Photosynthetic properties of spring geophytes assessed by chlorophyll fluorescence analysis

Since spring ephemerals are credited to be all "sun" species with unusually elevate photosynthesis, in contrast to shade-tolerant trees and understory geophytes with a long aboveground cycle, we examined the photosynthetic efficiency of 6 woody species, 9 long-cycle geophytes, and 8 spring ephemeral geophytes using blue flashes of increasing energy with the Imaging PAM fluorometer. Several parameters were obtained: quantum yield of electron transport (ΦETR) or of PSII (ΦPSII), maximum measured photosynthesis rate (ETR_hv), maximum extrapolated rate of photosynthesis (ETR_em), half-saturating photon flux density (K_PAR), and in some cases photochemical (qP) and non-photochemical quenching (NPQ). Results confirm the ecological consistency of the three plant groups, with internal differences. Woody species have low ETR_em and K_PAR values with good ΦETR; long-cycle herbs have low ETR_em and ΦETR and moderate K_PAR values; spring ephemerals have elevate ΦETR, ETR_em and K_PAR values. The mean ETR_em of ephemerals of 91 μmol m^-2 s^-1 exceeds that of long-cycle herbs 2.9-fold and woody species 4.8-fold, and corresponds to 19 μmol CO₂ m^-2 s^-1 by assuming an ETR/ΦCO₂ ratio of 4.7. Highest photosynthesis rates and K_PAR were exhibited by five ephemerals (Eranthis, Erythronium, Narcissus, Scilla, Tulipa) with peak ETR_em values equivalent to ∼40 μmol CO₂ m^-2 s^-1 or ∼60 μmol CO₂ (g Chl)^-1 s^-1 ("sun" species). According to a new, fluorescence based heliophily index, all trees and five long-cycle herbs were definitely "shade" species, while four long-cycle herbs and three ephemerals were intermediate shade-tolerant.