DYNAMICS OF PHOTOSYNTHETIC PHOTON FLUX DENSITY AND LIGHT EXTINCTION COEFFICIENT TO ASSESS RADIANT ENERGY INTERACTIONS FOR MAIZE CANOPY
Photosynthetic photon flux density (PPFD) is a measure of the rate of radiant energy per unit leaf area. Its quantification using the light extinction coefficient (K), which describes the transmissivity of light through the canopy, has been critical in predicting light interception above and beneath the canopy and the use of light interception in canopy radiation physics and plant productivity research. We measured incoming shortwave radiation (Rs), net radiation (Rn), radiation intercepted above (Rt) and beneath (Rtu) the canopy, and leaf area index (L) for a non-stressed maize canopy during partial and complete canopy periods to: (1) assess the radiation regimes and relationships between PPFD (sum of Rt and Rtu), Rt, Rtu, Rs, and Rn; (2) quantify the performance of Beer's law for estimating Rtu; (3) determine the diurnal and seasonal attenuation and augmentation of Bouguer-Lambert law-estimated variable daily maximum (Kmax) and average (Kavg) K values and compare the results with using a fixed K value; and (4) develop a relationship between Kavg and L for a non-stressed maize canopy during partial and complete canopy. The percentages of all radiation components (PPFD, Rt, Rtu, and Rn) 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 3.5, the percentages had fallen to 5.2, 26.5, and 32.1 for Rtu, Rt, and PPFD, respectively, and remained the same for Rn. Rs alone explained 93% of the variability in PPFD (PPFD = 0.1827Rs1.0969) for conditions when 1.2 < L < 5.30. A strong correlation was observed between Rs and Rt, and Rs explained 94% of the variability in Rt. The correlation between the Rs and Rtu was poor (r2 = 0.28) due to diffusion of the light beneath the canopy. The Beer's law Rtu estimates were poorly correlated with the data, with scatter increasing at higher Rtu values. Beer's law underestimated Rtu 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 Kmax varied from near zero to as high as 1.8 with a seasonal average of 0.73. Kavg ranged from 0.12 to 1.14 with a seasonal average of 0.44. Diurnal fluctuations and seasonal attenuation in Kavg were influenced by solar zenith angle (T). We made an attempt to quantify the effect of T on K and presented the results. Finally, we derived a variable Kavg 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 Kavg 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.