Increased prolificacy and reduced barrenness have been identified as physiological traits in maize (Zea mays L.) hybrids that are tolerant of environmental stresses induced by high plant density. The objective of this study was to investigate ear and kernel formation under a range of plant densities in old and new hybrids. Experiments were carried out during 1987 and 1988 at two locations in Ontario with nine maize hybrids representing three decades of yield improvement in Ontario, grown at 2, 4, 8, 10, and 13 plants m−2. Dry matter was measured at 1 wk presiiking, 3 wk postsilking, and at physiological maturity and plant growth rates calculated for both periods. Grain yield, kernel weight, and kernel number were determined at physiological maturity. Kernels per plant and plant growth rate for the period from 1 wk presilking to 3 wk postsilking could be fitted to a discontinuous function of multiple rectangular hyperbolae; a single rectangular hyperbola was associated with the kernel number on each grain‐bearing ear. The first rectangular hyperbola for each hybrid had a positive x‐axis intercept, which was interpreted as the threshold plant growth rate for ear formation. The analysis showed that grain yield improvement of Ontario maize hybrids was associated with an increase in kernel number per plant and higher plant growth rates from 1 wk presilking to 3 wk postsiiking. The increase in kernels per plant was associated with an increase in ears per plant. The increased prolificacy of recent hybrids appeared to be the combined result of higher rates of plant dry matter accumulation during silking and a greater inherent tendency to exhibit prolificacy. The lower barrenness of recent hybrids at high plant densities was associated with higher plant growth rates, but was not the result of a lower threshold plant growth rate for ear formation.
Remote-sensing techniques, in particular, multispectral visible and infrared (IR) reflectance, can provide Correlations between plant canopy reflectance and aboveground an instantaneous, nondestructive, and quantitative asbiomass can possibly be used for early prediction of crop yield. Field experiments were conducted in 1998 and 1999 on two soil types to sessment of the crop's ability to intercept radiation and assess whether measurements of canopy reflectance at given stages photosynthesize (Ma et al., 1996). The input of reflecof development could be used to discriminate high from low potential tance into yield production models has been shown to yields among genotypes with known differences in potential grain improve yield estimates (Clevers et al., 1994; Clevers, yield and whether a consistent relationship between yield and canopy 1997). Colwell (1956) was the first to use aerial IR phoreflectance could be used for screening and predicting soybean [Glytographs to monitor plant disease in the field. The cine max (L.) Merr.] yield in a variety trial. A 3-by-42 factorial experiamount of reflectance in the near IR (NIR) range ( ϭ ment, arranged in a randomized complete block design with three 700-1300 nm) is determined by the optical properties replications, was used on each soil type for both years. Three populaof the leaf tissues: their cellular structure and the air-cell tion densities (25, 50, and 75 seeds m Ϫ2 ) represented low, optimum, wall-protoplasm-chloroplast interfaces (Kumar and Silva, and high levels. Forty-two historical varieties represented nearly six decades (1934-1992) of soybean yield improvement in Canada. Can-1973). These anatomical characteristics are affected in opy reflectance was measured with a hand-held multispectral radiome-turn by environmental factors such as soil water and/or ter on three sampling dates (approximately R2, R4, and R5 stages) nutrient status (Gausman et al., 1969; Thomas et al., for each site. Grain yield at harvest was measured. Soybean grain yield 1971; Blackmer et al., 1994), soil salinity (Gausman and was highly positively correlated with canopy reflectance, expressed as Cardenas, 1968), and leaf age (Gausman et al., 1970).normalized difference vegetation index (NDVI), at all sampling dates. Reflectance in the visible red (R) range ( ϭ 550-675 Regression analyses showed a positive relationship between NDVI nm) has been used to estimate leaf chlorophyll and and grain yield, with R 2 up to 0.80 (P Ͻ 0.01) and progressive imcarotenoid (Benedict and Swidler, 1961; Thomas and provement from R2 to R5 stages. Population density did not affect Oerther, 1972;Filella et al., 1995) levels and, by extenthe yield-NDVI relationship at the development stages studied. Our sion, the photosynthetic capability of the crop. data suggest that canopy reflectance measured nondestructively between R4 and R5 stages adequately discriminates high-from low-The use of NIR or R spectral bands singly does not yielding genotypes and provides a reliable, fast, repeatable indicator account f...
Leaf area is important for crOJI light interception and therefore has a large influence on crop yield. In this study, a method was devised to characterize and predict the development process of maize (Zea mays L.) leaf area by separating the process into time of appearance of each mature leaf (leaf !itage) and leaf area of each mature leaf (leaf expansion). Leaf area was measured for several years on each of two soil types (a sandy loam of the Uplands association (Typic Haplorthod) and a clay of the Dalhousie association (Typic Haplaquoll)J at Ottawa, Canada. Analysis of field data indicated that leaf stage was highly correlated with growing degree days (base temp of l0°C} accumulated from planting (r = 0.98). However, air temperature alone did not account for the annual variability in leaf area of mature leaves. To separate leaf expansion from leaf stage, mature area per leaf was plotted as a function of leaf number. The resultant curve had a slightly skewed bell shape whose amplitude represented the leaf area of the largest leaf. When these curves were normalized with respect to their amplitudes they varied little from year to year. The amplitudes compared well (r=0.87) to plant-available water averaged from planting to development of the largest leaf and the sum of minimum daily temperature for the same period. Total mature plant leaf area for each year was then calculated by multiplying the integration of the normalized bell shaped curve times the amplitude calculated by the regression equation. These calculated total mature plant leaf areas were used to normalize total plant leaf areas (expanding plus mature leaf area in the absence of senescence). The normalized plant leaf areas were expressed as an Sshaped logistic function of growing degree days and total plant leaf areas were calculated over each growing season. An additional relationship related water deficit during growth to senescence and the amount of leaf area senesced was subtracted from total plant leaf areas to obtain actual plant leaf area. Estimates of actual plant leaf area for the six growing seasons used to develop the method compared well with measured values (r between 0.94 and 0.99). Estimates of actual plant leaf area for two independent years at the same location were also good (r=0.91 and 0.97). This approach reduces prediction of maize leaf area to relatively simple functions of temperature and available water.----------------Additional index words: Corn, Zea mays L., Plant water stress, Leaf expansion, Crop growth, Phenological development.
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.
Assessment of crop N requirements is necessary to develop production systems with optimal N input. A field experiment with six maize (Zea mays L.) hybrids grown at three N fertilizer rates (0, 100, and 200 kg N ha−1) was conducted on a well‐drained sandy loam of the Grenville series (coarse‐loamy, mixed, mesic Typic Eutrochrepts) on the Central Experimental Farm at Ottawa, ON, in Canada (45°23' N, 75°43' W) for 3 yr (from 1991 to 1993) to evaluate whether canopy reflectance and greenness can measure changes in maize yield response to N fertility. Canopy reflectance, leaf area and greenness were measured on 11 dates from 4 wk before to 4 wk after anthesis. Grain yield at harvest was also measured. Direct radiometer readings at the 600‐ and 800‐nm wavelengths or a derived normalized difference vegetation index [NDVI = (800 nm − 600 nm)/(800 nm + 600 nm)] best differentiated N and hybrid treatments at most sampling dates. Canopy light reflectance was strongly correlated with field greenness at almost all growth stages (field greenness being a product of plant leaf area and leaf greenness measured with a chlorophyll meter, in this case a SPAD‐502). Both canopy light reflectance and field greenness measured preanthesis were correlated with yield at harvest. Light reflectance measured after anthesis differentiated hybrid differences in leaf senescence. Our data suggest that light reflectance measurements prior to anthesis may predict grain yield response and provide in‐season indications of N deficiency.
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