Chlorophyll fluorescence characteristics and adaptability differences of two ephemeral plants under shaded conditions

In order to explore the photosynthetic adaptability of ephemeral plants, this paper took Malcolmia scorpioides and Arnebia decumbens as the experimental objects. The daily changes in chlorophyll fluorescence and the changes in chlorophyll content and biomass of two ephemeral plants were studied under four time gradients (0, 1, 7, 14 d) with a shading rate of 75%. The results showed that: 1) At 0 d, the maximum photochemical efficiency (F_v/F_m) and the actual photosynthetic efficiency Y (Ⅰ) and Y (Ⅱ) of the two ephemeral plants decreased significantly at mid-afternoon (14:00), showing a phenomenon of “midday photoinhibition”. 2) After being shaded for 1 and 7 d, the diurnal variation of photosynthetic efficiency of the two ephemeral plants showed a “single peak” curve. This increased to the peak value at 14:00, and the quantum yield of cyclic electron flow Y (CEF) increased, and the midday photoinhibition was improved. At 14 d, the quantum yield caused by PSⅠ acceptor-side limitation Y (NA) and the quantum yield of non-regulated energy dissipation in PSⅡ Y (NO) increased, the photosynthetic system was damaged, and the photosynthetic activity decreased. 3) Compared with the unshaded environment, at 14:00, the photosynthetic activity of M. scorpioides decreased by 95.3% and the degree of light damage increased by 95% under shade. However, the photosynthetic activity of A. decumbens increased by 4% and heat dissipation efficiency decreased by 18.6%. 4) After 14 d of shading, the chlorophyll content and biomass of the two ephemeral plants decreased, and the decline rate of M. scorpioides was higher than that of A. decumbens. In conclusion, the shade tolerance of A. decumbens is higher than that of M. scorpioides. Shading within 14 d can improve the light protection ability of A. decumbens and improve the light inhibition. Meanwhile, the improvement effect of M. scorpioides was only within 7 d, and the photosynthetic utilization rate decreased after 7 d.