Vicente I. Deltoro

Changes in chlorophyll a fluorescence, photosynthetic CO2 assimilation and xanthophyll cycle interconversions during dehydration in desiccation-tolerant and intolerant liverworts

Abstract. The interactions among water content, chlorophyll a fluorescence emission, xanthophyll interconversions and net photosynthesis were analyzed during dehydration in desiccation-tolerant Frullania dilatata (L.) Dum. and desiccation-intolerant Pellia endiviifolia (Dicks) Dum. Water loss led to a progressive suppression of photosynthetic carbon assimilation in both species. Their chlorophyll fluorescence characteristics at low water content were: low photosynthetic quantum conversion efficiency, high excitation pressure on photosystem II and strong non-photochemical quenching. However, dissipation activity was lower in P. endiviifolia and was not accompanied by a rise in the concentration of de-epoxidised xanthophylls as F. dilatata. The photosynthetic apparatus of F. dilatata remained fully and speedily recuperable after desiccation in as indicated by the restoration of chlorophyll fluorescence parameters to pre-desiccation values upon rehydration. A lack of recovery upon remoistening of P. endiviifolia indicated permanent and irreversible damage to photosystem II. The results suggest that F. dilatata possesses a desiccation-induced zeaxanthin-mediated photoprotective mechanism which might aid photosynthesis recovery when favourable conditions are restored by alleviating photoinhibitory damage during desiccation. This avoidance mechanism might have evolved as an adaptative response to repeated cycles of desiccation and rehydration that represent a real threat to photosynthetic viability.

Changes in net photosynthesis, chlorophyll fluorescence and xanthophyll cycle interconversions during freeze-thaw cycles in the Mediterranean moss Leucodon sciuroides

Abstract The tolerance to freezing and thawing of Leucodon sciuroides, a moss growing in mountainous areas of the Mediterranean (south-east Spain), was investigated by means of CO2 gas exchange, modulated chlorophyll (Chl) a fluorescence and pigment analysis by high-performance liquid chromatography. Evidence is presented for freezing-induced decreases in CO2 fixation that enhance non-radiative dissipation of absorbed light energy, a process which protects the photosynthetic apparatus. The photosynthetic apparatus of L. sciuroides remained fully recuperable after freezing, as indicated by the recovery of photosynthetic CO2 fixation and Chl fluorescence parameters to pre-freezing values during thawing. The rapid recovery of photosynthesis activity during thawing in L. sciuroides suggests that this moss is capable of tolerating freeze-thaw cycles in a manner similar to that found at higher latitudes or in the Antarctic. The resistance of the photosynthetic apparatus of this moss to freezing might be achieved, at least partially, through the employment of dissipative pathways, such as non-radiative dissipation of absorbed light energy.

Acclimation potential to high irradiance of two cultivars of watermelon.

The acclimation potential to high irradiance of two cultivars of watermelon, Reina de Corazones and Toro, calculated as the ratio of sun vs. shade activities of O2− and H2O2 scavenging enzymes and non-radiative energy dissipation, was similar. However, Reina de Corazones exhibited a higher capacity in absolute terms for photoprotection (harmless dissipation of absorbed light energy at PS 2 and ascorbate and O2− and H2O2 scavenging enzymes) suggesting a larger resistance of this cultivar to high irradiance. This could be seen as smaller decreases in fruit productivity and in lower oxidative injury as probed by malondialdehyde content in sun plants of Reina de Corazones than in Toro plants. Additionally, the results show that shading might be beneficial to both cultivars, presumably because it reduces the susceptibility of high irradiance-induced stress.