Preliminary selection of F1 maize hybrids for saline soils in the Yaqui Valley, Mexico¹

Seleção preliminar de híbridos de milho F1 para solos salinos no Vale Yaqui, México

ABSTRACT

Hybridization as part of a maize breeding program for salinity tolerance can contribute to increasing the profitability of saline soils and mitigating the deleterious effects of salt stress on plants. The present study aimed to evaluate the physiological and grain yield performance of 42 F1 hybrids obtained from a diallelic design based on Griffing’s method I to develop a preliminary selection of the best hybrids for use in moderately saline soil for future research in the Yaqui Valley, Sonora, Mexico. These crosses were evaluated under moderately saline soil conditions, in a lattice (7 × 7) design with four replications. Six variables related to plant gas exchange, and grain yield were evaluated. ANOVA was performed for all evaluated variables, and when significant differences were found between hybrids, means were compared by Tukey’s post hoc test at 1%. Pearson correlations were estimated between all variables. Most of the variables exhibited statistical differences, with the exception of the chlorophyll content and Normalized Difference Vegetation Index (NDVI). The differences within the variables maximum photosynthesis, transpiration, water use efficiency and stomatal conductance revealed a genetic variability within the hybrids under moderate salinity conditions. These results allowed us to propose hybrids with high photosynthesis (>27 µmol CO₂ m^-2 s^-1), medium transpiration (2-3 µmol H₂O m^-2 s^-1), high water use efficiency (>8 µmol CO₂ µmol H₂O m^-2 s^-1), and high yield (> 13 t ha^-1) as selection criteria for moderately saline soils.

Key words:
Zea mays L.; gas exchange; yield; diallelic

HIGHLIGHTS:

Salinity did not affect the Normalized Difference Vegetation Index and chlorophylls in maize hybrids in saline soil.

Transpiration and water use efficiency have a significant variability under moderately saline soil.

Photosynthesis and water use efficiency are hybrid selection criteria for salinity studies.

RESUMO

A hibridização como parte de um programa de melhoramento genético de milho para tolerância à salinidade pode contribuir para aumentar a rentabilidade dos solos salinos e mitigar os efeitos deletérios do estresse salino nas plantas. O presente estudo teve como objetivo avaliar o desempenho fisiológico e de produtividade de grãos de 42 híbridos F1 obtidos a partir de um delineamento dialélico baseado no método I de Griffing para desenvolver uma seleção preliminar dos melhores híbridos para uso em solo moderadamente salino para futuras pesquisas no Vale Yaqui, Sonora, México. Esses cruzamentos foram avaliados em condições de solo moderadamente salino, em delineamento em látice (7 × 7) com quatro repetições. Foram avaliadas seis variáveis relacionadas às trocas gasosas da planta e à produtividade de grãos. ANOVA foi realizada em todas as variáveis avaliadas e quando foram encontradas diferenças significativas entre os híbridos, as médias foram comparadas pelo teste de Tukey a 1%. Correlações de Pearson foram estimadas entre todas as variáveis. A maioria das variáveis apresentou diferenças estatísticas, com exceção do teor de clorofila e do NDVI. As diferenças nas variáveis fotossíntese máxima, transpiração, eficiência no uso da água e condutância estomática revelaram uma variabilidade genética dentro dos híbridos sob condições de salinidade moderada. Os resultados permitem propor como critérios de seleção para solos moderadamente salinos aqueles híbridos com alta fotossíntese (>27 µmol CO₂ m^-2 s^-1), transpiração média (2-3 µmol H₂O m^-2 s^-1), alta eficiência de uso de água (>8 µmol CO₂ µmol H₂O m^-2 s^-1) e alta produtividade (> 13 t ha^-1).

Palavras-chave:
Zea mays L.; trocas gasosas; produtividade; dialelo

Introduction

Maize (Zea mays L.) is a cereal considered one of the main components of the human food diet (Urango, 2018). Its genetic variability is widely related to the diversity of consumption forms and uses (López-Morales et al., 2023). White grain maize produced in Mexico is ranked first, representing about 88% of the national production (Núñez-Terrones et al., 2019; González-Cruz & Torres-Rojo, 2024).

The various strategies developed by plant breeders for hybrid formation are aimed at maximizing heterosis among the selected progenies (Bravo et al., 2021). The generation of hybrids with outstanding physiological and agronomic characteristics originates from the use of inbred lines. They are also obtained from lines with a high degree of homozygosity. These latter mentioned are generated through self-fertilization (Hernández-Caldera et al., 2023). Griffing (1956) developed four methods for obtaining maize hybrids, many of which are currently used to obtain single, double, and triple hybrids (Juárez et al., 2023). Nowadays, hybrid breeding allows increasing salt tolerance of maize in order to contribute to the national food security in Mexico (Guevara-Hernández & Mariaca-Méndez, 2023).

The effects of salinity on plant growth and yield performance of several crops have been described (Chachar et al., 2020). In maize for example, excess salts in the saturation extracts reduces the photosynthetic rate (Chachar et al., 2020). In addition, it affects starch mobilization from leaves to the grain, and subsequently grain yield decreases (Sabagh et al., 2021). Obtaining hybrids with salinity tolerance can contribute to the profitability of saline soils (Maqbool et al., 2020; Hussain et al., 2023) and increase yields in areas where other crops do not reach their genetic production potential (Miao et al., 2021).

The present study aimed to evaluate the physiological and grain yield performance of 42 F1 hybrids obtained from a diallelic design based on Griffing’s method I to develop a preliminary selection of the best hybrids for use in moderately saline soil for future research in the Yaqui Valley, Sonora, Mexico.

Material and Methods

The experiment was carried out under field conditions at an area 0.5 ha of the Tecnologico Nacional de México (TecNM), Campus Valle del Yaqui (ITVY), Bacum, Sonora, located on Calle 600, Block 611, 85276 San Ignacio Río Muerto, Sonora, at an altitude of 26 m above sea level. The climate of the experimental site is classified as dry and very warm in the summer months but cooler in the winter. The soil where the crosses were established corresponds to the Vertisol aridic soil grouping, according to the US Soil Taxonomy classification (Almazán et al., 2023).

The cumulative precipitation during the five months of cultivation (December, 2022 - May, 2023) was 15.3 mm. During this period, there was significant rainfall in December, January and March. Temperatures remained below 25 °C throughout the cycle, with the lowest average temperature occurring in January (16 °C). The relative humidity remained between 61 and 73%. The highest values of this variable were recorded in the months with the highest rainfall (Figure 1).

Figure 1
Variability in climatic variables recorded from December 2022 to May 2023 at the experimental site located in the Yaqui Valley, Sonora, Mexico

Previously, soil analyses were carried out at the Soil and Plant Nutrition Laboratory belonging to “Norman E. Borlaug” Experimental Field, located in Ciudad Obregon, Sonora, Mexico. For these analyses a total of 15 randomized soil samples were collected from the experimental area at a depth of 0-30 cm (Table 1).

Table 1
Results of the soil analysis corresponding to the experimental site at the Instituto Tecnologico del Valle del Yaqui (ITVY), Mexico

Site	Depth (cm)	pH	OM1 (%)	EC2:1 (dS m-1)	HC (cm h-1)	AD (g cm-3)	Phosphorus (ppm)	Nitrogen (kg ha-1)
ITVY	0-30	7.65	0.59	4.81	<0.04	1.39	34.2	128

• ¹OM - Organic matter; EC - Electrical conductivity; HC - Hydraulic conductivity; AD - Apparent density

To obtain the genetic materials (F1 hybrids), nine homozygous lines were used, which are described in Table 2. A 7 × 7 diallelic design was established in the experimental area of the Tecnologico Nacional de México, Campus Roque (ITR), Celaya Guanajuato, Mexico, using Griffing’s I method, developed by Saavedra et al. (2021). For the morphophysiological and yield studies, only the hybrids obtained without comparing homozygous lines were used and were evaluated under moderate saline soil conditions (Bannari et al., 2016) at the ITVY, Mexico.

Table 2
Origin and inbreeding level of the maize lines used in Griffing’s diallelic cross design I

Line	Level of inbreeding*	Origin of the line	Material origin
L1 TNM 38-R	S8	Celaya Creole	TecNM-Roque
L2 TNM 144-R	S8	Celaya Creole	TecNM-Roque
L3 TNM 111-DK	S9	Pob. F2 (DK2042)	Bayer
L5 TNM 72-P	S9	Pob. F2 (P3030 W)	Corteva
L6 TNM 6-P	S9	Pob. F2 (P3030 W)	Corteva
L7 TNM 107-C	S9	Pob. F2 (Cayman)	Bayer
L9 TNM 95-C	S9	Pob. F2 (Cayman)	Bayer

• *S₈ and S₉ represent lines that were selected and that are in the eighth and ninth generation of self-fertilization, respectively.

Thirty-six direct crosses, 36 reciprocal crosses, and nine self-pollinations were performed among the nine homozygous lines of white grain maize at the ITR. The hybrids were evaluated under a partially balanced lattice (7 × 7) experimental design (Rebolloza-Hernández et al., 2020) at ITVY, where hybrids from the L4 and L8 parents could not be evaluated because they did not generate viable seeds.

The treatments consisted of the 42 remaining F1 hybrids. These treatments were completely randomized and distributed in the experimental area. Sowing was carried out manually, on December 25, 2022, in furrows with a length of 5 m, and at a distance of 0.16 m between plants and 0.80 m between rows.

Crop fertilization management was carried out with four fertilizer applications: a pre-sowing application of urea (46%) (400 kg ha^-1) and a mixture of diammonium phosphate (DAP) (100 kg ha^-1) 15 days after sowing (Figueroa et al., 2021). During the experiment, three other fertilizer applications were made prior to the auxiliary irrigation at a rate of 150 kg ha^-1 of NPK (18-18-18).

The presence of pests, such as thrips (Frankliniella occidentalis) and the fall armyworm (Spodoptera frugiperda), was significant in the early stages of the crop. For its control, the insecticides Rimon^® Supra and Decís^® (deltamethrin) were preventively applied at a dose of 1.0 L ha⁻¹ in a single application.

For weed control and in order to provide aeration in the soil, two cultivator passes were carried out. Besides, when some other weeds were found they were removed manually (weeding, when necessary) to eliminate possible hosts of pests and diseases.

On December 25, 2022, immediately after sowing the first post-sowing surface irrigation was applied, with an average irrigation depth of 250 mm. Subsequently, four auxiliary irrigations were performed with irrigation intervals of 39, 30, 24 and 15 days for maintaining acceptable moisture (88%) during the phenological period of the crop, with irrigation depths of 110, 120, 110 and 110 mm, respectively.

Maximum photosynthesis [A(max)] (µmol CO₂ m⁻² s⁻¹) and transpiration (E) (µmol H₂O m⁻² s⁻¹) were measured, during the flowering phenophase, using an LI-6400XT system (Portable Photosynthesis System, LI-COR™, Lincoln, NE, USA). The measurements were carried out on five well irradiated leaves in ten plants per hybrid at 10:00 am - 12:00 pm. Three replicates per leaf were taken automatically and stored in the IRGA memory. Measurements were performed at a saturated photon flux density of at least 1,500 μmol m⁻² s⁻¹ and a CO₂ flux of 400 µmol CO₂.

Stomatal conductance (gs) was also measured with the same equipment (IRGA) during the A and E mensuration. Leaf water use efficiency (WUE) was calculated by the relation between A (max) and E and expressed as µmol CO₂ m⁻² s⁻¹ (Argentel-Martínez et al., 2019).

Chlorophyll (CHL) was measured within three replicates per leave in five well irradiated leaves for each hybrid, with a portable SPAD-type device. Measurement was carried out in the central region of leaves. This measurement was made at 10:00 am on the day of the flowering phenophase.

The normalized difference vegetation index (NDVI) was determined with a portable GreenSeeker device (Green Seeker, Trimble™ brand, Sunnyvale, CA, USA) (Govaerts & Verhulst, 2010), with three measurements per plant on a total of 10 plants per hybrid in the flowering phenophase. Higher NDVI values (-1<NDVI>1) indicate a better plant nutritional status (Inman et al., 2005).

For grain yield evaluations (at 165 days after sowing) of each hybrid, the methodology proposed by Medina-Méndez et al. (2019) was used (Eq. 1):

$$GY=\frac{10,000\times Wpp}{Ha}$$ (1)

where:

GY (kg ha⁻¹) - grain yield at 14% moisture;

Wpp - weight per plot at 14% moisture; and,

Ha - harvested area in ha.

Simple rank analysis of variance of fixed effects were performed to identify possible significant differences among the 42 F1 hybrids evaluated for all analyzed variables (Fisher, 1937). The statistical indicators mean square error (MSE), Fisher’s calculated value (F value), error probability when accepting the null hypothesis (p), standard error of the mean (SE), and unadjusted coefficient of determination were presented. When there were significant effects among hybrids, the Tukey’s multiple comparison of means test was used for a significance level of 1%. A Pearson correlation network analysis was performed to assess the relationships between all the evaluated variables. All analyses were performed with the professional statistical package STATISTICA, version 14.0 for Windows. The program Rbio version 166 (Bhering, 2017) was used to construct the Pearson correlation network.

Results and Discussion

Table 2 shows that the soil of the experimental area had an electrical conductivity of 4.81 dS m^-1, demonstrating the presence of moderate salinity level. According to the electrical conductivity of the saturation extract, the soil is classified as a moderately saline (Bannari et al., 2016; Trasviña et al., 2018). Even so, the soil pH is around neutrality (7.65), being within the range considered good for maize growth and yield (Sirisuntornlak et al., 2021; Campolo et al., 2022).

The organic matter content in the soil was very low, but the lack of organic fertilizer was counteracted by the application of the fertilization technology package for maize crop (López-Morales et al., 2019).

As it is shown in Table 3, maximum photosynthesis (A), transpiration (E), water use efficiency (WUE), stomatal conductance, and grain yield (GY) exhibited highly significant effects, demonstrating the existence of a great variability among the hybrids obtained (p ≤ 0.01). The variables with the least variability were chlorophyll content and the NDVI, with no significant differences among the hybrids (Table 3).

Table 3
Components of the analysis of variance: MSE (mean square error), F value (Fisher value), and P value (probability), standard error of mean (SE), coefficient of variation (CV) and unadjusted coefficient of determination (R²) of the response variables analyzed in the 42 F1 hybrids obtained in the Yaqui Valley, Mexico

Variables response	MSE	F value	P value	SE	CV (%)	R2
Maximum photosynthesis	5.52	2.303	0.00063	0.36	9.50	0.96
Transpiration	0.21	2.279	0.00072	0.07	21.62	0.98
Water use efficiency	3.63	2.37	0.00042	0.29	19.87	0.96
Stomatal conductance	117	2.96	0.00001	1.67	3.07	0.87
Chlorophyll	2.4	1.1	0.34472	0.24	2.42	0.45
NDVI	0.004	0.51	0.99087	0.01	7.11	0.49
Grain yield	0.44	9.01	0.00003	0.19	5.22	0.97

• NDVI - Normalized difference vegetation index

For the variables A, E, WUE, and GY, the linear fixed effects model used for the analysis of variance contributed more than 95% to the identification of differences between hybrids (R²) (Table 3). Only for the stomatal conductance variable the coefficient of determination was less than 0.9, but still, it was possible to identify differences between hybrids.

The chlorophyll content and NDVI were not significantly affected, and the linear mathematical model of fixed effects only explained the total variability in 45 and 49%, respectively, so it was not effective for the detection of significant differences among the hybrids. For the gas exchange variables (A, E, WUE, gs) and grain yield, the coefficient of variation ranged from 3 to 22%, which shows the precision of the data obtained in the experiment still under field conditions. In the present study, the highest standard error found was associated with stomatal conductance (1.67), which is due to the great variability of this variable, which manifests even within the same plant (Wang et al., 2018; Liao et al., 2022).

For NDVI and chlorophyll none of the 42 hybrids differed significantly but both variables had high values (0.81 and 64.11 SPAD units, respectively), which indicate good physiological and nutritional status of the plant in terms of nitrogen metabolism (Pan et al., 2024). Our results for these two variables indicate that all hybrids showed salt tolerance, and this stressor condition did not generate significant effect. However, some recent studies have reported that soil salinity has a negative effect on NDVI and chlorophyll content in maize (Shahzad et al., 2019; Qi et al., 2021; Vennam et al., 2024).

When evaluating maximum photosynthesis, the hybrids were divided into four groups (Table 4) according to Tukey’s test (p < 0.01). Group I, composed of only two hybrids, had the lowest average photosynthetic activity, equal to 19 µmol CO₂ m^-2 s^-1. This value is considered low for photosynthesis in maize during the flowering phenophase, according to reports by Meena et al. (2021). These authors found values between 27 and 40 µmol CO₂ m^-2 s^-1 in five maize hybrids evaluated between 20 and 50 days after anthesis. Group II, composed of 21 hybrids, had photosynthetic activity values of 23 µmol CO₂ m^-2 s^-1 on average. The third group included 18 hybrids, whose photosynthesis values were greater than 27 µmol CO₂ m^-2 s^-1. Group IV, formed by the hybrid obtained from the L6XL7 cross, had the highest photosynthetic activity, reaching 29 µmol CO₂ m^-2 s^-1. In a study carried out by Ahammed et al. (2020), they found significant reduction (6 µmol CO₂ m^-2 s^-1) in the photosynthetic activity of maize hybrids due to salinity effects when the plants were grown at EC= 3-4 dS m^-1. The variability found between groups of hybrids in the present study was 10 µmol CO₂ m^-2 s^-1, with the particularity that this variation was only obtained in two hybrids (L7XL9 and L7XL3).

Table 4
Maximum photosynthesis [A(max)] and transpiration (E) in the flowering phenophase in the formation of groups (G) according to the evaluation of 42 maize hybrids grown in the Yaqui Valley, Sonora, Mexico

Groups	Hybrids	A (max) (μmol CO2 m-2 s-1)	Hybrids	E (μmol H2O m-2 s-1)
I	L7XL9, L7XL3	19.8 ± 2.2 c	L2XL6, L3XL7, L1XL2, L6XL1	1.46 ± 0.1 c
II	L1XL2, L3XL7, L7XL1, L2XL1, L2XL9, L5XL3, L6XL3, L7XL5, L6XL9, L9XL7, L9XL6, L1XL6, L2XL5, L3XL9, L5XL7, L3XL6, L2XL3, L5XL1, L3XL1, L1XL7, L9XL5	23.10 ± 4.7 bc	L6XL7, L6XL9, L3XL9, L2XL5, L9XL5, L2XL3, L2XL9, L1XL3, L3XL6, L3XL5, L5XL1, L9XL6, L6XL2, L7XL5, L5XL3, L9XL1, L1XL7, L6XL3, L7XL2, L7XL6, L7XL9, L7XL3, L7XL1, L1XL5, L1XL6, L5XL7, L9XL3, L1XL9, L3XL1, L2XL1, L5XL6, L3XL2, L9XL7, L9XL2, L2XL7	2.11 ± 0.2 abc
III	L7XL6, L9XL2, L5XL6, L3XL5, L9XL1, L1XL5, L6XL2, L1XL9, L7XL2, L6XL1, L3XL2, L5XL2, L2XL6, L2XL7, L9XL3, L1XL3, L5XL9, L6XL5	27.03 ± 4.6 abc	L5XL2, L6XL5	3.13 ± 0.4 ab
IV	L6XL7	29.05 ± 4.6 abc	L5XL9	3.37 ± 0.2 a

Increasing photosynthetic efficiency in breeding programs is important for ensuring better grain yields, and it has been proven that there is a positive correlation between these two variables (Li et al., 2015; Yan et al., 2021). Photosynthesis is one of the most important physiological processes affecting maize yield (Hessini et al., 2019).

Transpiration measurements allowed us to divide the hybrids into four groups (Table 4). Groups 1 and 2 had transpiration values of 1.5 and 2.1 µmol H₂O m^-2 s^-1, respectively; therefore, they can be classified as hybrids of low transpiration (Liu et al., 2023b). The low transpiration obtained was possibly due to the adverse effects of salinity, which generated greater tension in the xylem and hindered normal water absorption (Feng et al., 2021).

In Group 1, the hybrids L2XL6, L3XL7, L1XL2 and L6XL1 were grouped, which had the lowest transpiration values. This result could favor water use efficiency in hybrids with high photosynthetic activity, which would presumably translate into high intrinsic water use efficiency (Liu et al., 2023b; Wang et al., 2023). Group II had the highest number of hybrids (35) and exhibited moderate transpiration (Table 4). Groups III and IV had values higher than 3 µmol H₂O m^-2 s^-1 and were formed by two hybrids (L5XL2 and L6XL5) and one hybrid (L5XL9).

Water use efficiency showed variability, forming three groups (Table 5). In all the cases, the water use efficiency was greater than 7 µmol CO₂ µmol H₂O m^-2 s^-1, which indicated high efficiency for CO₂ entering into the substomatal chamber of the leaves (Fátima et al., 2023). The high water use efficiency values found in the hybrids resulted from the low transpiration (Table 5) exhibited by the hybrids. The water use efficiency values were high in hybrid Groups II and III, and this increase was due to low transpiration combined with high maximum photosynthesis (Table 4). These results are in agreement with those obtained by Han et al. (2023), who studied the effect of water regime regulation and high CO₂ concentrations on photosynthetic parameters and stomatal conductance in maize and obtained values of 8-13 µmol CO₂ µmol H₂O m^-2 s^-1.

Table 5
Leaf water use efficiency (WUE) of 42 maize hybrids evaluated in the flowering phenophase grown in the Yaqui Valley, Sonora, Mexico

Groups	Hybrids	WUE (μmol CO2 µmol H2O m-2 s-1)
I	L2XL1, L2XL7, L5XL9, L9XL2, L9XL7, L7XL3, L7XL1, L7XL9, L5XL6, L3XL1	7.68 ± 1.18 c
II	L3XL2, L6XL5, L5XL7, L1XL9, L6XL3, L1XL6, L1XL5, L6XL7, L5XL3, L7XL5, L2XL9, L9XL1, L9XL6, L7XL6, L5XL1, L7XL2, L3XL6, L6XL2, L3XL5, L6XL9, L1XL7, L2XL5, L1XL3, L9XL5, L3XL9, L1XL2, L9XL3, L6XL1, L2XL3, L5XL2, L3XL7	10.22 ± 1.4 ab
III	L2XL6	14.67 ± 1.4 a

Four homogeneous groups were formed based on stomatal conductance values (Table 6). These conductance values in maize are considered high, which allows better gas exchange within the plant to favor photosynthetic activity and the future accumulation of dry matter in the leaves.

Table 6
Stomatal conductance (gs) of 42 maize hybrids evaluated in the flowering phenophase grown in the Yaqui Valley, Sonora, Mexico

Groups	Hybrids	gs (μmol m-2 s-1)
I	L2XL5, L3XL9	279.1 ± 9.2 c
II	L1XL9, L7XL3, L6XL9, L2XL9, L2XL7, L1XL2, L5XL7, L5XL1, L9XL5, L5XL3, L7XL9	316.1 ± 8.4 b
III	L1XL6, L1XL6, L6XL2, L3XL2, L3XL1, L7XL6, L7XL1, L6XL3, L5XL6, L2XL3, L9XL1, L7XL5, L7XL2, L9XL6, L1XL5	387.2 ± 4.2 ab
IV	L9XL3, L9XL3, L3XL6, L6XL7, L9XL7, L9XL7, L3XL5, L5XL2, L2XL1, L1XL3, L6XL5, L1XL7, L6XL1, L3XL7, L2XL6	392.2 ± 5.1 a

Stomatal conductance is an important variable in initial breeding programs. High gs values indicate the ability of plants to maintain, under any condition, an adequate water status, as obtained by Ma et al. (2023) under salinity stress conditions in cotton (Gossypium herbaceum). Similarly, Malini et al. (2023) reported similar results under high-temperature stress conditions. However, the regulation of stomatal conductance is important for ensuring water use efficiency. A transcription factor (ZmNAC20) recently identified in maize controls gs under drought stress conditions and activates the expression of drought stress response genes to avoid water imbalance (Liu et al., 2023a).

Grain yield was the variable with the greatest variability (Table 7), and five homogenous groups were formed. There were 15 hybrids (35%) with yields between 13 and 14 t ha^-1 in the groups IV and V (Table 7). On the other hand, 18 hybrids (42%) had an average yield between 12 and 12.8 t ha^-1 (group III), and only nine hybrids (21%) had yields between 10 and 11.9 t ha^-1 (groups I and II). These results showed that more than 90% of the F1 hybrids obtained had a yield potential higher than 11 t ha^-1.

Table 7
Grain yield of 42 maize hybrids grown in the Yaqui Valley, Sonora, Mexico

Groups	Hybrids	Grain yield (t ha-1)
I	L3XL9, L7XL3	10.63 ± 0.6 e
II	L7XL9, L2XL7, L9XL3, L9XL2, L5XL1, L9XL7, L5XL7	11.35 ± 2.6 d
III	L5XL3, L5XL6, L9XL5, L3XL5, L3XL7, L1XL9, L7XL2, L1XL2, L1XL6, L3XL1, L2XL5, L2XL9, L9XL6, L6XL5, L5XL9, L2XL3, L1XL5, L6XL2	12.06 ± 1.3 c
IV	L2XL1, L7XL1, L6XL9, L6XL3, L3XL6, L3XL2, L7XL6, L9XL1	13.59 ± 0.7 b
V	L1XL3, L2XL6, L7XL5, L6XL7, L1XL7, L5XL2, L6XL1	14.47 ± 0.8 a

The obtained yield from these hybrids is considered high compared to the average yield of maize in Mexico in 2023, which was 4.6 t ha^-1 under irrigated production systems (González-Abraham et al., 2023). Grain yields of the present experiment are also similar to those of some hybrids that have recently been obtained and evaluated in different high valleys of Mexico under non-saline soil (Martínez-Gutiérrez et al., 2018; Sierra-Macías et al., 2023).

The greatest yield difference was found in the present study between groups V and I, being approximately 4 t ha^-1, which shows that salinity does modify the agronomic response in some hybrids. The essential physiological response to low water availability is a function of low transpiration in some hybrids (Riveros-Burgos et al., 2023). For example, in our study, L7XL9 and L3XL7 caused a decrease in the maximum photosynthesis of the plants, leading to lower yields compared to the others. A similar result was reported by Song et al. (2020) for maize under drought conditions. Salinity produces a significant reduction in soil water potential, which leads to associated water stress conditions in plants (Liu et al., 2022); this behavior could have occurred in these hybrids in the present investigation.

The present research constitutes an advance in understanding the actions developed to increase grain yield in saline soils by monitoring germplasm with tolerance to this type of stress, since maize is classified as a crop moderately susceptible to soil salinity (Beyaz & Xin, 2023; Ali et al., 2023). Based on the obtained grain yield, the best hybrids for future agronomic performance evaluation programs were L1XL3, L2XL6, L7XL5, L6XL7, L1XL7, L5XL2, and L6XL1 (Table 7). These hybrids had also the highest values of gas exchange.

Figure 2 shows the correlations obtained between the variables evaluated in the maize hybrids. Chlorophyll exhibited a low correlation (0.29) with NDVI, but it was significant (p < 0.01). In addition, chlorophyll content was correlated with gs, with a correlation of 0.21 (p < 0.05). These correlations, although positive and low, indicate that the hybrids exhibited better gas exchange, favoring photosynthetic activity and dry matter accumulation in the leaves. These findings are also associated with adequate NDVI values, which are indirect indicators of good mineral and carbon nutrition.

Figure 2
Pearson correlations between the variables studied in the 42 white grain maize hybrids obtained in the Yaqui Valley, Sonora, Mexico

Photosynthesis showed moderate and highly significant correlations with transpiration (0.56, p < 0.001) (Figure 2). These relationships confirmed that the hybrids obtained exhibited high photosynthesis values at the same time as high values of E and gs, thus increasing their capacity to adapt to saline soils if we consider that the experiment was carried out on a moderately saline soil (Table 1).

Transpiration showed a low positive and highly significant correlation with gs (0.33, p < 0.001), while it showed a high negative and highly significant correlation with WUE (-0.76, p < 0.001) (Figure 2). These relationships indicate that, at the same time, the hybrids that had low transpiration exhibited low gs and high WUE, thus allowing improved adaptation to the growing conditions and improved photosynthetic efficiency.

Finally, the values obtained for the hybrids revealed a positive correlation between the variables GY and WUE, with low correlation that was weak significant (r =0.20, p < 0.05) (Figure 2). This relationship indicates that, as the grain yield of the hybrids increased, the WUE increased, which favors the response to the experimental conditions, thus allowing better adaptation. The greater efficiency of water use guarantees better survival under conditions of water stress, characteristic of the region where the experiment was conducted.

The results of the present study allow us to propose hybrids with high photosynthesis (A>27 µmol CO m^-2 s^-1), medium transpiration (2 < E < 3 µmol H₂O m^-2 s^-1), high water use efficiency (WUE>8 µmol CO₂ µmol H₂O m^-2 s^-1), and high grain yield (GY > 13 t ha^-1) as salt tolerant. These variables can be used as effective initial criteria for selection of hybrids under moderately saline soils.

Conclusion

1. The soil salinity condition of the Yaqui Valley caused a significant variability of physiological and agronomic traits in the 42 F1 hybrids evaluated.

2. The gas exchange variables correlated positively with grain yield in the evaluated hybrids, demonstrating the contribution of these variables to the identification and selection of new hybrids.

3. This approach allowed the selection of the hybrids L1XL3, L2XL6, L7XL5, L6XL7, L1XL7, L5XL2, and L6XL1 as tolerant to moderate soil salinity.

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• ¹ Research developed at Tecnológico Nacional de México, Campus Valle del Yaqui, Bácum, Sonora, México

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Edited by

• Editors: Ítalo Herbet Lucena Cavalcante & Hans Raj Gheyi

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Publication Dates

• Publication in this collection
  10 Feb 2025
• Date of issue
  2025

History

• Received
  15 Feb 2024
• Accepted
  11 Dec 2024
• Published
  15 Jan 2025