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High hydrostatic pressure has the potential to affect food-related enzymes and microorganisms while retaining the produce's characteristic properties. Although many studies on effects of high pressure on quality attributes of fruit and vegetables have been published, experimental results on the impact of high-pressure treatment on the physiological activity of products are rare. To characterize changes of the samples fast and noninvasive methods as well as a sensitive biological model system are required for this purpose. In this study, fresh lamb's lettuce (Valerianella olitoria Poll.) was used as a model produce. For each treatment, two leaves were carefully inserted to small plastic pouches, sealed and then subjected to pressure (up to 7.5 min at 200 MPa) or thermal treatment (up to 1 min at 50°C). Chlorophyll fluorescence imaging was applied to measure the local and temporal dynamics of the physiological postprocessing effects. Measurements of the maximum photochemical efficiency F v /F m allows the immediate evaluation of photosynthetic activity as an indicator for cell and tissue vitality. Thermal treatment at pressure below 125 MPa and temperatures lower than 45°C showed minor fully reversible effects but a pronounced decline in the maximum photochemical efficiency was obtained after pressure treatment of 150 MPa and temperatures of 45°C or higher values. These changes were irreversible within 24 h of recovery time. Above these thresholds, high pressure and heat treatment may not be applicable for mild processing of highly perishable fresh produce. Chlorophyll fluorescence analysis has been proven to be a valuable tool for the rapid and comprehensive evaluation of postharvest processing of green perishables.

The small-scale distribution pattern of macroalgae in the river Ilm, in Germany was monitored. These patterns were then related to abiotic factors and tested to discover whether the distribution of the common macroalgae, Cladophora glomerata (L.) Kütz. and Vaucheria sp., was linked to differences in their photosynthetic plasticity. Cladophora glomerata revealed higher maximum photosynthetic electron transport rates after acclimation to high light (HL) compared with low light (LL) acclimated samples. By contrast, Vaucheria sp. did not acclimate to different growth light conditions. The photosynthetic performance of both algae also varied according to diurnal conditions. High light caused a reversible decrease of the dark-adapted quantum yield (F(v)/F(m)) in C. glomerata and a concomitant reversible decrease of the light-adapted quantum yield (DeltaF/F'(m)). In Vaucheria sp., F(v)/F(m) remained mostly unchanged over the day, whereas DeltaF/F'(m) decreased during the morning at low light. Photosynthetic pigments confirmed acclimational differences between the species. HL C. glomerata showed increased chlorophyll a:chlorophyll b ratios, and higher amounts of xanthophyll-cycle pigments compared with LL samples, whereas Vaucheria sp. did not reveal differences between the light treatments. While preferences for substrate size, water velocity, and depth are similar for C. glomerata and Vaucheria sp., the physiological responses to light conditions are different. It is concluded that light conditions significantly affect the small-scale spatial distribution of macroalgae and that fitness is enhanced in species with a higher plasticity in photosynthetic acclimation in unstable environments.

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