| Abstract: | Histograms of the number of hours in the day in which the photosynthetic photon flux density (PPFD) of solar radiation was within 5 preset bands, were constructed from readings made with a miniature battery-operated millivolt histogram recorder, connected to a photon sensor. The number of histograms showing a bimodal distribution was much fewer than was expected from a review of the literature, even in a site shaded by trees. This was attributed to smoothing of the data over a whole day, compared with published data which were mostly taken near noon. Bimodal distributions were found in a greenhouse under clear or partly cloudy conditions.The histograms were combined with published data for the instantaneous gross photosynthetic responses to PPFD of two bermudagrass swards, one of low leaf area index (LAI) and highly non-linear response, and the other of high LAI and more linear response. The daily photosynthesis totals of the swards were calculated by summing the products of the number of hours accumulated in each band by the photosynthetic rate in the center of the band.Because of the non-linearity of response, the photosynthesis total in the greenhouse was 75–82% of that calculated for an open site on a clear day, while the daily photon total was only 70% of the open site value. Also, for the same daily photon total, the daily photosynthesis total of each sward on the open site was greater on a day of light clouds (moderate PPFD) than on a day of sunshine alternating with heavy clouds (bimodal histogram of PPFDs). The difference between the two days was greater for the sward with the more non-linear response. These interactions between non-linearity of photosynthetic response and type of radiation histogram were in the expected direction, however, the differences were not very large (+ 10% for the sites and responses studied).The histogram recorder provides an inexpensive and convenient means of obtaining solar radiation data that can be used to calculate daily photosynthesis totals, either from experimental photosynthesis data or from canopy photosynthesis models. Because the response of photosynthesis is non-linear, linear integrals of solar irradiance do not provide sufficient information for such photosynthesis studies. |
