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Abstract

A sensor for measuring photosynthetically active radiation was constructed using a silicon photocell in combination with a glass absorption filter. A trimmer potentiometer was used for standardization of sensor output. The sensors were calibrated using a commercially available quantum sensor. Average correlation coefficient between constructed sensors and the standard was 0.94. The sensors had a quantum response, were sensitive only in the wavelengths between 400 and 700 nm, exhibited a linear response to varying PAR light levels, and were inexpensive to construct.

Open Access

( Fig. 1 ). Fig. 1. Mobile platform measurement system retrofitted on a utility vehicle (model 610 Mule; Kawasaki Heavy Industries, Tokyo) for measurement of photosynthetically active radiation ( PAR) interception in orchards; IRT = IR thermometers, GPS

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/below biomass (A/B), total biomass (TB), shoot number (SN), root number (RN), and total leaf surface area (TLSA) in the trials in the study of the effects of photosynthetically active radiation on vegetative growth of Texas wild rice. Significant differences in

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Photosynthetically active radiation (PAR) was measured at two times of day (8:00 am and noon Central Standard Time) in a 915 × 915-cm area of a 1006 × 915-cm gable roof greenhouse. PAR measurements were taken across a grid at 40-cm intervals, a total of 529 data points. Spatial variation of PAR in the greenhouse was evaluated through contour plots and the geostatistical technique of semivariogram construction. Semivariograms provide a visual guide to the degree of spatial correlation of a variable, allowing a quantification of the distance at which variables cease to be spatially correlated (the range) Measured PAR contained distinct zones of lowered values, a function of overhead greenhouse structures, wall-hung electrical boxes, and tall plants in adjacent greenhouses. Although the amount of PAR changed over time, zones of high and low PAR remained relatively constant, except at the sides of the greenhouse. Constructed semivariograms revealed that PAR contained strong spatial correlation (up to a 350-cm separation) as measured in the north-south direction, moving parallel to greenhouse bench placement. When PAR measurements perpendicular to benches (east-west) were used in directional semivariograms PAR was found to be completely random, plotting as a horizontal line called a nugget effect. Thus, plants placed perpendicular to the greenhouse benches (east-west) would not be affected by the spatial correlation of PAR.

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plot variability increases when LAI obs >4 m 2 ·m −2 ( Fig. 2D ). Fig. 2. Relationship between observed leaf area index (LAI obs ) and light indices. ( A ) Near-infrared radiation (700–1000 nm) to photosynthetically active radiation (PAR; 400

Open Access

This report describes a system for integrating photosynthetically active radiation (PAR) using fiberoptics. Many photoelectric sensors or 1-m-long line sensors that integrate individual interception points for spatial averaging were replaced with fiberoptics, which integrate interception points. Depending on the positioning of optical fibers and the amount of fibers terminated at a PAR sensor, whole-plant, canopy layer, and individual leaf light interception can be determined. The use of fiberoptics has the added advantage of being very small in comparison to the bulk of a typical quantum sensor. The fiberoptic-based system potentially is a more accurate, less expensive method to integrate PAR throughout plant canopies than PAR sensors.

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Abstract

Several summer pruning treatments were applied in August to ‘Stayman’/Malling Merton (MM) 111 apple (Malus domestica Borkh.) trees measuring 4m high and 5m wide to examine the resulting changes in light climate. Diffuse photosynthetically active radiation (PAR) increased immediately by about 1/5 on the periphery and 1/10 within the canopy from 2 types of summer pruning. These changes in PAR had no effect on net photosynthetic (Pn) potential, dark respiration (Rd), or specific leaf weight (SLW) determined 21 and 54 days after pruning. One method of summer pruning reduced penetration of PAR into inner canopy positions the year following treatment.

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Abstract

Citrus trees were defoliated to obtain measurements of radiation penetration at various canopy densities. Tree size measurements were used to calculate the leaf-area index (LAI) and leaf-area to canopy-area ratio (LAC) of trees. Penetration of photosynthetically active radiation (PAR) increased curvilinearly as defoliation increased and LAC decreased. Observed relationships between PAR penetration and LAC were used to estimate LAC values from PAR penetration measurements. Correlations between PAR penetration and LAC were slightly higher than with LAI. LAI and LAC appeared to vary independently from tree size as measured by tree diameter or leaf area.

Open Access

Abstract

The influence of photosynthetically active radiation (PAR) and temperature on net photosynthesis and dark respiration was determined on leatherleaf fern [Rumohra adiantiformis (G. Forst) Ching] grown under 73% polypropylene shade. The maximum photosynthesis rate of 7.5 mg CO2 dm-2 hr-1 occurred at 500 μE m-2s-1 PAR. The light compensation point was 44 μE m-2s-1 PAR. A temperature of 20°C and 1000 μE m-2s-1 PAR was most favorable for net photosynthesis of leatherleaf fern. Use of the antitranspirant, a paraffin wax emulsion (Mobilcer A), at a 2% concentration reduced dark respiration by more than 50% and reduced net photosynthesis considerably compared to the control at 40°.

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A mathematical description for the relationship between the rate of rose (Rosa hybrida L.) leaf net photosynthesis and photosynthetically active radiation, leaf temperature, and leaf age is developed. The model provides a tool for the prediction of these rates for leaves growing in a rose crop canopy.

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