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This article in AJ

  1. Vol. 77 No. 6, p. 841-845
    Received: Aug 10, 1984

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Transmitted and Absorbed Photosynthetically Active Radiation in Grain Sorghum1

  1. W. D. Rosenthal,
  2. G. F. Arkin and
  3. T. A. Howell2



Relationships defining transmitted (TPAR) and absorbed (APAR) photosynthetically active radiation (PAR) in sorghum [Sorghum bicolor (L.) Moenchj canopies do not account for senesced leaves, stalks, and panicles after anthesis. Absorbed PAR by senesced leaves does not directly contribute to photosynthesis, whereas PAR absorbed by panicles does. In addition, senesced leaves, stalks, and panicles reduce transmitted solar radiation reaching the soil and thereby affect soil evaporation. The objective of the study was to examine the combined effect of senesced leaves, stalks, and panicles on TPAR and APAR in a developing sorghum canopy before and after maximum green leaf area index (GLAI) was attained. The study was conducted on a Panoche clay loam (fine-loamy, mixed, thermic Typic Torriorthent) at the West Side Field Station (CA) near Five Points, CA during 1982 and on a Houston black clay (fine-montmorillonitic, thermic Udic Pellustert) at the Blackland Research Center (TX) in Temple, TX, during 1983. The experimental designs were three replications of four and five altered canopy treatments (panicles and layers of green and senesced leaves removed) at TX and CA, respectively. Transmitted and absorbed PAR were calculated from PAR measurements above and below the canopy, before and after maximum green leaf area was attained. Transmitted and absorbed PAR differed significantly (95% level) before and after maximum leaf area index was attained. Panicles intercepted 8% of the incoming PAR stalks 14%; and senesced leaves, 23%. Relationships derived between TPAR and GLAI compared favorably with other published data. Absorption of PAR by panicles, stalks, and senesced leaves should be considered in photosynthesis, evapotranspiration, and crop growth estimates.

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