Gas exchanges and photochemical efficiency of West Indian cherry cultivated with saline water and potassium fertilization

Trocas gasosas e eficiência fotoquímica da aceroleira cultivada com águas salinas e adubação potássica R E S U M O O problema da falta de recursos hídricos adequados para agricultura vem se intensificando nos últimos anos, tornando-se necessário o uso de águas com relativamente alta concentração de sais para a irrigação das culturas por todo o mundo. Com isso, objetivou-se avaliar a influência da adubação potássica como atenuador do estresse salino sobre as trocas gasosas e eficiência fotoquímica da aceroleira. O cultivo das aceroleiras foi realizado em condição de casa de vegetação no município de Campina Grande, PB, em lisímetros preenchidos com 250 kg de solo de textura franco-arenosa. Os tratamentos foram distribuídos em blocos casualizados, em esquema fatorial 2 x 4, sendo dois níveis de condutividade elétrica da água de irrigação CEa (0,8 e 3,8 dS m-1) e quatro doses de potássio (50, 75; 100 e 125% da recomendação), sendo a dose correspondente a 100% igual a 19,8 g de K2O por planta ano -1, com três repetições e uma planta por repetição. Foram utilizadas mudas de acerolas cultivar BRS 366-Jaburu, enxertadas sobre o porta-enxerto Crioulo proveniente do jardim de sementes da EMBRAPA Agroindústria Tropical, em Pacajus, CE. A irrigação com água salina (condutividade elétrica de 3,8 dS m-1) comprometeu as trocas gasosas e a eficiência fotoquímica das plantas de acerola. A adubação potássica não foi eficiente na redução do estresse promovido pela salinidade da água em plantas de aceroleira.


Introduction
West Indian cherry (Malpighia emarginata D.C.) is a fruit crop that has stood out as an excellent natural source of vitamin C, besides anthocyanins and carotenoids, compounds with beneficial effects on human health due to their recognized antioxidant action (Maciel et al., 2010).According to Freitas et al. (2006), Brazil is the largest producer, consumer and exporter of this fruit in the world, and its Northeast region stands out as the region were West Indian cherry is best adapted to, due to the conditions of soil and climate.
In the Northeast region, West Indian cherry production reaches 22,964 tons in an area of 7,237 ha, which represents about 70% of the Brazilian production (Caetano, 2010).Thus, the cultivation of West Indian cherry becomes highly promising in the Northeast region, having good prospects for the fruit grower market.Despite the good adaptability of West Indian cherry, the Brazilian semi-arid region poses risks to its cultivation, due to the scarcity of water resources, where the available sources of water for irrigation commonly have high concentrations of salts, especially sodium (Jiang et al., 2012;Souza et al., 2017).
The excess of salts in the soil causes reduction in the osmotic potential, resulting in water deficit in the crops.This deficit causes stomatal closure, limiting CO 2 assimilation and transpiration in plants, and consequently reduces their photosynthetic rate (Silva et al., 2010).In addition, plants subjected to saline stress undergo changes in the functional state of the thylakoid membranes of chloroplasts, which cause alterations in the characteristics of the fluorescence signals (Chaum & Kirmanee, 2011;Silva et al., 2011a;Freire et al., 2014), thus diminishing growth and yield (Sousa et al., 2012).
As a consequence, in an attempt to mitigate the deleterious effects caused on crops by the excess salts present in irrigation water and/or soil, some management practices have been developed.These practices include fertilization based on the use of fertilizers which favor the acquisition of nutrients by plants under saline conditions (Silva et al., 2011b).Works conducted by Gurgel et al. (2005Gurgel et al. ( , 2010) ) concluded that potassium (K) fertilization led to a decrease in the deleterious effects of irrigation with high-salinity water on the melon crop.
The beneficial effect of potassium is due to the fact that it is vital for photosynthesis, so that situations of deficiency cause reduction in the photosynthetic rate and increase in respiration.In addition, this nutrient favors the formation and translocation of carbohydrates, improving fruit quality (Araújo et al., 2012).Although the benefits of K fertilization are known, studies providing evidence of its mitigating action under conditions of saline stress on plants, mainly in West Indian cherry, are still scarce in the literature.Therefore, this study aimed to evaluate the influence of K fertilization as a mitigating factor of saline stress on gas exchanges and photochemical efficiency of West Indian cherry.

Material and Methods
The experiment was carried out under greenhouse conditions from July 2016 to July 2017, in recipients adapted as drainage lysimeters, at the Center of Technology and Natural Resources of the Federal University of Campina Grande (CTRN/UFCG), located in the municipality of Campina Grande -PB, Brazil (7° 15' 18'' S; 35° 52' 28'' W; ~550 m).
The experimental design was in randomized blocks, with three replicates, using a 2 x 4 factorial scheme, and treatments consisted of two levels of irrigation water electrical conductivity -ECw (0.8 and 3.8 dS m -1 ) and four K doses (50, 75, 100 and 125% of recommendation).The 100% dose corresponded to 19.8 g of K 2 O plant -1 year -1 (Musser, 1995).
The bottom of each lysimeter was connected to a 4-mmdiameter hose to drain the leachate into a container, in order to evaluate the drained water and determine water consumption by plants.The tip of the drain inside the pot was involved with a nonwoven geotextile (Bidim OP 30) to avoid clogging by soil particles.
The experiment used West Indian cherry seedlings, cultivar BRS 366-Jaburu, grafted onto locally developed rootstocks from the Embrapa Tropical Agroindustry, Pacajus, CE.At transplantation, seedlings were 240 days old.During the acclimation period in the greenhouse, plants were irrigated with low-salinity water (0.8 dS m -1 ).The cultivar BRS 366-Jaburu stands out for its high yield (57 t ha -1 ) and production of ascorbic acid (2,648 mg 100g -1 ).These plants are about 1.87 m tall with crown diameter of 2.18 m.Fruits are shiny when ripe, weighing on average 4 to 5 g when unripe, adequate stage to obtain vitamin C, and 6 to 7 g after ripening (EMBRAPA, 2012).
Prior to seedling transplantation, the soil was brought to field capacity using the respective water of each treatment.After transplantation, irrigation was carried out every day by applying in each lysimeter a water volume to maintain the soil near field capacity, and the applied volume was determined according to the water requirement of the plants, estimated by the water balance: volume applied minus the volume drained in the previous irrigation, plus a leaching fraction of 0.10, applied at intervals of 30 days.
Water with electrical conductivity of 3.8 dS m -1 was prepared by dissolving the salts NaCl, CaCl 2 .2H 2 O and MgCl 2 .6H 2 O, at equivalent proportion of 7:2:1, respectively, in public-supply water (ECw = 1.40 dS m -1 ) in the municipality of Campina Grande, PB, based on the relationship between ECw and concentration of salts (10 * mmol c L -1 = ECw dS m -1 ), according to Richards (1954).On the other hand, the ECw level of 0.8 dS m -1 was prepared by mixing rainwater (ECw = 0.02 dS m -1 ) in the municipal-supply water (ECw = 1.40 dS m -1 ).
The data were subjected to analysis of variance by F test and, when significant, means were compared by Tukey test at 0.05 probability level for water salinity levels and regression analysis was conducted for K doses, using the statistical program SISVAR-ESAL (Ferreira, 2011).

Results and Discussion
There was significant influence (p < 0.05) of the interaction between levels of irrigation water salinity and K fertilization on variable fluorescence (Fv).The other parameters of gas exchanges and chlorophyll a fluorescence were only significantly influenced (p < 0.05) by the irrigation water salinity, except the instantaneous water use efficiency (WUEi), which was not significantly affected by any of the factors studied.Potassium doses significantly influenced (p < 0.05) only the variable fluorescence (Table 1).
Irrigation water salinity of 3.8 dS m -1 negatively affected gas exchanges in West Indian cherry, leading to accentuated reductions of the order of 47.58, 33.14, 41.03 and 56.75% in stomatal conductance, transpiration, CO 2 assimilation rate and instantaneous carboxylation efficiency, respectively, in comparison to the ECw level of 0.8 dS m -1 (Figures 1A, B, C  and E).However, internal CO 2 concentration increased by 38.03% as a function of the increment in ECw levels from 0.8 to 3.8 dS m -1 (Figure 1D).According to Freire et al. (2014), such increment in Ci inside the leaves indicates that CO 2 is not being used for the synthesis of sugars in the photosynthetic process, suggesting that probably a non-stomatal factor was interfering with this process, which can be confirmed by the reduction in the photosynthetic rate of plants irrigated with 3.8 dS m -1 water (Figure 2A).Despite the drastic reduction in stomatal conductance, and consequently in transpiration, it did not influence the instantaneous water use efficiency or internal CO 2 concentration, since there was no reduction in CO 2 influx in plants irrigated with high-salinity water.
Thus, the reduction in stomatal conductance can be related to the tolerance mechanism of the species, aiming to reduce water loss and consequently the absorption of water and salts from the soil solution, without compromising photosynthetic activity.According to Taiz & Zeiger (2013), plants under salt stress have as the first response to salinity the reduction in stomatal opening, to reduce water loss through the leaves, which is induced by the reduction in the osmotic potential of the soil solution, resulting in higher resistance to water absorption by the roots.Accumulation of salts in the leaves also affects, significantly, essential physiological processes, including gs (Hussain et al., 2012).
The reductions observed in CO 2 assimilation rate and intrinsic carboxylation efficiency coincided with the increase in internal CO 2 concentration of plants cultivated under high salinity (Figures 1C, D and E).Reductions in intrinsic carboxylation efficiency are related to metabolic restrictions in the Calvin cycle, where the received carbon is not being fixed in the carboxylation stage in mesophyll cells (Larcher, 2006;Sousa et al., 2016).According to Taiz & Zeiger (2013), as the stress becomes severe there is a dehydration in mesophyll cells, the mesophyll metabolism is damaged and, consequently, carboxylation efficiency is compromised.This result is an indication that non-stomatal factors act on the ns, **, * Respectively not significant, significant at p < 0.01 and p < 0.05 photosynthetic activity of West Indian cherry, such as low activity of the enzyme Ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBisCO), probably due to the low availability of substrate (ATP and NADPH) for activation and regeneration of the enzyme, which come from the photochemical phase of photosynthesis, leading to reduction in CO 2 carboxylation and increase in its internal concentration (Hussain et al., 2012;Silva et al., 2015;Sá et al., 2017).Initial fluorescence (Fo) and maximum fluorescence (Fm) of West Indian cherry were reduced by 21.49 and 15.48% as water salinity increased from 0.8 to 3.8 dS m -1 respectively (Figures 2A and B).Initial fluorescence is characterized as the minimum fluorescence emitted when all reaction centers, dark adapted, are open (Gorbe & Calatayud, 2012).Maximum fluorescence is an indicator that represents the maximum intensity of the fluorescence emitted, when almost all quinone is reduced and the reaction centers reach their maximum capacity of photochemical reactions (Suassuna et al., 2010;Silva et al., 2015).The reduction in initial and maximum fluorescence indicates reduction in the photochemical capacity of West Indian cherry under saline stress conditions, which combined with the low CO 2 carboxylation efficiency confirms that the low photosynthetic yield of this crop under high salinity results from the damages caused by the excess of salts to the chlorophyll a, reducing its activity and thus the synthesis of ATP and NADPH.

Source of variation F test
Variable fluorescence in West Indian cherry plants irrigated with low-salinity water (0.8 dS m -1 ) decreased linearly as K doses increased, with a reduction of 22.78% in the Fv in plants fertilized with 125% of the K recommendation in comparison to those subjected to 50%, which represents a reduction of  ) Water salinity (dS m -1 ) A.
162.50 electrons quantum -1 (Figure 3A).In plants irrigated with high-salinity water (EC = 3.8 dS m -1 ) there was a quadratic behavior for variable fluorescence (Figure 3A), and highest Fv was obtained when plants were fertilized with K dose of 87% (501.93 electrons quantum -1 ).From this dose on, the increment in K fertilization led to reduction in this variable and corresponded to 362.145 electrons quantum -1 at the highest K dose, which represents a decrease of 139.79 electrons quantum -1 .Such initial increase in Fv due to the increment in K dose is related to the greater competition of this cation with other ions, such as Na ions, thus reducing the deleterious effects of saline stress on chlorophyll a activity.However, as the dose increased, K may have intensified even more the salt stress.
As previously observed for Fo and Fm (Figures 2A and B), the potential quantum efficiency of photosystem II (Figure 3B) also decreased with the increase in salinity, and irrigation with high-salinity water (3.8 dS m -1 ) caused reduction in the Fv/Fm ratio from 0.75, in plants irrigated with 0.8 dS m -1 , to 0.63 electrons quantum -1 .The Fv/Fm ratio reflects the efficiency relative to the absorption of light energy by the antenna complex of the PSII and to its conversion into chemical energy, being considered as a sensitive indicator of photosynthetic performance of the plant, as well as an indicator of disturbances in the photosynthetic system caused by stresses and its reduction consists in the limitation of photochemical activity (Lucena et al., 2012).Fv/Fm values between 0.75 and 0.85 electrons quantum -1 suggest that the photosynthetic apparatus is intact, as opposed to values lower than 0.75 electrons quantum -1 (Reis & Campostrini, 2011;Silva et al., 2015).Therefore, it can be inferred that the photosynthetic apparatus of West Indian cherry plants irrigated with 3.8 dS m -1 water (Figure 3B) is compromised by salt stress, since these plants had Fv/Fm values of 0.63 electrons quantum -1 .

Figure 1 .
Figure 1.Stomatal conductance -gs (A), transpiration -E (B), CO 2 assimilation rate -A (C), internal CO 2 concentration -Ci (D) and intrinsic carboxylation efficiency -CEi (E) of West Indian plants irrigated with saline water at 180 days after transplanting Bars represent the standard error of the mean (n=3).Mean followed by different letter indicates that the treatments differ by Tukey test, p < 0.05

Figure 2 .
Figure 2. Initial fluorescence -Fo (A) and maximum fluorescence -Fm (B) of West Indian cherry plants irrigated with saline water at 180 days after transplanting Bars represent the standard error of the mean (n=3).Mean with different letter indicates that the treatments differ by Tukey test, p < 0.05

Figure 3 .
Figure 3. Variable fluorescence -Fv (A) as a function of the interaction between irrigation water salinity and potassium doses, and potential quantum efficiency of photosystem II -Fv/Fm (B) of West Indian cherry plants irrigated with saline water at 180 days after transplanting Bars represent the standard error of the mean (n=3).Mean with different letter indicates that the treatments differ by Tukey test, p < 0.05