Variation in photosynthetic photon flux density within a tropical seasonal rain forest of Xishuangbanna, south-western China

The effects of canopy development, solar angle, and weather conditions on temporal variation in photosynthetic photon flux density（PPFD） at three heights within a tropical rain forest canopy in Xishuangbanna, China, were examined. PPFD was measured every second and stored as 10-min averages from 1 December 2002 to 30 November 2003. PPFD variability was examined at three different temporal scales. Specific days in March, September, and December with clear and overcast sky conditions were selected to separate the effects of leaf area index（LAI） and solar angle on diurnal variability. On both clear and overcast days, mean daily average PPFD was significantly different between March and September at all heights, except 10 m on clear days, suggesting that LAI directly influences PPFD. In contrast, the differences in daily average PPFD among three heights between September and December were likely due to variation in solar angle. In addition, daily average PPFD at all locations were significantly lower under overcast than clear sky conditions in March, September and December. Over the year-long study, the mean daily total PPFD at 2! m, 10 m and 4 m was 2.8, 2.7 and 0.7 mol/（m^2·d）, which accounted for 9.7%, 9.4% and 2.4% of the daily PPFD above the canopy, respectively. Significant differences in mean daily total PPFD occurred at the same heights among different seasons, and diurnal, day-to-day and seasonal PPFD varied at different heights within the canopy. The possible effects of light variability on physiological and morphological responses of plants are discussed.

Single-turnover and multiple-turnover measurement of phytoplankton photosynthesis parameters using variable light pulse induced fluorescence

Using a measurement system based on fluorescence induced by variable pulse light, photosynthesis parameters of chlorella pyrenoidosa are obtained, employing single-turnover and multiple-turnover protocols under darkadapted and light-adapted conditions. Under the light-adapted condition,σ’PSII is larger, and F’v/F’m（ST） and F’v/F’m（MT） are smaller than those of the dark-adapted condition, but the corresponding parameters possess good linear correlations.Fm（MT）, F’m（MT）, Fv/Fm（MT） and F’v/F’m（MT） which are measured using the multipleturnover protocol, are larger than those of the single-turnover protocol. The linear correlation coefficient between Fm（ST） and Fm（MT） is 0.984,and Fv/Fm（MT） = 1.18 Fv/Fm（ST） The linear correlation coefficient between F’m（ST） and F’m（MT） is 0.995, and F’v/F’m（MT） = 1.36 F’v/F’m/（ST）.