At the whole plant level, the effect of stress is usually perceived as a decrease in photosynthesis and growth. That is why this review is focused mainly on the effect of drought on photosynthesis,...

Phenylamides, secondary metabolites, conjugates of aliphatic polyamines or arylmonoamines and hydroxycinnamic acids, combining the properties of both parent compounds, are a subject of increasing...

Blue light has many direct and indirect effects on photosynthesis. The impact of blue light on mesophyll conductance (g_m), one of the main diffusive limitation to photosynthesis, was investigated in leaves of Nicotiana tabacum and Platanus orientalis, characterized by high and low g_m, respectively. Leaves were exposed to blue light fractions between 0% and 80% of incident light intensity (300 μmol photons m⁻² s⁻¹), the other fraction being supplied as red light. Leaves exposed to blue light showed reduced photosynthesis and unaltered stomatal conductance. The g_m, measured using the chlorophyll fluorescence-based method, was strongly reduced in both plant species. Such a reduction of g_m may not be real, as several assumptions used for the calculation of g_m by fluorescence may not hold under blue light. To assess possible artefacts, the electron transport rate measured by fluorescence (J_f) and by gas-exchange (J_c) were compared in leaves exposed to different fractions of blue light under non-photorespiratory conditions. The two values were only equal, a prerequisite for correct g_m measurements, when the illumination was totally provided as red light. Under increasing blue light levels an increasing discrepancy was observed, which suggests that J_f was not correctly calculated, and that such an error could also upset g_m measurements. Blue light was not found to change the absorbance of light by leaves, whereas it slightly decreased the distribution of light to PSII. To equate J_f and J_c under blue light, a further factor must be added to the J_f equation, which possibly accounted for the reduced efficiency of energy transfer between the pigments predominantly absorbing blue light (the carotenoids) and the chlorophylls. This correction reduced by about 50% the effect of blue light on g_m. However, the residual reduction of g_m under blue light was real and significant, although it did not appear to limit the chloroplast CO₂ concentration and, consequently, photosynthesis. Reduction of g_m might be caused by chloroplast movement to avoid photodamage, in turn affecting the chloroplast surface exposed to intercellular spaces. However, g_m reduction occurred immediately after exposure to blue light and was complete after less than 3 min, whereas chloroplast relocation was expected to occur more slowly. In addition, fast g_m reduction was also observed after inhibiting chloroplast movement by cytochalasin. It is therefore concluded that g_m reduction under blue light is unlikely to be caused by chloroplast movement only, and must be elicited by other, as yet unknown, factors.

We used a localized ozone (O₃) fumigation (LOF) system to study acute and short-term O₃ effects on physiological leaf traits. The LOF system enabled investigation of primary and secondary metabolic responses of similarly and differently aged leaves on the same plant to three different O₃ concentrations ([O₃]), unconfounded by other influences on O₃ sensitivity, such as genetic, meteorological and soil factors. To simulate the diurnal cycle of O₃ formation, current-year and 1-year-old Quercus ilex (L.) and Quercus pubescens (L.) leaves were fumigated with O₃ at different positions (and hence, different leaf ages) on the same branch over three consecutive days. The LOF system supplied a high [O₃] (300 ± 50 ppb) on leaves appressed to the vents, and an intermediate, super-ambient [O₃] (varying between 120 and 280 ppb) on leaves less than 30 cm from the vent. Leaves more than 60 cm from the O₃ vent were exposed to an [O₃] comparable with the ambient concentration, with a 100 ppb peak during the hottest hours of the day. Only leaves exposed to the high [O₃] were affected by the 3-day treatment, confirming that Mediterranean oak are tolerant to ambient and super-ambient [O₃], but may be damaged by acute exposure to high [O₃]. Stomatal and mesophyll conductance and photosynthesis were all reduced immediately after fumigation with high [O₃], but recovered to control values within 72 h. Both the intercellular and chloroplast CO₂ concentrations ([CO₂]) remained constant throughout the experiment. Thus, although treatment with a high [O₃] may have induced stomatal closure and consequent down-regulation of photosynthesis, we found no evidence that photosynthesis was limited by low [CO₂] at the site of fixation. One-year-old leaves of Q. ilex were much less sensitive to O₃ than current-year leaves, suggesting that the low stomatal conductance observed in aging leaves limited O₃ uptake. No similar effect of leaf age was found in Q. pubescens. Dark respiration decreased during the treatment period, but a similar decrease was observed in leaves exposed to low [O₃], and therefore may not be an effect of O₃ treatment. Light respiration, on the other hand, was mostly constant in ozone-treated leaves and increased only in leaves in which photosynthesis was temporarily inhibited by high [O₃], preventing them from acting as strong sinks that recycle respiratory CO₂ in the leaves. There was no evidence of photochemical damage in Q. ilex leaves, whereas Q. pubescens leaves exposed to a high [O₃] showed limited photochemical damage, but recovered rapidly. Biochemical markers were affected by the high [O₃], indicating accumulation of reactive oxygen species (ROS) and increased denaturation of lipid membranes, followed by activation of isoprene biosynthesis in Q. pubescens leaves. We speculate that the high isoprene emissions helped quench ROS and normalize membrane stability in leaves recovering from O₃ stress.

Effects of high-temperature stress (HTS) and PEG-induced water stress (WS), applied separately or in combination, on the functional activity and ultrastructure of the photosynthetic apparatus (PSA)...