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

摘要

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 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 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_s1.0969) 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 (r2=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 ( Theta ). We attempted to quantify the effect of Theta 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.