Analysis of chlorophyll fluorescence transients in mandarin leaves during a photo-oxidative cold shock and recovery

In temperate climates, plants are more frequently exposed to sudden and strong decrease of temperature combined with high light intensity that compromise the photosynthetic efficiency and, often, the development and their survival. To investigate deeper the effects of such a photo-oxidative cold shock on photosynthesis of mandarin (Citrus reticulata Blanco) we used transient fluorescence analysis and gas exchange techniques. Mandarin plants were acclimated at 13 °C or grown at 25 °C for 30 days, then single leaves were exposed for 6 h at 5 °C and 1200 μmol photons m−2 s−1 and again exposed for 72 h under growth conditions. Acclimated leaves showed lower photosynthesis and higher total carotenoid contents than non-acclimated leaves. The photo-oxidative cold shock inhibited photosystem II (PSII) efficiency, as suggested by the reduction of Fv/Fm ratios, in all leaves. Carotenoid content was reduced in non-acclimated leaves. A time-dependent increase in the initial fluorescence level (F0) and a faster decline in Fv/Fm ratio indicated a stronger PSII inhibition in non-acclimated leaves in comparison with acclimated leaves. The chlorophyll a fluorescence kinetic showed in all leaves complex changes in the O-J, J-I and I-P phases. The I and P steps of the fluorescence rise were particularly affected in non-acclimated leaves. Photosynthesis, fluorescence and pigment contents, measured in leaves maintained for 2 h in the dark and then returned for 72 h to growth conditions, showed a different pattern of recovery between leaves. Photosynthesis and fluorescence completely recovered in acclimated leaves after 72 h under growth conditions. On the contrary, non-acclimated leaves still showed very low values of photosynthesis and Fv/Fm, suppression of the sigmoidal shape of fluorescent transients and further reduction of chlorophyll contents, indicating the presence of irreversible damages to PSII and, probably, an anticipated senescence. It is suggested that carotenoids have a relevant role in protecting leaves of mandarin from the effects of a photo-oxidative cold shock.