Active and passive chlorophyll fuorescence measurements at canopy level on potato crops. Evidence of similitude of diurnal cycles of apparent fuorescence yields

Abstract

We performed active and passive measurements of diurnal cycles of chlorophyll fuorescence on potato crops at canopy level in outdoors conditions for 26 days. Active measurements of the stationary fuorescence yield (Fs) were performed using Ledfex, a fuorescence micro-LIDAR described in Moya et al. (Photosynth Res 142:1–15, 2019), capable of remote measurements of chlorophyll fuorescence under full sun-light in the wavelength range from 650 to 800 nm. Passive measurements of solar-induced fuorescence (SIF) fuxes were performed with Spectrofex, an instrument based on the method of flling-in in the O2A and O2B absorption bands at 760 nm (F760) and 687 nm (F687), respectively. Diurnal cycles of Fs showed signifcant variations throughout the day, directly attributed to changes in photosystem II yield. Contrasting patterns were observed according to illumination conditions. Under cloudy sky, Fs varied in parallel with photosynthetically active radiation (PAR). By contrast, during clear sky days, the diurnal cycle of Fs showed a “M” shape pattern with a minimum around noon. F687 and F760 showed diferent patterns, according to illumination conditions. Under low irradiance associated with cloudy conditions, F687 and F760 followed similar diurnal patterns, in parallel with PAR. Under high irradiance associated with clear sky we observed an increase of the F760/F687 ratio, which we attributed to the contributions in the 760 nm emission of photosystem I fuorescence from deeper layers of the leaves, on one end, and by the decrease of 687 nm emission as a result of red fuorescence re-absorption, on the other end. We defned an approach to derive a proxy of fuorescence yield (FYSIF) from SIF measurements as a linear combination of F687 and F760 normalized by vegetation radiance, where the coefcients of the linear combination were derived from the spectral transmittance of Ledfex. We demonstrated a close relationship between diurnal cycles of FYSIF and Fs, which outperformed other approaches based on normalization by incident light.