Solar radiation in crop canopies.
Total solar, near infra-red, and photosynthetically active radiation were measured by tube solarimeters at Rothamsted in barley, sugar beet, field beans, kale and spring wheat. The measurements were used to obtain the sunfleck parameters (s) [see FCA 24, 2814(5)] and the general extinction coefficient of Kasanaga & Monsi (K'). Mean integral leaf transmission coefficients needed in the analyses were calculated from published transmission spectra. Crop transmission parameters calculated from the daily radiation totals changed little when crop cover was full, and the average sunfleck and extinction values, respectively, for total solar radiation were 0.60 and 0.48 for barley, 0.48 and 0.63 for sugar beet, 0.64-0.68 and 0.38 for field beans, 0.42-0.48 and 0.70 for kale, and 0.55-0.65 and 0.40 for wheat. In barley s changed slightly with depth in the canopy, but the change was often difficult to detect because of scatter in the experimental points. On cloudless days s changed somewhat in all crops when solar elevation, beta , was less than about 20 deg , but in the dense foliage of kale s was less sensitive to beta and showed no detectable change down to about 10 deg . Within the canopy, the photosynthetically active fraction of total solar radiation, eta L, changed rapidly; the shape of the curve describing the change with increasing LAI(L) was slightly different for the 3 main crops examined in detail, but was such that eta L was about half eta 0 at L = 5 for beans, 3.5 for kale and 4 for wheat.