Effects of periodic fluctuations of photon flux density on anatomical and photosynthetic characteristics of soybean leaves

The development of soybean leaves grown at fluctuating photon flux density between 100 and 1500M m^-2s^-1 with a period of 160 sec were compared to leaves developed under continuous light with the same mean photon flux density. Number of epidermal cells and stomata, leaf area and specific leaf weight were not affected by the periodic fluctuation of photon flux density. Chloroplastic pigment concentration and chlorophyll fluorescence reveal some photoinhibitory effects of the high photon flux density phase. Stomatal and internal CO₂ conductance and the quantum yield were not affected by the light regime. In contrast ribulose 1.5 bisphosphate carboxylase/oxygenase activity before in vitro activation by CO₂ and Mg⁺⁺ was stimulated by the periodic illumination whereas the total amount of the enzyme and the internal leaf CO₂ conductance remained steady. In conclusion, there was no major difference between leaves of plant grown either under a steady or under a periodic fluctuation of the photon flux density except some photoinhibitory symptoms under fluctuating illumination, and a higher in vivo level of activation of the Rubisco.