Chlorophyll fluorescence, especially the optimum quantum yield Fv/Fm, is used to quantify the physiological state of plants, because stress, infections or wilting may lower the optimum quantum yield. The characteristics of chlorophyll fluorescence have attracted our attention for its use to determine the quality of spinach, because the measurement of Fv/ Fm during storage can be performed quickly and non-destructively, delivering information about spinach quality and its sale suitability. Applicability of modern techniques for quick and non-invasive in vivo determination of such parameters as quality and freshness of spinach under simulated sale conditions was tested in our study. In this experiment, application of chlorophyll fluorescence for quantification of quality as well as physical and physiological changes which occur in fresh products was verified. The experiment was conducted on spinach, a product characterized by rapid appearance of postharvest losses. Spinach was harvested as leafy vegetable for direct consumption and stored for 5 days (16 h in a cooling room at 2-4°C, 96-98% relative humidity, followed by 8 h at room temperature: ca 22oC, 65% relative humidity). Variants were represented by different cultivation conditions (fertilizer, leaf application of Previcur, CaCl2 and Cerone). The investigations revealed that fresh mass losses in the postharvest phase could be described by changes of Fv/Fm spinach leaves adapted to darkness by a linear relationship of parameters (r=0.77). The disadvantage of chlorophyll fluorescence measurement was the less precise determination of the critical loss of fresh mass of 3%. Instead, losses of 4 to 5% resulted in a the decrease of optimum quantum yield below the critical value of 0.80, which indicates the occurrence of postharvest stress. Leaf application of pesticides negatively influenced Fv/Fm. This relationship may be used to detect the application of plant protection products shortly before harvest, before the grace period is over. Hence, the determination of freshness using chlorophyll fluorescence offers, to a limited extent, protection of consumers. Further investigations showed that optimum quantum yield was linearly and positively correlated with the calcium content in spinach leaves. This relationship may indicate worse cell membrane integrity and reduced cell wall stability postharvest. Additionally, the decrease of Fv/Fm during storage may indicate the occurrence of cold stress as well as altering processes (senescence) and loss of quality of spinach, independent of its cultivation method. In these investigations, the relationship between chlorophyll fluorescence and color changes of leaves, content of chlorophyll, total nitrogen and water soluble carbohydrates, as well as respiration intensity and ethylene emission could not be found.
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