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Crop Science Abstract -

The Solar Tracking Pattern in a Closed Alfalfa Canopy1


This article in CS

  1. Vol. 23 No. 4, p. 664-668
    Received: Sept 7, 1982

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  1. R. L. Travis and
  2. R. Reed2



The average cosine of the solar incidence angle for all leaflets within a closed alfalfa canopy [leaf area index (LAI) = 4 to 5] was relatively high during the morning hours, decreased to a minimum level during the early afternoon, and increased again during late afternoon hours. High values for cosine of the solar incidence angle indicate the leaflets were exposed near normal to the sun's rays. This resulted from diaheliotropic leaflet movements. The lower, midday values were associated with leaf cupping movements. Leaf cupping began about 3 h after the beginning of the midday decline in xylem water potential but the recovery from cupping, as evidenced by the return of diaheliotropic movements, occurred in the absence of the recovery of xylem water potential. An evaluation of tracking ability revealed an increasing average cosine of incidence angle with increasing leaflet age resulting in a canopy architecture which would enhance light interception by the lower strata. Light flux density measurements indicated that most leaflets in the upper canopy were exposed to sufficient photosynthetic photon flux density (PPFD) to saturate nearly saturate the photosynthetic process. However, PPFD available to leaflets in the lower canopy ranged from 45% required for saturation of photosynthesis for the average nontracking leaflet to about 85% for leaflets with a cosine of incidence of 1.0. Since most leaflets in the lower strata maintained relatively high cosine of incidence angle values, the results suggest that the observed tracking gradient (increasing ability with leaflet age) may provide the alfalfa plant with a mechanism for maintaining photosynthetic productivity in lower leaves. The data are discussed in terms of the potential carbon gain benefit to the alfalfa plant resulting from the tracking gradient within the canopy.

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