Light quality beneath field‐grown maize, soybean and wheat canopies – red:far red variations

Abstract

Using artificial canopies, several authors have shown that horizontally propagated and overall propagated radiation beneath the canopy differ substantially in spectral distribution in the red (R) and far red (FR) wavelengths. Given the lack of information about light quality under real crop canopies, the R:FR ratio of vertical and horizontal radiation beneath field‐grown maize, soybean and wheat was monitored until leaf area index (LAI) reached 4, 2.5 and 6.9, respectively. A Li‐Cor 1800 spectroradiometer with a remote cosine receptor fitted with a quartz fibre‐optic light‐guide was used. To isolate radiation coming from a given direction, a black coated tube was fitted to the cosine receptor. The viewing angle was 15°. In open conditions, the values of R:FR from the upper hemisphere were between 1.07 and 1.20. For vertically and horizontally‐propagated light, average values were 1.22 and 0.75 respectively. Beneath the canopy, both R:FR and photosynthetic photon flux density (PPFD) from the entire upper hemisphere decreased in relation to LAI and crop height. R:FR of the horizontal component were found to be generally much lower than the vertical, which decreased significantly only in the later measurements. The lowest R:FR values were recorded under wheat and soybean canopies. Even the very low LAIs present at early development stages were enough to cause a sharp decrease of R:FR in the horizontal fluxes. Referring to the entire upper hemisphere, PPFD transmittance and R:FR as a percentage of the external references appeared well correlated.

Simulation of the three‐dimensional distribution of the red:far‐red ratio within crop canopies

“…Thorough evaluation of the model for the strong sensitivity of to sky conditions would have required a precise characterization of sky conditions during measurements. However, measurements from the Wageningen 2004 experiment as well as data from Sattin et al (1994) and Sparkes et al (2006) did fall within the range of the simulated ratios. Moreover, simulation and measurements agreed closely in the cases where LAI > 5, that is when sky conditions had limited impact on ζ.…”

“…This explanation is supported by the fact that ζ ref was higher for the blue part of clear skies (1.7) than for the sun and overcast skies. Sattin et al (1994) also found that ζ ref was higher when measured with a 15° FOV than with a hemispherical FOV. The difference in ζ ref among clear skies, overcast skies, and sun also influenced the downward ζ inside the canopy when the degree of scattering was low, i.e.…”

“…ζ* decreased slowly until LAI was 2; then it decreased rapidly until LAI was 8, at which point there was a slow decrease from a LAI of 8 onwards. Measured ζ* was more variable than experimental data (from Sattin et al, 1994) and simulated data. This was probably a consequence of the fact that the normalization procedure could not compensate for variations in ζ ref between the times when measurements were taken above and inside the canopy.…”

Long‐term Low Radiation Decreases Leaf Photosynthesis, Photochemical Efficiency and Grain Yield in Winter Wheat

“…It should be noted that, in natural shading scenarios, light spectrum differs the non-shading (Sattin et al 1994). The variations in spectral quality also affect crop physiological processes, growth and yield (Causin et al 2006).…”

Survey of Plant Density Tolerance in U.S. Maize Germplasm

“…Light interception through the canopy is key to maintaining high photosynthetic capacity in plants as increased penetration of light through the canopy is responsible for increased dry matter production on whole (Sinclair, 1998) and assimilate partitioning to the ear (Hammer et al, 2009), contributing toward higher grain yields (Dwyer et al, 1991;Stewart et al, 2003). Competition for light can occur before canopy shading begins from neighboring plants (Sattin et al, 1994). As plants are increasingly crowded at high plant density, the canopy begins to self-shade, which results in decreased light penetration into the canopy and leads to decreased grain yield (Duncan, 1971;Hammer et al, 2009;Stewart et al, 2003).…”