Some plant growth models require estimates of leaf area and absorbed radiation for simulating evapotranspiration and photosynthesis. Previous studies indicated that spectral reflectance, absorption of photosynthetically active radiation (PAR), and leaf area index (LAI) are interrelated. The objective of this study was to establish a procedure by which spectral reflectance can be used to simultaneously estimate PAR absorption and LAI. A method is presented for estimating the quantity of absorbed PAR by wheat (Triticum aestivumL.) plants and their LAI based on the normalized difference (ND), transformation of the near infrared (ρn= 800 to 1100 nm) and red (ρr= 600 to 700 nm) canopy reflectances. The results, from a theoretical analysis and field measurements, indicated that ND correlates with the fraction of PAR absorbed by wheat canopies. Bare soil reflectance and scattering of near infrared radiation by foliage elements were the major factors that influenced the relation between ND and PAR absorption. The estimated PAR absorption values, based on the ND, and four classes of assumed leaf angles (45°, 60°, 75°, and spherical), were used to indirectly evaluate LAI of wheat for three different geographical locations. The standard deviation on mean predicted to measured LAI's for the three locations varied from 0.5 to 0.9 for a range of 0 to 6 LAI. The method is considerably less sensitive in predicting LAI above 6.0 since the sensitivity of ND to changes in LAI becomes small (<0.01), due to small changes in canopy reflectance.

Abstract. Modifications of the design and calibration procedure of a diffusion porometer permit determinations of stomatal resistance which agree well with results obtained bv leaf energy balance. The energy balance and the diffusion porometer measurements indicate that the boundary layer resistances of leaves in the field are substantially less than those predicted from hcat transport formulas based on wind flow and leaf size.

The efficient breeding and selection of corn (Zea mays L.) genotypes for different climatic regions requires a quantitative understanding of the plant's developmental responses to environmental factors such as temperature and photoperiod. This information is also essential if reliable and meaningful crop simulation models are to be developed. Plants of two corn hybrids, XL45 and W346 were grown in controlled environments under 18 day/night temperature combinations ranging from 16/6 to 38/33°C (12‐h photoperiod) and under three photoperiods (12,14, and 16 h) at two selected temperatures (constant 18 and 28°C). Data defining the temperature response curves, including the minimum and optimum temperature limits, for germination and emergence and for the development periods from sowing to tassel initiation and sowing to anthesis were obtained. A minimum temperature of 9°C was predicted for germination and emergence, and a requirement of 62.5 degree‐days was determined for this growth stage. The optimum temperature was approximately 30°C. Minimum temperatures of 8 and 7°C were determined for tassel initiation and anthesis, respectively, and the optimum temperature for both was 28°C above which the development rates declined. These temperature limits compared with minima and maxima of 10 and 30°C, respectively, used in most current heat‐sum methods. Between the limits of 7 and 28°C, the number of degree‐days required to reach tassel initiation and anthesis were, respectively, 208 and 736 for hybrid W346, and 245 and 816 for XL45. Tassel initiation occurred at approximately one‐third of the time between sowing and anthesis when calculated either on the basis of heat‐sums (degree‐days) or from calendar‐days under the steady‐state temperature conditions used. An increase in photoperiod lengthened both the time between sowing and tassel initiation and that between tassel initiation and anthesis in a similar, almost equal, manner for both cultivars. Sensitivity to the photoperiod response was not altered by temperature.

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