A Fiberoptic-based System for Integrating Photosynthetically Active Radiation in Plant Canopies

This report describes a system for integrating photosynthetically active radiation (PAR) using fi beroptics. Many photoelectric sensors or 1-m-long line sensors that integrate individual interception points for spatial averaging were replaced with fi beroptics, which integrate interception points. Depending on the positioning of optical fi bers and the amount of fi bers terminated at a PAR sensor, whole-plant, canopy layer, and individual leaf light interception can be determined. The use of fi beroptics has the added advantage of being very small in comparison to the bulk of a typical quantum sensor. The fi beroptic-based system potentially is a more accurate, less expensive method to integrate PAR throughout plant canopies than PAR sensors. Characterization of the light environment in a spatially and temporally variable plant canopy or orchard requires adequate description of the repeating unit of the planting pattern (Jackson, 1980). A single point photosynthetically active radiation (PAR) sensor measures the fl ux density of quanta within a selected spectral waveband, within the PAR wavelength range (400 to 700 nm) (Pearcy, 1989). However, this method is limited because it only quantifi es light at a single point. As is often the case when using single point PAR sensors, inadequate spatial sampling procedures occur (Kyle et al., 1977; Baldocchi and Collineau, 1994; Salimen et al., 1983). Alternatively, integrated light measurements can be obtained from line quantum sensors, which are often used in crops that are row oriented (Pearcy, 1989). Trying to collect information about a spherical object with light bars necessitates the use of multiple linear light bars positioned orthogonally, which can be arduous. Regardless of technique, measurements of the radiation transfer through canopies strive to adequately characterize the foliage interception in canopies. The most economic photoelectric sensors with spectral responses in the PAR region, silicon cells (Si) (Pearcy, 1989) and gallium arsenide phosphide (GaAsP) (Pearcy, 1989) sensors, do not integrate PAR over many points and require a large amount of data channels and logger memory. In addition, such PAR data are subject to question due to the shiny surface of photoelectric sensors, a potential cause of poor responses at low angles of incidence (Pearcy, 1989). When mounted directly on several leaves and at different canopy layers, GaAsP photocells are capable of sampling various points within a canopy (Gutschick et al. 1985). However, fi beroptics offer an advantage since they allow integration of 2 to >100 light Paired line and fi beroptic PAR measurements. On 22 July 2002 three uniform red maple trees in the central portion of the plot were randomly chosen for intensive measurement of PAR interception. Each tree was placed within a telescopic poly vinyl chloride (PVC) rack (Fig. 1) measuring 1 m wide and 1 m long for 9 d. PVC pipe 5 cm in diameter cut into 1-m lengths was used for the bottom four legs and then 3.75-cm and 2.5-cm-diameter PVC was slid inside of the 5-cm tubes, respectively. The four legs had peg holes drilled every 10 cm on three sections. When fully extended the rack was 3 m high. The rack supported fourline quantum sensors (LI-COR Inc., Lincoln, Neb.) placed orthogonal in a square grid. Each line sensor was placed 40 cm perpendicular to the tree stem in an north, south, east, and west orientation. The canopies were each divided into three layers (I, II, and III) by splitting the length of the live crown into equal thirds. The telescopic rack was moved to the median of each layer for 3 d per layer and after the three layers were sampled, the rack was moved to the next sample tree. In all, three trees were sampled at three layers for 3 d per canopy layer. The line sensors sampled PAR every 1 min and then recorded a 15-min average (CR10x; Campbell Scientifi c, Logan, Utah). The fiberoptic technique to integrate PAR used multimode fi beroptic bundles that sampled canopy light interception incident at three layers of the canopy boundary. The multimode optical fi ber was specifi cally chosen not to interfere with wavelength transmission (Clifford R. Pollock, Cornell University, personal communication). To manufacture the optical sensors, we followed Secore Recommended Procedure SRP-005-005 and SRP-005-006 (Corning Cable Systems, 2001). The specifi c technical details are available from the author. To sample PAR interception via fi beroptics, each of the three intensive light sample trees were fi t with three Corning optical cables (one per canopy layer) (Fig. 1). The four sets of twelve microfi bers per cable were placed at the boundary edge of the median of each of the three canopy layers per tree at a 45 angle facing each cardinal direction (north, south, east, and west), fastened to and supported by a 7-mm wooden dowel, and secured to the main stem with plastic fasteners. This integrated 48 62.5-μm irradiance interception points per canopy layer. All sensors were sampled every 1 min at a 5000-μV sensitivity and 15-min averages were logged (CR7x, Campbell Scientifi c, Logan Utah). At any one time, three trees and three canopy layers were continuously sampled for PAR interception by the fi beroptic-based system, whereas the orthogonal grid of line quantum sensors could sample only one layer on one tree. PAR sensor scaling. Due to line quantum sensors comprising a square grid, two of the four sensors rested on the opposing orthogonal two at the distal portions of the sensors. To correct the data for self imposed line sensor shading, the exact confi guration was placed in an open fi eld on the PVC telescopic rack during solar noon to derive a shading correction interception points into one PAR and or solar radiation sensor reading. Immature or widely spaced crop canopies such as woody nursery vegetation and orchards vary in either two or three dimensions. Models that characterize the light environment within such canopies are complicated and validation of the probability of beam penetration through foliage, such as the Norman and Wells (1983) three-dimensional radiation transfer model, justify the need for an inexpensive and integrated PAR measurement technique. Measurement of canopy light interception with an inexpensive fi beroptic-based system overcomes the common limitations mentioned above, especially when the focus is to measure different canopy layers and validate a multilayer model. Integration of the quantity of light over numerous points in as large an area as necessary is not restricted by geometric shapes and therefore, drastically reduces the number of PAR sensors required. This report is part of a larger study (Bauerle et al., 2004) on intracanopy interactions of PAR in Acer rubrum L. (red maple). In this report, we test a fi beroptic-based PAR integration system and examine three canopy layers, in three independent trees, and in comparison to paired line quantum sensor values in red maple canopies. Validation was achieved by comparing measurements of fi beroptic PAR to an orthogonal grid of PAR line sensors. Materials and Methods The study was conducted at the Clemson University Calhoun Field Laboratory site in Clemson, S.C., during 2002. A full description of the site and plant material is given in Bauerle et al. (2002). Briefl y, containerized red maple saplings were spaced 1.25 m center to center. Three-year-old stock in 57-L Spin Out (Nursery Supplies Inc.) treated plastic pots contained a mixture of pine bark and sand (20:1, v/v), fertilized with 8.3 kg·m of Nutricote 20N–3.0P–8.3K type 360 (ChisoAsahi Inc.). HORTSCIENCE 39(5):1027–1029. 2004. Received for publication 1 Apr. 2003. Accepted for publication 9 Sept. 2003. Technical contribution of the South Carolina Agriculture and Forestry Research System, Clemson University. Corresponding author; e-mail [email protected].