Physiology and production of naturally-colored cotton under irrigation strategies using salinized water

Fisiologia e produção de algodoeiro naturalmente colorido submetido a estratégias de irrigação com água salinizada

Abstract:

The objective of this work was to evaluate the effects of irrigation management strategies, using low (0.8 dS m^-1) and high (9.0 dS m^-1) salinity water, on the physiological processes, production variables, and fiber productivity of naturally-colored cotton (Gossypium hirsutum) genotypes during different phenological stages. Three naturally-colored cotton cultivars (BRS Rubi, BRS Topázio, and BRS Safira) were subjected to seven irrigation management strategies, in a 3x7 factorial arrangement, in a randomized complete block design with three replicates. Plants were cultivated in pots in a protected environment. Gas exchange, chlorophyll a fluorescence, fiber production, and productivity and yield variables were evaluated. Salt stress causes greater reductions in gas exchange in natural cotton plants during the production stage. Plume cotton production is not compromised by the use of highly salinized water during the initial developmental stages of naturally-colored cotton.

Index terms:
Gossypium hirsutum; fiber productivity; quantum yield; salinity

Resumo:

O objetivo deste trabalho foi avaliar os efeitos de estratégias de manejo da irrigação, com água de baixa (0,8 dS m^-1) e alta salinidade (9,0 dS m^-1), sobre os processos fisiológicos, as variáveis de produção e o rendimento de fibras de genótipos de algodoeiro (Gossypium hirsutum) naturalmente colorido, em diferentes fases fenológicas. Foram avaliadas três cultivares de algodoeiro colorido (BRS Rubi, BRS Topázio e BRS Safira), submetidas a sete estratégias de manejo de irrigação, em arranjo fatorial 3x7, em delineamento de blocos ao acaso, com três repetições. As plantas foram cultivadas em vasos, em ambiente protegido. Foram avaliadas trocas gasosas, fluorescência da clorofila a, e variáveis de produção e de rendimento de fibra. O estresse salino ocasiona maiores reduções nas trocas gasosas do algodoeiro durante a fase de formação da produção. A produção de algodão em pluma não é comprometida pelo uso de água de alta salinidade nas fases iniciais de desenvolvimento do algodoeiro colorido naturalmente.

Termos para indexação:
Gossypium hirsutum; rendimento de fibra; eficiência quântica; salinidade

Introduction

Salinity is considered one of the main limiting factors for agricultural productivity and sustainability, especially in arid and semiarid regions characterized by elevated evapotranspiration demand, reduced rainfall, reduced water availability, and soil resources with variable salt content (Dong et al., 2010). In these regions, it is essential to generate technology that enables the use of water with salinity restrictions for crop irrigation, taking into account that highly salinized water increases soil salinity due to ion accumulation in the root zone (Letey & Feng, 2007).

In general, decreases in the productivity of plants in highly salinized environments can be attributed, in part, to limitations in the physiological processes of the plant, mainly gas exchange and a chlorophyll fluorescence because of stomatal closure and a reduction in carbon fixation activity (Zhang et al., 2014; Rady et al., 2016). Furthermore, salinity leads to nutritional imbalances, since excessive amounts of Na⁺ in the soil solution result in absorption disorders that affect other nutrients, particularly Ca, Mg, and K (Rosales et al., 2012).

Cotton (Gossypium hirsutum L.) plants are capable of adapting to semiarid environments due to their tolerance to salt stress of up to 5.1 dS m^-1 for irrigation water (Ayers & Westcot, 1999; Oliveira et al., 2013). Despite this tolerance, several researchers have reported negative effects of salinity on naturally-colored cotton plants (Santos et al., 2016; Lima et al., 2017; Silva et al., 2017). It should be noted, however, that the level of tolerance to salinity may vary among species and/or genotypes of a species, or even among the phenological stages of a same genotype (Neves et al., 2009).

Previous studies on the use of salinized water for cotton irrigation have evaluated in which developmental stage the physiological processes and, consequently, cotton production were less affected (Chen et al., 2010; Zhang et al., 2013). In white plume cotton plants, different results were obtained according to genotype, soil, and climate conditions; at times, plants were more susceptible during germination and initial plant growth and, at others, during flowering and fructification (Khorsandi & Anagholi, 2009).

However, the effects of salinity during each developmental stage of cotton plants and the reproductive compensation after salt stress exposure are still poorly understood. This shows the importance of studies on the recovery of cotton plants after salt stress during each developmental stage, from germination to production (Ashraf et al., 2008; Shaheen & Shahbaz, 2012). Physiological and production aspects should be considered when selecting new tolerant materials; salt ions, for example, may affect the photosynthetic and production apparatuses, which may be related to cotton fiber productivity.

The objective of this work was to evaluate the effects of irrigation management strategies, using low (0.8 dS m^-1) and high (9.0 dS m^-1) salinity water, on the physiological processes, production variables, and fiber productivity of naturally-colored cotton genotypes during different phenological stages.

Materials and Methods

The experiment was conducted in a protected environment of the center of technology and natural resources of Universidade Federal de Campina Grande, located in the municipality of Campina Grande, in the state of Paraíba, Brazil (07°15'18"S, 35°52'28"W, at an average altitude of 550 m).

Three cotton plant genotypes - BRS Rubi, BRS Topázio, and BRS Safira - were subjected to different irrigation management strategies using salinized water, in the following phenological stages: vegetative (A), from the emission of the first leaf until the anthesis of the first flower; flowering (B), from the first anthesis until the opening of the first boll; and fructification/production (C), from the opening of the first boll until harvest.

A total of seven irrigation management strategies were applied, using water with two electrical conductivity levels (ECw): low (ECw = 0.8 dS m^-1) and high salinity (9 dS m^-1), according to Lima et al. (2016). The different strategies evaluated were: A₁B₁C₁, irrigation with water with low ECw during the entire cycle, identified by index 1 in the phenological stages; A₂B₁C₁, salt stress during the vegetative stage, represented by index 2 during stage A, with plants receiving ECw = 9 dS m^-1 water from 16 days after sowing (DAS) until flowering at 37 DAS, followed by irrigation using water with low ECw until the end of cycle; A₁B₂C₁, salt stress during the flowering stage, i.e., irrigation with water with high ECw from 37 DAS until the beginning of fructification at 59 DAS and with water with low ECw during the remaining stages; A₁B₁C₂, use of water with low ECw during the vegetative and flowering stages, and with high ECw until the end of the cycle at 113 DAS; A₂B₁C₂, irrigation with water with high ECw during the vegetative and fructification stages, and without salinity during flowering; A₂B₂C₁, irrigation with salinized water during the vegetative and flowering stages, and with water with low ECw during fructification; and A₁B₂C₂, water with low ECw during the vegetative stage and irrigation with water with high ECw during the flowering and production stages. The experimental design was a randomized complete block in a 3×7 factorial arrangement, totaling 21 treatments with three replicates and three plants per plot.

The plants were cultivated in 20-L plastic pots (height of 35 cm, top diameter of 31 cm, and bottom diameter of 20 cm), lined with a fine screen at the bottom to avoid soil loss; a hose was also placed at the bottom and coupled to a container to collect drainage water. A 3-cm layer of gravel and 24.5 kg of previously screened Neossolo Regolítico Eutrófico (Santos et al., 2013), i.e., Entisol, with sandy loam texture, were placed on the screen. The soil physical-hydraulic attributes were: 38.59% total porosity; 11.48 and 2.41 kPa water content at field capacity (33.42 kPa) and permanent wilting point (1,519.5 kPa), respectively; and 1.51 available water. The other chemical and physical attributes are presented in Table 1. The pots were arranged in simple rows spaced at 1.0 m from each other; the spacing within each row was 0.6 m.

Table 1
Chemical and physical attributes of the soil used in the experiment, before the treatments were applied⁽¹⁾.

Attributes		Value
pHsp		5.8
SOM (dag kg-1)		21.20
P (mg kg-1)		45
K+ (cmolc kg-1)		0.18
Na+ (cmolc kg-1)		0.37
Ca2+ (cmolc kg-1)		2.37
Mg2+ (cmolc kg-1)		3.09
Al3++H+ (cmolc kg-1)		1.78
ECse (dS m-1)		0.20
Particle size fraction
	Sand (g kg-1)	808.6
	Silt (g kg-1)	80.5
	Clay (g kg-1)	110.9
Textural class		Sandy loam
Density (kg dm-3)		1.67

• ⁽¹⁾
  pH_SP, pH of the saturation paste; SOM, soil organic matter determined by the Walkley-Black wet digestion method; K⁺ and Na⁺ extracted using 1 mol L^-1 NH₄OAc, pH 7.0; Ca²⁺ and Mg²⁺ extracted with 1 mol L^-1 KCl, pH 7.0; ECse, electrical conductivity of the saturation extract.

Base dressing was performed using N-P-K, according to recommendations for experiments in pots (Novais et al., 1991); a total of 100 mg kg^-1 N as ammonium sulfate, 300 mg kg^-1 P as single superphosphate, and 150 mg kg^-1 K as potassium chloride was applied. The base dressing provided the full amount of P, but only 1/3 of N and K; the other 2/3 were applied through irrigation water at 45 and 65 DAS. In addition, 500 g worm castings were added to each pot. Foliar fertilization was done at the beginning of the flowering stage at 45 DAS, in order to improve plant nutrition, using a commercial product containing: 0.05% B, 0.5% Cu, 0.5% Fe, 0.05% Mn, 0.02% Mo, and 0.5% Zn.

The water used for low-salinity irrigation (0.8 dS m^-1) was obtained by diluting the water supplied by the public system of Campina Grande, in the state of Paraíba, Brazil, with rain water; the preparation of the water with high ECw (9.0 dS m^-1) and of the irrigation managements was similar to that described by Lima et al. (2016). Sowing was carried out after soil moisture was elevated to maximum retention in all experimental units, using low-salinity water (0.8 dS m^-1) and five seeds per pot at a 3-cm depth. Thinning was performed at 30 DAS, and only one plant was kept in each pot.

Physiological variables were evaluated at 70 and 93 DAS, when plants reached full fructification and all salinity management strategies could be included in the statistical analysis, since all salinized treatments had already started. The following parameters were assessed on the third leaf from the top of the plant: CO₂ assimilation rate (μmol m^-2 s^-1), transpiration (mmol H₂O m^-2 s^-1), stomatal conductance (mol H₂O m^-2 s^-1), and inner CO₂ concentration (μmol mol^-1) using the LCpro⁺ gas exchange meter (ADC Bioscientific Ltd., Hoddesdon, United Kingdom). The obtained data were used to estimate the instantaneous water use efficiency (WUE) (CO₂ assimilation rate/transpiration) [(μmol m^-2s^-1) (mmol H₂O m^-2 s^-1)^-1] and the intrinsic carboxylation efficiency (iCE) (CO₂ assimilation rate/inner CO₂ concentration ) [(μmol m^-2 s^-1) (μmol mol^-1)^-1].

At 93 DAS, the same leaves used for the gas exchange analysis were clipped, and, after adaptation to the dark for 30 min, a chlorophyll was determined. Initial fluorescence (Fo), maximum fluorescence (Fm), variable fluorescence (Fm-Fo), and quantum yield of photosystem II (Fv/Fm) were also obtained using the pulse-modulated fluorometer, model OS5p-Opti Science (Hansatech Instruments Ltd, Norfolk, United Kingdom) (Maxwell & Johnson, 2000; Konrad et al., 2005).

Production was quantified by the harvest of bolls until 113 DAS. Treatments were applied until the end of cycle, and plume mass and plume fiber productivity were determined using the equation described by Albrecht et al. (2009): FP (%) = FM/TM × 100, where FP is fiber productivity, FM is cotton fiber mass (g), and TM is total boll mass (g).

Data were evaluated using the F-test, and, when significant, means were compared by Scott-Knott’s grouping test, at 5% probability, for salinity strategies, and by Tukey’s test, also at 5% probability, for cotton plant genotypes (Ferreira, 2011).

Results and Discussion

Based on the analysis of variance (Table 2), salinity management strategies had significant effects on stomatal conductance, transpiration, CO₂ assimilation rate, WUE, inner CO₂ concentration, and iCE at 70 and 93 DAS, when plants were in full fructification. Among cotton genotypes, only inner CO₂ concentration and iCE differed at 93 DAS. The interaction between factors (management strategies × genotypes) showed no significant effects, i.e., gas exchange did not vary according to the stage in which the genotypes were subjected to salt stress, indicating that the negative effects of irrigation water salinity on cotton plants are significantly more intense during prolonged exposure or exposure during the entire crop cycle (Jafri & Ahmad, 2002).

Table 2.
Analysis of variance of the physiological parameters evaluated for naturally-colored cotton (Gossypium hirsutum) at 70 and 93 days after sowing (DAS) according to different salinity management strategies and genotypes.

Variable(1)	DF	Mean square	DF	Mean square	DF	Mean square	CV (%)	Overall mean
		Management strategy (MS)		Genotype (G)		MS x G
	70 days after sowing (DAS)
gs	6	0.071**	2	0.001ns	12	0.0009ns	21.78	0.19
E	6	16.718**	2	0.296ns	12	0.350ns	23.74	4.20
A	6	249.59**	2	9.723ns	12	3.880ns	14.66	17.57
WUE	6	1.010**	2	0.287ns	12	0.100ns	19.34	4.42
Ci	6	2,587.851**	2	659.377ns	12	385.435ns	18.42	147.72
iCE	6	0.0008**	2	0.0003ns	12	0.0005ns	27.35	0.12
	93 days after sowing (DAS)
gs	6	0.033**	2	0.0004ns	12	0.0002ns	20.96	0.13
E	6	10.442**	2	0.072ns	12	0.084ns	22.1	2.98
A	6	109.087**	2	6.465ns	12	2.272ns	13.93	11.90
WUE	6	1.882**	2	0.868ns	12	0.230ns	14.45	4.428
Ci	6	6,642.769**	2	1,652.994*	12	378.708ns	13.51	163.841
iCE	6	0.001**	2	0.001**	12	0.0004ns	24.06	0.0765

• ⁽¹⁾
  gs, stomatal conductance; E, transpiration; A, CO₂ assimilation rate; WUE, instantaneous water use efficiency; Ci, inner CO₂ concentration; iCE, intrinsic carboxylation efficiency; DF, degree of freedom; and CV, coefficient of variation. * and **Significant by the F-test, at 5 and 1% probability, respectively. ^nsNonsignificant.

Stomatal conductance at 70 DAS (Figure 1 A) showed reductions of 61.13, 47.19, and 67.27% in plants irrigated with salinized water in the A₁B₂C₁, A₂B₂C₁, and A₁B₂C₂ management strategies, respectively, when compared with plants irrigated with low-salinity water in A₁B₁C₁. However, there was an average recovery in stomatal conductance of 0.13 and 0.19 mol m^-2 s^-1, respectively, using the A₁B₂C₁ and A₂B₂C₁ management strategies at 93 DAS. It should be pointed out that these plants had already been irrigated with low-salinity water (0.8 dS m^-1) during 34 days (Figure 1 A). According to Gratan et al. (2004), the tolerance of plants to salinity usually increases with age, after the vegetative growth stages. Similar results were observed for barley (Hordeum vulgare L.) by Pandya et al. (2005), cowpea [Vigna unguiculata (L.) Walp.] by Neves et al. (2009), and white-plume cotton (Gossypium hirsutum L.) by Zhang et al. (2013).

Figure 1.
Means related to: A, stomatal conductance (gs); B, transpiration (E); C, CO₂ assimilation rate (A); D, inner CO₂ concentration (Ci); E, instant water usage effectiveness (WUE); and F, instant carboxylation efficiency (iCE) in naturally-colored cotton (Gossypium hirsutum) plants at 70 and 93 days after sowing (DAS) under different salinity management strategies. Distinct lowercase letters indicate significant differences among salinity management strategies by Scott-Knott’s test, at 5% probability. Bars represent the standard error of the mean (n=9). 1, A₁B₁C₁; 2, A₂B₁C₁; 3, A₁B₂C₁; 4, A₁B₁C₂; 5, A₂B₁C₂; 6, A₂B₂C₁; and 7, A₁B₂C₂, where A₁, B₁, C₁ represent no salinity during the vegetative, flowering, and fructification stages, and A₂, B₂, C₂, salinity during the vegetative, flowering, and fructification stages.

As a result of partial stomatal closure, when plants were subjected to salt stress, foliar transpiration showed similar reductions (Figure 1 B) of 50, 36.13, and 51.75% for A₁B₂C₁, A₂B₂C₁, and A₁B₂C₂, respectively, at 70 DAS, compared with A₁B₁C₁. At 93 DAS, however, reductions of 54.15, 58.84, and 43.53% in transpiration were observed for A₁B₁C₂, A₂B₁C₂, and A₁B₂C₂, respectively. Similarly, Lima et al. (2017) verified that water salinity (9.1 dS m^-1) reduced the stomatal conductance and foliar transpiration of 'BRS Rubi' in 33.20 and 27.72%, respectively, during the production stage, at 83 DAS; the authors also associated these reductions with partial stomatal closure, as well as with osmotic and ionic toxicity effects on the metabolism of plants subjected to salinity conditions.

The CO₂ assimilation rate was compromised for plants irrigated with highly salinized water (9.0 dS m^-1), and the most significant reductions were observed at 93 DAS for the A₁B₁C₂, A₂B₁C₂, and A₁B₂C₂ management strategies (Figure 1 C). Therefore, when salt stress was applied during fructification, there was an increase in the resistance to CO₂ diffusion due to a decrease in stomatal conductance, as previously discussed. Desingh & Kanagaraj (2007) also reported a decrease in CO₂ assimilation while evaluating the Arya-Anubam and LRA-5166 cotton varieties subjected to different salinity levels - 0 (control), 50, 100, and 150 mmol L^-1 NaCl, MgSO₄, and CaCl₂, respectively - at 50 DAS, confirming the negative effect of salinity on cotton plants.

A higher inner CO₂ concentration was observed for A₁B₁C₁ using 168.55 μmol mol^-1, but it did not differ significantly from that of A₂B₁C₁, A₁B₁C₂, and A₂B₁C₂ (Figure 1 D). Partial stomatal closure at 93 DAS caused a decrease in foliar transpiration and in inner CO₂ concentration in A₁B₁C₂ and A₂B₁C₂, when salinized water was applied during the production stage (Figure 1 D). These results are probably related to the decrease in the potential energy of water in the roots and/or in the transportation of abscisic acid to the leaves, resulting in increased stomatal resistance and decreased carbon concentration in the substomatal chamber (inner CO₂ concentration). This is explained by the direct relationship between gas exchange (with implicit CO₂ absorption) and water loss, in which stomatal closure results in a reduction in transpiration and, consequently, in a lower inner CO₂ concentration (Shimazaki et al., 2007).

In alignment with the results obtained for stomatal conductance, transpiration, and CO₂ assimilation, a higher WUE was observed at 93 DAS (Figure 1 E) for A₁B₁C₂, A₂B₁C₂, A₂B₂C₁, and A₁B₂C₂, with an average of 4.69 [(μmol m^-2 s^-1) (mmol H₂O m^-2 s^-1)^-1]. Therefore, plants that are more capable of increasing water usage under salinity conditions possibly show higher tolerance to salt stress during these stages, because reductions in water consumption result in a reduction in the absorption of specific ions, avoiding toxic effects on the plant (Nobre et al., 2014).

A higher iCE due to salinized water (Figure 1 E) was observed for A₂B₁C₁ and A₂B₁C₂ at 70 DAS, and for A₁B₂C₁ and A₂B₂C₁ at 93 DAS, notably during the vegetative and flowering stages, in which the A₂B₂C₁ management strategy showed an average of 0.098 [(μmol m^-2 s^-1) (μmol mol^-1)^-1] at 93 DAS. The values obtained for transpiration, CO₂ assimilation rate, and instant carboxylation were within the variation range reported for cotton plants (Zhang et al., 2014; Lima et al., 2017), i.e., the photosynthetic apparatus was not compromised by stress during the initial crop developmental stages.

Genotype data analysis at 93 DAS indicated higher internal CO₂ concentration (Figure 2 A) for 'BRS Topázio' (172.40 μmol mol^-1) than for the 'BRS Rubi' (164.65 μmol mol^-1) and 'BRS Safira' (154.69 μmol mol^-1) genotypes. The average iCE values observed for 'BRS Rubi' and 'BRS Safira', however, were 0.085 and 0.072 [(μmol m^-2 s^-1) (μmol mol^-1)^-1], greater than that observed for 'BRS Topázio' (Figure 2 B). This is probably attributed to the low CO₂ assimilation in the substomatal chamber of 'BRS Rubi' and 'BRS Safira'; however, the obtained values are higher than the average of 0.03 [(μmol m^-2 s^-1) (μmol ol^-1)^-1] iCE found by Pereira (2012) for these same genotypes.

Figure 2.
Means related to: A, internal CO₂ concentration (Ci); and B, instant carboxylation efficiency (iCE) of naturally-colored cotton (Gossypium hirsutum) genotypes 93 days after sowing. Distinct lowercase letters indicate significant differences among cotton plant genotypes by Tukey’s test, at 5% probability. Bars represent the standard error of the mean (n=21).

Regarding fluorescence, no significant differences were verified among the naturally-colored cotton genotypes for photosynthetic apparatus efficiency (Table 3), only for fiber productivity and plume cotton mass. Furthermore, salinity management strategies caused significant effects (p<0.01) on Fm, Fv, Fv/Fm, fiber productivity, and plume cotton mass. The unfolding of the factor management strategies × genotype revealed significant effects only for plume cotton mass at 113 DAS.

Table 3.
Analysis of variance for initial fluorescence (Fo), maximum fluorescence (Fm), variable fluorescence (Fv), and quantum yield of photosystem II (Fv/Fm) in naturally-colored cotton (Gossypium hirsutum) plants at 93 days after sowing (DAS), as well as plume fiber productivity (FP) and plume cotton mass (Plcotton) at 113 DAS according to salinity management strategies and genotypes⁽¹⁾.

Source of variation	DF	Mean square
		Fo	Fm	Fv	Fv/Fm	FP	Plcotton
Management strategy (MS)	6	239.36ns	52,058.04**	52,301.79**	0.0003**	5.59**	4,710.77**
Genotype (G)	2	440.23ns	4144.10ns	2,224.53ns	0.00006ns	147.64**	4,618.92**
MS x G	12	168.77ns	6236.48ns	5,621.43ns	0.00007ns	0.74 ns	136.25*
Block	2	1,761.32ns	189,882.37ns	23,207.75ns	0.0007ns	0.30 ns	6.39ns
Error	40	167.53	3,985.76	3,216.02	0.00004	0.98	55.58
CV (%)		4.48	3.73	4.04	0.77	3.66	11.55
Mean		288.941	1691.460	1,403.465	0.828	27.196	64.55

• ⁽¹⁾
  DF, degrees of freedom; and CV, coefficient of variation. * and **Significant by the F-test, at 5 and 1% probability, respectively. ^nsNonsignificant.

There were no significant differences for Fo when salt stress was applied during the distinct phenological stages evaluated (Table 4). This result indicates that the exposure of plants to salinized irrigation water during the phenological stages did not affect the integrity of the complex light receptor of photosystem II (FS II) in naturally-colored cotton, which may be related to tolerance to salinity, as reported in other studies (Oliveira et al., 2014; Dias et al., 2016).

Table 4.
Mean test for initial fluorescence (Fo), maximum fluorescence (Fm), variable fluorescence (Fv), and quantum yield of the photosystem II (Fv/Fm) in cotton (Gossypium hirsutum) plants under different salinity management strategies at 93 days after sowing⁽¹⁾.

Management strategy	Fo	Fm	Fv	Fv/Fm
A1B1C1	292.74±10.54a	1,814.48±47.32a	1,529.70±40.88a	0.839±0.004a
A2B1C1	293.66±6.38a	1,679.29±35.17b	1,385.63±34.69b	0.824±0.004c
A1B2C1	285.77±9.09a	1,628.70±31.84c	1,346.03±27.13c	0.825±0.004c
A1B1C2	285.77±8.90a	1,685.00±34.05b	1,399.22±32.51b	0.829±0.005b
A2B1C2	293.25±4.32a	1,771.63±41.04a	1,478.37±43.51a	0.833±0.005b
A2B2C1	282.18±7.67a	1,658.81±32.38b	1,376.48±25.73b	0.829±0.003b
A1B2C2	292.18±4.32a	1,602.29±19.55c	1,308.81±22.21c	0.818±0.003d

• ⁽¹⁾
  Means followed by distinct lowercase letters indicate significant differences among salinity management strategies by Scott-Knott’s test, at 5% probability, and followed by distinct uppercase letters indicate significant differences among cotton plant genotypes by Tukey’s test, at 5% probability. Number±standard error. A₁, B₁, C₁, no salinity during the vegetative, flowering, and fructification stages, respectively; and A₂, B₂, C₂, salinity during the vegetative, flowering, and fructification stages.

The mean grouping tests performed for maximum fluorescence as affected by the salinity management strategies (Table 4) indicated that A₁B₁C₁ was statistically superior; however, plants subjected to A₂B₁C₂ received 1,771.63 electrons per quantum, not differing from those irrigated with low-salinity water during their life cycle. Similar tendencies were obtained for Fo and Fm; therefore, salt stress in A₂B₁C₂ did not impair quinone A photoreduction or electron flow among photosystems (Tatagiba et al., 2014).

During the flowering and fructification phenological stages (Table 4), plants irrigated with highly salinized water, subjected to A₁B₂C₁ and A₁B₂C₂, exhibited Fv values of 1,346.03 and 1,308.815 electrons per quantum, respectively, which were 9.41 and 14.43% lower than those obtained for plants without salt stress during the cycle in A₁B₁C₁.

The quantum efficiency of photosystem II for plants under salt stress in the A₂B₁C₁ and A₂B₂C₁ management strategies decreased an average of 1.70%, compared with plants irrigated with low-salinity water during the entire cycle in A₁B₁C₁ (Table 4). However, the most significant reduction was found in A₁B₂C₂, in which Fv showed 0.818 electrons per quantum, as observed for gas exchange. Decreases in Fv/Fm were also reported for peanut (Arachis hypogaea L.) and safflower (Carthamus tinctorius L.) plants (Correia et al., 2009; Çulha Erdal & Çakirlar, 2014); however, the decreases verified in the present study did not cause the degradation of the photosynthetic apparatus, since the obtained values were above 0.75 (Silva et al., 2014).

Fiber productivity decreased in 6.34, 5.41, and 4.65% in the A₁B₂C₁, A₂B₂C₁, and A₁B₂C₂ management strategies, respectively, compared with A₁B₁C₁, when plants were subjected to salt stress during the flowering and fructification stages (Figure 3 A). It should be highlighted that there was an average recovery of 27% in fiber productivity in plants subjected to salt stress in A₂B₁C₁, A₁B₁C₂, and A₂B₁C₂. However, Khorsandi & Anagholi (2009) verified that irrigation with 10 dS m^-1 during the vegetative stage negatively affected cotton productivity during the end of the cycle. Among the analyzed genotypes, 'BRS Topázio' presented the highest fiber productivity (30.21%), which was 16.49 and 13.46% greater than that of 'BRS Rubi' and 'BRS Safira', respectively, regardless of the adopted salinized water management strategy (Figure 3 B).

Figure 3.
Means related to the fiber productivity of naturally-colored cotton (Gossypium hirsutum) plants under different salinity management strategies during phenological stages (A) and among genotypes (B) at 113 days after sowing. Distinct lowercase letters indicate significant differences among salinity management strategies by Scott-Knott’s test, at 5% probability, and distinct uppercase letters indicate significant differences among cotton plant genotypes by Tukey’s test, at 5% probability. Bars represent the standard error of the mean for management strategies (n=9) and genotypes (n=21). 1, A₁B₁C₁; 2, A₂B₁C₁; 3, A₁B₂C₁; 4, A₁B₁C₂; 5, A₂B₁C₂; 6, A₂B₂C₁; and 7, A₁B₂C₂, where A₁, B₁, C₁ represent no salinity during the vegetative, flowering, and fructification stages, and A₂, B₂, C₂, salinity during the vegetative, flowering, and fructification stages.

The highest plume cotton mass values were observed for the 'BRS Topázio' genotype at 113 DAS (Table 5). Regarding salinity management, the highest means were obtained for the A₁B₁C₁ and A₂B₁C₁ strategies. It is important to emphasize that 'BRS Safira' plants irrigated with salinized water during flowering in A₁B₂C₁ showed the highest means of 72 g per plant; the A₁B₁C₂, A₂B₁C₂, A₂B₂C₁, and A₁B₂C₂ management strategies resulted in the lowest means.

Table 5.
Mean test for the unfolding of the interaction between cotton (Gossypium hirsutum) plant genotypes and management strategies for plume cotton mass at 113 days after sowing⁽¹⁾.

Management strategy	Genotype
	BRS Rubi	BRS Topázio	BRS Safira
A1B1C1	84.33±5.53aB	125.33±3.54aA	87.66±3.32aB
A2B1C1	73.33±2.51aB	114.66±1.04aA	81.00±6.87aB
A1B2C1	54.66±4.07bC	90.33±3.05bA	72.00±2.29aB
A1B1C2	44.66±2.02cB	63.00±3.27cA	49.66±0.57bAB
A2B1C2	40.33±3.54cB	62.66±2.56cA	43.33±2.56bB
A2B2C1	42.00±6.94cB	68.00±3.60cA	43.00±3.60bB
A1B2C2	34.33±2.25cA	45.66±1.60dA	35.66±4.07bA

• ⁽¹⁾
  Means followed by distinct lowercase letters indicate significant differences among salinity management strategies by Scott-Knott’s test, at 5% probability, and followed by distinct uppercase letters indicate significant differences among cotton plant genotypes by Tukey’s test, at 5% probability. Number ± standard error. A₁, B₁, C₁, no salinity during the vegetative, flowering, and fructification stages; and A₂, B₂, C₂, salinity during the vegetative, flowering, and fructification stages.

The results obtained in the present study revealed that the effects of salinity on naturally-colored cotton production are related to stress during the flowering and production stages. Production decreases due to ECw increases may be attributed to lower water absorption by plants under salt stress because of the lower soil water potential, as concluded by Cordão Sobrinho et al. (2007). This was confirmed by the results of the present study, in which the effects of salinity were greater on gas exchange than on a chlorophyll fluorescence, i.e., water restriction due to increased water salinity caused reductions in stomatal conductance, transpiration, and liquid photosynthesis, but did not cause irreversible damage to the photosynthetic apparatus, since the quantum yield values for photosystem II were over 0.75, showing that ionic effects were less expressive.

Conclusions

1. Irrigation with salinized water significantly reduces gas exchange in naturally-colored cotton (Gossypium hirsutum) plants during the production stage, but increases plume cotton production in the vegetative stage.

2. Irrigation with salinized water during different phenological stages of naturally-colored cotton genotypes does not compromise the quantum yield of their photosystems.

3. Among the studied genotypes, ‘BRS Topázio’ stands out due to its higher fiber productivity and plume cotton production.

Acknowledgments

To Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Capes), for scholarship granted to the first author; and to Embrapa Algodão, for providing the seeds.

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Publication Dates

• Publication in this collection
  June 2018

History

• Received
  05 June 2017
• Accepted
  04 Sept 2017