Effects of potassium in Myracrodruon urundeuva, Libidibia ferrea and Mimosa tenuiflora

This study aimed to evaluate the effects of potassium fertilization on Myracrodruon urundeuva, Libidibia ferrea and Mimosa tenuiflora seedlings submitted to a short period of water deficit. Three doses of K (0, 150 and 300 mg dm KCl) and three water regimes (without water deficit, moderate water deficit and severe water deficit) were evaluated. The plants were kept in black plastic bags containing soil from the Caatinga and, 30 days after the beginning of the water regimes, the relative water content (RWC), transpiration (E), stomatal conductance (gs) and photosynthesis (A) were evaluated. High moisture content in the soil was unfavorable to plants, promoting reduction in RWC and gas exchange. The water deficit increase E, gs and A, but the intensity of these responses was variable between species. The water deficit favored L. ferrea, with elevated RWC and reduced alterations in gas exchange, demonstrating its greater tolerance in relation to the others species evaluated, however it was the species most benefited by K. The supply of 300 mg dm of KCl positively influenced the water status and gas exchange of the studied species with an increase in the water deficit. We Research, Society and Development, v. 9, n. 5, e97953269, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i5.3269 3 suggest the development of research subjecting the plants to a longer period of time, in order to establish a better relationship between potassium and the increased tolerance of these plants to water deficit.

suggest the development of research subjecting the plants to a longer period of time, in order to establish a better relationship between potassium and the increased tolerance of these plants to water deficit. Keywords: Water stress; Caatinga; Gas exchange.
Las plantas se mantuvieron en bolsas de plástico negras que contenían tierra de Caatinga y, 30 días después del comienzo de los regímenes hídricos, se evaluó el contenido relativo de agua, la tasa de transpiración, la conductancia estomática y la tasa de fotosíntesis. El alto contenido de humedad en el suelo fue desfavorable para las plantas, lo que provocó una reducción en el TRA y el intercambio de gases. Se encontró que el déficit de agua aumentó la tasa de transpiración, la conductancia estomática y la tasa de fotosíntesis de las plantas, sin embargo, la intensidad de estas respuestas fue variable entre las especies. El déficit hídrico favoreció a las plantas de jucá, con un aumento en el contenido relativo de agua y un pequeño cambio en el intercambio de gases en relación con los niveles de déficit hídrico impuestos, lo que demuestra su mayor tolerancia en relación con los demás, sin embargo, fue la especie más beneficiada por K. O El suministro de 300 mg dm -3 de KCl influyó positivamente en el estado del agua y el intercambio de gases de las especies estudiadas con el aumento del déficit hídrico. Sugerimos el desarrollo de investigaciones que sometan a las plantas a un período de tiempo más largo, a fin de establecer una major relación entre el potasio y la mayor tolerancia de estas plantas al déficit hídrico.
The water, essential for maintaining cell turgor and elongation, and gas exchange between plant and environment (Souza, Soares & Regina, 2001;Flexas et al., 2004;Chaves et al, 2009), constitutes a limiting factor for growth and establishment of plants in the Caatinga.
When its availability in soil becomes limiting, stomatal closure begins as a strategy to control water loss (Flexas et al., 2004;Reich, 2014), reducing leaf transpiration and preventing tissue death from dehydration (Peak et al., 2004). However, this stomatal closure decreases gas exchange between the leaf and the atmosphere, reducing the intercellular CO2 concentration, with direct reflexes on photosynthesis (Chaves et al., 2002;Tang et al., 2002;Peeva;Cornic, 2009;Hu, Wang & Huang, 2010).
This stomatal control mechanism has been reported in several studies (Cakmak, 2005;Jin et al., 2011;Mendes et al. 2013;Wang et al., 2013;Shabala;Potossin, 2014) and is associated with the entry of potassium ions (K + ) in guard cells (Marenco;Lopes, 2011). The K + accumulation promotes changes in the cells osmotic potential, leading to water absorption and, consequently, changes in turgor pressure, resulting in the opening of stomata (Silveira, 2000;Mendes et al., 2013).
Extreme weather events, such as increased periods and drought cycles occurring more frequently, are expected as a result of climate change caused by anthropogenic actions (Easterling et al., 2000;IPCC, 2014), and how plants will behave in the face of this scenario the study has been the focus of several researchers (Fu et al., 2013;Silva, et al.;Ciemer et al., 2019). In these studies, strategies are being tested to mitigate the effects of drought in plants, increasing their tolerance, among them is the supplementary use of K, given its role as an osmotic agent (Kerbauy, 2013;Taiz;Zeiger, 2013;Martineau et al., 2017). Its accumulation in the cells vacuole, together with organics solutes such as sugars (Costa et al., 2015), amino acids  and proteins (Silva, 2011) aims to decrease the cellular water potential, promoting greater absorption of water and, consequently, maintenance of metabolic activities essential to plant growth, even under low water availability in the soil.

Plant material and growth conditions
This field research, of quali-quantitative nature (Pereira et al. 2018), was conducted at the containing 5 kg of soil from Caatinga with the following chemical characteristics: pH (H2O) 6.5; P 2.9 mg kg -1 ; Ca 5.0 cmolc dm -3 Mg 2.0 cmolc dm -3 ; K 4.6 cmolc dm -3 ; H + Al 0.22 cmolc dm -3 ; and T 94.1 cmolc dm -3 . The soil fertilization was carried out with N and P, according to Furtini Neto et al. (1999) recommendation, while K was according to treatments.
Before sowing, the seeds of M. tenuiflora and L. ferrea were subjected integumentary breaking dormancy, respectively, by immersion in hot water (85 °C) during 30 seconds and later washing in running water during one minute (Bakke et al., 2006) and by acid scarification in concentrated sulfuric acid during 30 minutes and later washing in running water. The seedlings were irrigated daily (100% soil retention capacity (SC)), determined by weighing.

Relative water content
Relative water content was assessed according to Barrs and Weatherley (1962). After stomatal evaluations, to determine the relative water content (RWC), four leaves per repetition were collected and subjected to weighing to determine the weight of fresh matter (FM). Then, they were floated in distilled water and, under a sheet of filter paper, placed in Petri dishes and kept in the refrigerator (5° C) for 72 h. After this period, leaf discs were weighed, to obtain the weight of the turgid mass (TM). Subsequently, they were dried in an oven at 65 ° C for 72 hours and weighing obtain the dry mass (DM).

Statistical analysis
All analyses were carried out using the SISVAR software, version 5.6 (FERREIRA, 2011). The data were submitted to analysis of variance (ANOVA). Tukey's HSD test was used, where applicable, to distinguish between treatment mean values. Differences were considered significant at P < 0.05.

Relative water content
There was a significant interaction of treatments on RWC in M. urundeuva and M. tenuiflora, while in L. ferrea, the effect was isolated from water treatments and potassium fertilization. In the M. urundeuva (Table 1), in potassium treatments, there was no statistical difference in the RWC between WI and MD treatments. In SD, the lowest RWC was verify in plants without K, indicating that this nutrient improved the water status of these plants when under severe water deficit. Research, Society and Development, v. 9, n. 5, e97953269, 2020 (CC BY 4. In M. tenuiflora, there was no statistical difference between K treatments at MD, while in WI and SD treatments, RWC decreased with increase of K provided (Table 3). Probably the amount of potassium in the substrate (4.0 cmolc dm -3 K) may have been sufficient for the Research, Society and Development, v. 9, n. 5, e97953269, 2020 (CC BY 4. and Trischidium molle (Benth.) H. E. Ireland. Water stress is not caused only by water deficiency, but also by excess (Fernandes, 2012), because in saturated environments there is less diffusion of O2, lead to low ATP production and the accumulation of toxic substances under these anaerobic conditions (Drew;Lynch, 1980), resulting in less water absorption by plants (Oliveira;Gualtiere, 2017).
In relation of potassium, its role in cell osmotic regulation is very known, helping photosynthesis and also the transport of carbohydrates from the leaves to the roots (Martineau et al., 2017) which, together with the possible roots K accumulation, may have exerted osmotic effect on the vacuole, favoring water absorption (Premachandra et al., 1992). Such factors provide conditions for the plants to maintain their physiological activities under limited water availability, thus making it possible to increase the plants' tolerance to drought.

Gas exchange
Transpiration (E) in M. urundeuva increase when the amount of water supplied decreased (Table 4), with a positive influence of potassium fertilization. Analyzing the plants that did not receive K, the values of E increase from 11.7 µmol H2O m -2 s -1 (WI) to 14 µmol H2O m -2 s -1 (SD), equivalent to increment of 20%, while in the treatment 150 mg dm -3 KCl, this increase was only 12%. The highest rate of transpiration was found in plants at 50% cv Development, v. 9, n. 5, e97953269, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i5.3269 and 300 mg dm -3 KCl, with an increase of 33%, going from 11.1 µmol H2O m -2 s -1 (0 mg dm -3 KCl) to 14.8 µmol H2O m -2 s -1 (300 mg dm -3 KCl). In L. ferrea and M. tenuiflora (Table 4), the differences in E became more evident, and in L. ferrea, plants maintained at MD showed the highest E in relation to other water treatments. As for potassium fertilization, within the water treatments, as the K increased, there was an increase in E, corresponding to 76% (WI), 73% (MD) and 64% (SD), comparing 0 and 300 mg dm -3 KCl treatments. In M. tenuiflora, there was a progressive increase in E when the water availability decreased in all fertilization treatments. The most pronounced positive effect of K supply was found in plants at MD, in which plants that received 150 and 300 mg dm -3 KCl showed, respectively, 64% and 72% greater transpiration than plants that did not receive K.

Tabel 4 -
There was a decrease in stomatal conductance (gs) (  Development, v. 9, n. 5, e97953269, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i5.3269 verified in E (Table 4), plants that received 300 mg dm -3 KCl, in WI and MD treatments, the gs was higher, while at SD, the 150 mg dm -3 KCl treatment provided the highest gs. Increase in A in M. urundeuva was observed when the water availability decreased, which was favored by the addition of K (Tabel 6). This also was observed in L. ferrea, in which under water deficit, when 150 mg dm -3 KCl was supplied, A increase 50% and 46% under MD and SD treatments, when compared to WI. When the amount of K was doubled (300 mg dm -3 KCl), the increments were 66% and 60% in the same line of comparison. In relation to M. tenuiflora, the behavior was also similar, except in plants under severe deficit, in which the addition of 150 mg dm -3 KCl provided the highest A.
The RWC decrease in M. urudeuva (Table 1), when the water added to the substrate decreased, resulted in stomatal closure, reducing E (Table 4), gs (Table 5) and A (Table 6).

Final Considerations
High moisture content in the soil was unfavorable to plants, promoting reduction in RWC and gas exchange.
The water deficit increase E, gs and A, but the intensity of these responses was variable between species.
The water deficit favored L. ferrea, with elevated RWC and reduced alterations in gas exchange, demonstrating its greater tolerance in relation to the others species evaluated, however it was the species most benefited by K.
The supply of 300 mg dm -3 of KCl positively influenced the water status and gas exchange of the studied species with an increase in the water deficit.
We suggest the development of research subjecting the plants to a longer period of time, in order to establish a greater relationship between potassium and the increased tolerance of these plants to the water deficit.