Abstract
Fluorine (F-) stands out for its phytotoxic potential, because it accumulates in plants, changes enzymes activity, reduces chlorophyll content and, consequently, affects growth and yield of crop plants. An experiment was conducted to evaluate the effects of F- on leaf gas exchange in coffee and sweet orange plants, compared to sensitive (gladiolus) and tolerant (ryegrass) reference species. Plants grown in pots were exposed to F- in a semi-open mist chamber. The experimental design was completely randomized with treatments defined by the combination of plant species and two intensities of exposure to atmospheric F, with nebulization of HF solutions (low = 0.065 mmol·m-3 and high = 0.260 mmol·m-3) in a mist chamber, as well as with non-exposed control samples. CO2 assimilation (A), transpiration (E), stomatal conductance (gs) and chlorophyll fluorescence rates were measured after 27 days of treatment application. The leaf gas exchange variables in ryegrass and orange plants did not vary in response to the increase in atmospheric F, while an increase in gs and E values was observed in gladiolus and coffee plants. A decrease in A and potential quantum efficiency of photosystem II (Fv/Fm) was found for gladiolus plants. On the contrary, an increase of A for coffee plants was associated with the apparent effect previously reported about the loss of leaf stomatal regulation related to the short assessment period of plants in this experiment. Damages caused to the photosynthetic system were reflected in the susceptibility of the evaluated species to the contamination by the element.
Highlights
Air pollutants can affect plants by multiple metabolic pathways and mechanisms, and impair the productivity of agricultural crops [1]
F < 0.01; CV = 2.4%), with values of Fv/Fm = 0.83 in the control plants, Fv/Fm = 0.82 in plants exposed to a low concentration of F (0.065 mmol·m−3) and Fv/Fm = 0.58 in those exposed to a high F concentration (0.260 mmol·m−3), which suggested the occurrence of photoinhibition [19] and possible damage to the photochemical system for this species
This later is possibly explained by the fact that excess F causes similar injuries to thylakoids membranes and consequent reduction of the electron transport chain between photossistems as observed in Spinacea oleracea plant treated with the DCMU photosynthesis inhibitor [20]
Summary
Air pollutants can affect plants by multiple metabolic pathways and mechanisms, and impair the productivity of agricultural crops [1]. The response of plants to pollution depends on the toxicity of the chemical element, the plant exposure and the species sensitivity [2] Among these pollutants, F− stands out because of its high phytotoxic potential, with absorption occurring preferentially through the stomata [1,3]. In Brazil, this problem began to be studied in 1990, due to the losses recorded on soybean production in the state of Minas Gerais [9] and in forest trees [10,11] located in areas adjacent to the Cubatão industrial hub-SP These studies expanded the knowledge on the Atlantic Forest biome [12], as well as on fruit crops and forages as bioindicators of environmental pollution [8,13]. Despite the importance of the subject, there is only limited information on the response of agricultural perennial species of economic interest and (sub) tropical origin to the atmospheric F pollution [14]
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