Spatial heterogeneity and diurnal course of photon flux density on paddy field water surface under rice plant canopy

Abstract

Detailed measurements of photon flux density (PFD) on the paddy field water surface were conducted under sunny sky conditions using a photosensor array of 100 small photodiodes under rice canopies at the heading and ripening stages. PFD on paddy water surface was not uniform and spatially variable even within a short distance of a few centimeters, showing that the irregular penetration of direct sunlight significantly affects the light environment under the canopies. Although occasional high peaks of PFD were observed, most of the water surface was exposed to a PFD of less than 200 µmol m−2 s−1, even around midday. Mean PFD were high at midday and low in early morning or late evening. Mean ratios of the PFD under the canopy to that outside the canopy (RPFD) were also high at midday and low in early morning or late evening, which can be explained by Poisson's model of light penetration. Maximum mean PFD and mean RPFD were often observed not around noon, but when the azimuth angle of the sun coincided with the row direction. Moreover, under rice canopies at the heading stage, rapid increases and decreases of mean RPFD were observed immediately before and after the peak time. These results indicate that significant high photosynthetic carbon fixation by the paddy field weeds can take place only during the few hours when the azimuth angle of the sun coincides with the row direction.

Grapevine row orientation affects light environment, growth, and development of black nightshade (Solanum nigrum)

“…At those times, PAR at the WCZ in NS rows increased rapidly from about 35 mol m Ϫ2 s Ϫ1 , in both years, to peaks of about 500 mol m Ϫ2 s Ϫ1 , in 2003, and 700 mol m Ϫ2 s Ϫ1 , in 2004 (about 30 to 40% of full sun; data not shown). Rapid increases and decreases of irradiance, similar in magnitude and corresponding to the time of day when the row orientation allowed direct penetration of sunlight, have also been observed under the canopy of rice (Oryza sativa L.) growing in rows oriented north-east to south-west (Nishimura and Itoh 2003).…”

Geometrical similarity analysis of photosynthetic light response curves, light saturation and light use efficiency

“…Small photodiodes (G1118; Hamamatsu Photonics, Hamamatsu, Japan) were used to estimate PPFD on the leaves (Nishimura et al 1998;Nishimura and Itoh 2003). Each photodiode had been calibrated against a quantum sensor (IKS-27; Koito, Yokohama, Japan).…”