Dynamics of photosynthetic photon flux density and light extinction coefficient to assess radiant energy interactions for maize canopy

We measured incoming shortwave radiation (R _s), net radiation (R _n), radiation intercepted above (R _t) and beneath (R _tu) the canopy, and leaf area index (L) for a non-stressed maize canopy during partial and complete canopy periods to: (1) assess the relationships between photosynthetic photon flux density (PPFD; sum of R _t and R _tu), R _t, R _tu, R _s, and R _n; (2) quantify the performance of Beer's law for estimating R _tu; (3) determine the diurnal and seasonal attenuation and augmentation of Bouguer-Lambert law-estimated variable daily maximum and average light extinction coefficient (K _max and K _avg, respectively) and compare the results with using a fixed K value; and (4) develop a relationship between K _avg and L for a non-stressed maize canopy during partial and complete canopy. The percentages of all radiation components relative to Rs were highest early in the season before the full canopy and gradually decreased as L increased. Early in the season, when L < 2.0, the PPFD was as high as 43% of R _s. PPFD decreased to 31% at 64 days after planting (DAP), when L = 4.4, and stayed relatively constant until 98 DAP (L = 4.9). Similar trends were observed for R _t and R _tu with lower magnitudes. When L < 3.5, the average percentages of R _s for R _tu, R _t, PPFD, and R _n were 8.4, 29.3, 38.0, 29.2, respectively. By midsummer, when L > 3.5, the percentages had fallen to 5.2, 26.5, and 32.1 for R _tu, R _t, and PPFD, respectively, and remained the same for R _n. R _s alone explained 93% of the variability in PPFD (PPFD = 0.1827R _{s 1.0969}) when 1.2 < L < 5.30. A strong correlation was observed between R _s and R _t, and R _s explained 94% of the variability in R _t. The correlation between the R _s and R _tu was poor (r ² = 0.28) due to diffusion of the light beneath the canopy. The Beer's law R _tu estimates were poorly correlated with the data, with scatter increasing at higher R _tu values. Beer's law underestimated R _tu in the range of 10 to 40 W m ^-2 and overestimated for values greater than 40 W m ^-2 (due to using a constant K) with an overall root mean square difference (RMSD) of 11.3 W m ^-2. We showed that K not only changed during the season but also fluctuated significantly within a day due to change in the sun angle and other factors. Daily K _max varied from near zero to as high as 1.8 with a seasonal average of 0.73. K _avg ranged from 0.12 to 1.14 with a seasonal average of 0.44. Diurnal fluctuations and seasonal attenuation in K _avg were influenced by solar zenith angle (Θ). We attempted to quantify the effect of Θ on K and present the results. Finally, we derived a variable K _avg equation as a function of L. There was a logarithmic and very strong dependence between the transmissivity of light through the canopy and L akin to the original logarithmic decay function of Beer's law. The derived function (K = -0.439·ln(L) + 1.016) accounted for 76% of the variability in K _avg using L alone. The model represents conditions when 1.2 > L > 5.30 for non-stressed maize canopy, and extrapolating it beyond these boundaries may not provide particularly accurate estimates of K.