b; Begna et al., 1997; Modarres et al., 1997), has leaf area and leaf angle patterns. The extreme phenotypes, leafy and required reassessment of optimum plant populations reduced stature, were included to vary plant height and number of leaves above the ear. Measurements of average PAR at various levels and planting patterns to maximize production. The obwere made in seven different canopies and compared with calculations jectives of this study were to develop methods to quanfrom the model (R 2 of 0.68 and 0.92 for two sets of data). As well, tify two-dimensional leaf area distribution and to use measurements of PAR at 20-cm increments on transects perpendicular this distribution to calculate light interception and canto the row were made in three canopy types at three levels and opy photosynthesis. This methodology was used to charcompared with theoretical calculations (R 2 ϭ 0.74). A simple numeriacterize the two-dimensional distribution of leaf area cal experiment was run to demonstrate the utility of the model where of maize hybrids with contrasting architecture and to daily canopy photosynthesis was calculated for two row widths and compare calculations of light penetration into these canseven plant types. One result was that depending on row widths, opies with measurements. We also used the theory to plants with very upright leaves can have both the smallest and largest calculate PAR flux densities on leaf surfaces, which daily canopy photosynthesis.

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Chilling temperatures increase the amounts of potentially lethal toxic oxygen compounds present within plants. These toxic oxygen compounds can be scavenged by antioxidant compounds such as ascorbate and β‐carotene. Three developmental stages (first, third and fifth leaf) of four inbred lines of maize (Zea mays L.) exhibiting differential sensitivity to chilling were examined in order to determine if the chilling‐sensitive line had lower concentrations of antioxidant compounds than did the tolerant lines. Plants were exposed to one of three treatments: (1) control (25°C constant), (2) control treatment plus a short‐term chilling exposure of 11°C one day prior to harvesting, and (3) long‐term (11°C constant) chilling exposure. Total ascorbate, total glutathione, β‐carotene, α‐tocopherol and chlorophyll contents were quantified, and ratios of dehydroascorbate/ascorbate and reduced/oxidized glutathione were determined. Lower concentrations of β‐carotene were found in the chilling‐sensitive relative to those in the chilling‐tolerant lines for the first‐leaf stage under both short‐ and long‐term chilling treatments. Concentrations of total ascorbate and glutathione and β‐carotene in the chilling‐sensitive line increased as the chilling treatment progressed and as the plants developed until they ultimately became either significantly higher or no different relative to the tolerant lines. Results suggest that this sensitive line became less sensitive to chilling‐induced oxidative stress with development.

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Measurement of relatively small (<100 m total length, <6 g fresh wt.) root systems has been simplified by image analysis, but measuring larger root systems remains time‐consuming and inaccurate. Reliability of root estimation can be improved through identification of effective sampling methods. We devised a system for the collection of homogeneous root subsamples by air‐stirring in water. We optimized the subsampling technique and used a scanner‐based image analysis system to measure total root length, mean root diameter, and root surface area of three maize (Zea mays L.) genotypes with contrasting root morphologies: leafy reduced stature (LRS), leafy normal stature (LNS), and Pioneer 3905 (P3905), a commercial hybrid. Root length was determined for 957 subsamples. Confidence intervals were generated by software using the bootstrap resampling approach for optimizing sample size. Confidence intervals for mean estimates of each sample size were defined by ordering the evaluation function values from the smallest to the largest in a set of 5000 iterations. The lower and upper bounds of confidence intervals were also calculated using the standard procedure. This system allowed collection of homogeneous subsamples. Calculations showed that ∼10% of total root volume should be analyzed for estimation of the entire root system to be accurate within 10%. Although unreplicated, these data suggest that maize genotypes with the leafy trait have greater root lengths (1.75 km for LRS, 2.37 km for LNS, and 0.49 km for conventional commercial hybrid P3905) and a greater proportion of fine roots than the nonleafy type.

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