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In this doctoral thesis, the effects of moderately reduced water supply and seasonal variations was studied regarding the chemical and nutritionally relevant composition of the edible harvest organs of three different vegetable crops, i.e. cucumber fruit (Cucumis sativus L.), radish roots (Raphanus sativus L. var. sativus), and spinach leaves (Spinacia oleracea L.). The study was conducted to demonstrate putative plant-physiological alterations in their chemical composition that may be evoked by moderate water deficits during cultivation under open field conditions. The vegetable crops were selected according to differing harvest organs (leaves, roots and fruit), being cultivated during three consecutive years under open field conditions and two different water supply treatments within a randomized block design with multiple sets of six plots each. Depending on the vegetable, the provided water amount of the moderately reduced water supply treatment varied between 80-97% of the well-watered control treatment. Chapter 1 dealt with the effects of moderate water reduction of 3-20% on radish. Including one cultivation set per year and an additional set in 2016, i.e. a total of four separate sets, a dry matter-based increase of myo-inositol was observed, whereas the levels of selected minerals and trace elements like iron and manganese as well as those of phenolic compounds and glucosinolates decreased or remained unchanged upon moderately reduced water supply. Only insignificant effects of reduced water supply were seen on the levels of acylated anthocyanins, which represent a highly important commercial quality trait due to their importance for the roots’ or hypocotyls’ color. In contrast, the levels of fresh matter-related analytes were mainly enhanced by reduced water supply due to higher contents of dry matter. Thus, the overall quality of radish tubers seemed to be only marginally affected by moderate water limitation.
Similar results were observed for spinach as presented in Chapter 2. Nevertheless, reducing the total water amount by only 4-10% during cultivation led to various significant alterations in the chemical composition of the leaves. Similar to radish, higher dry matter-based levels of myo-inositol were found in spinach grown under low water reduction. Moreover, increased contents of zinc and manganese were found, while the levels of malic acid, fumaric acid, phosphate and chloride decreased upon slight water reduction in spinach dry matter. In contrast, spinach fresh matter was expectedly characterized by significantly higher contents of polyols, ascorbic acid, potassium, nitrogen, phosphorous, zinc and manganese, as well as total flavonoids and carotenoids, including nutritionally highly relevant compounds like the provitamin A-active β-carotene and the macular pigment lutein. Noteworthy, strong seasonal variations have been observed for numerous minor phytochemicals, such as for example for a series of specific flavonoids, whose contents seemed to be dependent on the respective irradiation conditions of the cultivation set.
In contrast to radish and spinach, cucumber from open field cultivation reacted differently to slightly reduced water supply, as summarized in Chapter 3. When cucumber fruits obtained from plants received 85-90% of the total water amount of the control treatment, they exhibited significantly higher levels of malic acid, calcium, and magnesium, while those of phosphate, phosphorous, nitrogen, and iron decreased based on dry matter. Fruit dry matter-content did not significantly change due to moderate water restriction, and, as a result, the concentration of most analytes tended to decrease in the cucumber fresh matter when grown under reduced water supply. The chemical analyses of single cucumber fruit compartments (exocarp, mesocarp and endocarp) from fruits grown under 60% of the full supply in a separate greenhouse experiment confirmed that malic acid, certain minerals and trace elements, as well as carotenoids like the most abundant lutein were relatively sensitive to moderate water reduction.
Altogether, a moderately reduced water supply was shown to significantly alter the chemical composition of the aforementioned vegetable crops. Considering the broad range of analytes from, e.g., minerals and anions to polyols and acids to micronutrients like provitamin A, it became clear that water supply reduction as studied herein did not lead to generalizable effects such as an undesired decrease or a desired enhancement of valuable compounds. For instance, the levels of selected health-promoting constituents in the fresh matter of radish and spinach were increased, while those of the same compounds contained in cucumber did not benefit at all. Generally, the overall quality of vegetables was clearly shown to be influenced not only by water supply, but also by the interaction with other abiotic factors, especially when cultivation took place in open field. Further study in the future should target interactions of moderately reduced water supply situations with changes in other abiotic factors, such as elevated CO2 concentrations.