Dynamics of Photosynthetic Photon Flux Density and Light Extinction Coefficient to Assess Radiant Energy Interactions for Maize Canopy

摘要

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 (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 radiation regimes and relationships between 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 (K max ) and average (K avg ) K values 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 (PPFD, R t , R tu , and R n ) relative to R s 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 amount of 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 ) for conditions 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 ( T ). We made an attempt to quantify the effect of T on K and presented 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.