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Charles Barnes, Theodore Tibbitts, John Sager, Gerald Deitzer, David Bubenheim, Gus Koerner, and Bruce Bugbee

Photosynthesis is fundamentally driven by photon flux rather than energy flux, but not all absorbed photons yield equal amounts of photosynthesis. Thus, two measures of photosynthetically active radiation have emerged: photosynthetic photon flux (PPF), which values all photons from 400 to 700 nm equally, and yield photon flux (YPF), which weights photons in the range from 360 to 760 nm according to plant photosynthetic response. We selected seven common radiation sources and measured YPF and PPF from each source with a spectroradiometer. We then compared these measurements with measurements from three quantum sensors designed to measure YPF, and from six quantum sensors designed to measure PPF. There were few differences among sensors within a group (usually <5%), but YPF values from sensors were consistently lower (3 % to 20 %) than YPF values calculated from spectroradiometric measurements. Quantum sensor measurements of PPF also were consistently lower than PPF values calculated from spectroradiometric measurements, but the differences were <7% for all sources, except red-light-emitting diodes. The sensors were most accurate for broad-band sources and least accurate for narrow-band sources. According to spectroradiometric measurement, YPF sensors were significantly less accurate (>9% difference) than PPF sensors under metal halide, high-pressure sodium, and low-pressure sodium lamps. Both sensor types were inaccurate (>18% error) under red-light-emitting diodes. Because both YPF and PPF sensors are imperfect integrators, and because spectroradiometers can measure photosynthetically active radiation much more accurately, researchers should consider developing calibration factors from spectroradiometric data for some specific radiation sources to improve the accuracy of integrating sensors.

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Jens N. Wünsche, Alan N. Lakso, and Terence L. Robinson

Four methods of estimating daily light interception (fisheye photography with image analysis, multiple-light sensors, ceptometer, and point grid) were compared using various apple (Malus domestica Borkh.) tree forms: slender spindle, Y- and T-trellises, and vertical palmette. Interactions of tree form, time of day, and atmospheric conditions with light interception estimates were examined. All methods were highly correlated to each other (r 2 > 0.92) for estimated daily mean percent total light interception by the various tree forms, except that the point grid method values were slightly lower. Interactions were found among tree form, time of day, and diffuse/direct radiation balance on estimated light interception, suggesting that several readings over the day are needed under clear skies, especially in upright canopies. The similar results obtained by using the point grid method (counting shaded/exposed points on a grid under the canopy) on clear days may allow rapid, simple, and inexpensive estimates of orchard light interception.

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Richard J. Campbell and Richard P. Marini

Percent instantaneous incident photosynthetic photon flux density (%INPPFD) was measured within an apple (Malus domestica Borkh.) canopy for various sky conditions and used to predict the percent cumulative incident photosynthetic photon density (PPD) for the last 10 weeks of the growing season (%CPPDLS) and the total growing season (%CPPDTS). Instantaneous measurements from overcast conditions were superior to measurements from clear or hazy conditions for the prediction of %CPPDLS in 1989 and 1990. A one-to-one relationship between %INPPFD and %CPPDLS was found for overcast conditions in both years, even though there was an 11% difference in total cumulative PPD between the years. The models had good predictive accuracy, with prediction coefficients of determination (R 2 Pred) >0.83 in both years (n = 30). %lNPPFD from overcast conditions also yielded accurate predictive models for %CPPDTS (R > 0.84, n = 30), which differed from the models for %CPPDLS. Predictive models (for both %CPPDLS and %CPPDTS) from %lNPPFD made before the canopy was fully developed differed from the models developed after canopy development was complete. The models still had good predictive accuracy, with R 2 Pred >0.76 (n = 30). Predictive models developed for cloudless conditions had inferior predictive accuracy (R 2 Pred = 0.49 to 0.80, n = 30) compared to models for overcast conditions. R 2 Pred were higher for hazy than for clear conditions. Time of day (1000 to 1400 hr) had no consistent effect on the development of predictive models for any weather condition. The most reliable models resulted from the average of several measurements within a day, particularly for cloudless conditions.

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C. Campillo, M.H. Prieto, C. Daza, M.J. Moñino, and M.I. García

. Nevertheless, the three proposed methods enable a good estimation of LI applying traditional methods such as the line quantum sensor. We therefore have a valid tool that can be used to rapidly characterize plant canopies and easily study the incidence of

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William L. Bauerle and Joseph D. Bowden

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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Richard J. Campbell and Richard P. Marini

Integrative measurements of photosynthetically active radiation (PAR) were made at 30 `Delicious' canopy positions throughout the season to characterize the canopy light environment. Instantaneous measurements (IM) of PAR were made at the same positions with a quantum sensor on clear and overcast days and correlated with integrated seasonal PAR. Hourly (1100, 1200, 1300, and 1400 hrs) IM made on clear days were influenced b sunflecks and had variable relationships with integrated values (R2 = 0.52-0.90). This was improved by using the average of the four IM measured during the day (R2 = 0.92). Hourly IM on overcast days were consistent and highly correlated to integrated values (R2 = 0.97). IM from overcast days were reliable predictors of seasonal PAR and could be used to characterize the canopy light environment.

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James E. Faust and Royal D. Heins

Quantum sensors were placed at plant canopy height inside and outside a glass greenhouse. Photosynthetic photon flux (PPF) was measured during September for a 3-hour period near sunrise and sunset, which were determined from US Naval Observatory Circular #171. Under clear skies, the PPF at the canopy exceeded 0.25 μmol·m-2·s-1 for nearly 20 minutes before sunrise through 20 minutes after sunset. Under heavy overcast, the duration was only 5 minutes before sunrise through 5 minutes after sunset. The PPF at the canopy reached 0.25 μmol·m-2·s-1 approximately 12 minutes later in the morning and 12 minutes earlier in the evening than it did outside the greenhouse. The length of the dark period perceived by plants in a greenhouse on September 21st (assuming plants perceive light at 0.25 μmol·m-2·s-1) can range from 11:37 (hr:min) during cloudy conditions to 11:15 during clear ones, a difference of 22 minutes. At 43°N latitude, the maximum difference in date of flower initiation because of an extended period of heavily overcast versus clear weather on a crop such as poinsettias would be one week since the night length during September increases by 3 minutes per day. The actual difference from year to year is probably less because a seven-day duration of heavily overcast weather is unlikely.

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Steven F. Price, Marc L. Schuette, and Elizebeth Tassie

Digital imaging and analysis was used to quantify and characterize the light exposure patterns of photo-sensitive paper tubes placed in representative cluster postions in two grape (Vitis `vinifera L.) canopies: a minimally pruned and a vertically trained canopy. Blue pixel values of the captured video images had a strong negative correlation with the log of irradience from an integrating quantum sensor (R2=0.9308). Histograms of incident light distribution on individual paper tubes were developed using imaging software. Histograms were able to quantify the spatial distribution of light on individual tubes and were clearly related to exposure in the canopy. Average population curves of light distribution were able to differentiate the typical cluster light environment in the two canopies. Tubes in the minimally pruned canopy had a larger proportion of their surface exposed to irradiences greater than 50 μmol s-1m-2 and 65% higher average irradience than the vertical canopy. Image analysis of photo-sensitive paper appears to he a workable method to record variation in spatial and temporal light in plant canopies.

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Donald T. Krizek, Roman M. Mirecki, and Alton L. Fleming

A controlled-environment study was conducted in separate growth chambers with the wall surface covered either with white enamel paint (WEP) or polished aluminum (PA). `Williams' soybean were grown under 1500 mA cool white fluorescent lamps and internodes measured at 7, 14, and 21 days. Photosynthetic photon flux (PPF) levels in the center of each chamber were set at 320 μmol m-2 s-1 with a quantum sensor. Means ± SD for PPF levels in the WEP and PA chambers were 286 ± 28 and 307 ± 11 μmol m-2 s-1, respectively. This increase in mean PPF and decrease in variance of PPF in the PA chamber was reflected in: a) a decrease in hypocotyl, first internode, and total shoot elongation: and b) an increase in enlargement of the primary and the first trifoliolate leaves. These findings demonstrate that plants can detect small differences in irradiance within a growth chamber and suggest the advantages of using a highly polished wall surface to improve uniformity of irradiance and reduce variability in growth.

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Mark R. LeBlanc and James N. McCrimmon

Establishing and maintaining turfgrass in the shade is one of the most challenging problems facing turfgrass managers and home owners. A greenhouse study was initiated to determine the shade tolerance of centipedegrass [Eremochloa ophiuroides (Munro.) Hack.], carpetgrass [Axonopus affinis Chase], and selected St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] cultivars (`Floratam', `FX-10', `Seville', and `TR 6-10'). Plants were grown under artificial shade (85% polypropylene shade cloth) and full sun. Actual percent shade (%shade={PAR under shade/PAR under sun}*100) was determined by measuring photosynthetically active radiation (PAR) under shade cloth and full sun adjacent to the shade structure using a quantum sensor. Pots were arranged in a completely randomized block design with four replications. All turfgrasses, except `TR 6-10', had a significant reduction in total dry weight in the shade compared to those in the sun. `TR 6-10' had the highest root, leaf, and total dry weight in the shade. `FX-10' had the lowest root, leaf, and total dry weight in the shade. Plants grown under the shade treatment compared to those in the sun resulted in an average decrease in stolon number of 13 and in total stolon length of 170 cm. In the shade, `Floratam' and `Seville' had the longest stolon internode lengths, while `Floratam' had the longest in the sun. There were significant differences for leaf length between the shade and sun treatments, except for carpetgrass and `FX-10'. `Floratam' and `FX-10' had differences in leaf width between the sun and shade.