Physiological parameters of soybeans under water stress

Soybean production in Brazil has been growing annually, becoming the world leader in soybean production and exports as of 2020. Commercial competitiveness requires the soybean sector to invest in and develop research and technologies to increase production per area (m²) and efficient management practices, making soybean cultivation increasingly sustainable and efficient. Therefore, understanding the factors that affect soybean productivity plays a fundamental role in directing research.

Among the main factors that reduce soybean productivity, we can highlight the non-use of good quality seeds, low use of fertilizers, low efficiency in controlling weeds, diseases and pests and the “dreaded” water deficiency. Water is one of the most important elements for maximum soybean productivity to be achieved in a crop. Soybean crops have different water needs throughout the cycle, with the reproductive phase being the one with the greatest demand.

Depending on the stage of development the plant is in during this stress, the damage can become irreversible and significantly reduce production potential. In the southern region of Brazil, this phenomenon has been occurring more frequently, in the last three harvests 2019/2020, 20/21 and 21/22 losses reached average values ​​between 40 and 50%, according to data from EMATER/RS.

Water demand for soybean crops

Soybean cultivation demands between 450 and 700 mm of water throughout the cycle, being considered sensitive to water stress, especially in the critical period at the beginning of reproductive development, causing the abortion of vegetables and grains. In order to ensure good plant germination in the field, during emergence it is necessary to absorb at least 50% of the seed's weight in water. The water requirement increases throughout the development of the crop, reaching approximately 7 to 8 mm/day during the flowering and grain filling phases.

Based on this information, we can manage the critical period of the crop with the positioning of sowing time, irrigation, cultivars, among other strategies, aiming to reduce the effect of water deficit in the most sensitive phenological stages.

Tests with new genetic materials in Brazil

A good drought-tolerant soybean cultivar can escape the effects of water stress by increasing the depth of the roots in the soil, reducing the expansion of the leaf area, closing the stomata, maintaining a higher relative water content, greater water potential and turgor pressure. Leaf water potential has a smaller reduction in new soybean cultivars under stress, higher concentrations of Abscisic Acid (ABA), inducing earlier stomatal closure, reducing water losses, increasing the concentration of solutes and generating a greater osmotic adjustment, when We compare it with the oldest ones.

In a study carried out by the Federal University of Rio Grande do Sul (UFRGS), cultivars called Conventional TMG, Transgenic TMG (Gene HaHb4) and commercial cultivars (Vmax RR and TMG 7262 RR, which is resistant to water deficiency) were used. these were subjected to water deficit in the reproductive stage. The aim of the work was to evaluate the physiological parameters of cultivars under the effects of water deficit. Based on the evaluations, there was no statistical difference for the cultivars in relation to water deficit, only for days with water deficit.

The treatments used to evaluate the cultivars' response to physiological parameters were: (I) it consisted of ceasing irrigation of the cultivars for six consecutive days and after this period, irrigating them for four days. In the second treatment (II) - control - there was daily irrigation during the 10 days of the experiment.

Effect of water deficit on physiological parameters of soybean crops

The physiological parameters of soybean cultivars under water stress during the crop's reproductive period can be measured by some soybean development indices, such as: photosystem II quantum yield (Yield), relative water content in the leaf (CRAF), water in the leaf and dry mass accumulated in the tissues (dry biomass of the aerial part), variables shown in figure 2.

The quantum yield of photosystem II (Yield), which evaluates the photochemical activity of the plant, representing the capture of excitation energy by the reaction centers of photosystem II and can represent the efficiency of electron transport through this photosystem, that is, we measure the efficiency of plant photosynthetic activity with or without water deficit.

Yield results indicated that during the beginning of water stress, plants showed similar behavior between irrigated and stressed. However, from the 4th day under water stress, the plants showed a significant reduction in photosynthetic capacity assessed by “Yield” and even after the return of irrigation, we found that the crop was unable to return the same photosynthetic rate as plants without stress. , this is due to the damage caused by water deficit in plants (Figure 3).

Another physiological parameter of great importance is the relative water content in the leaf (CRAF). This parameter is an indicator of the state of water in the plant, which when subjected to water stress, reduces the water content in the leaves. Reducing the CRAF rate reduces cell growth and, consequently, the plant's height and leaf area. Therefore, CRAF is a growth parameter and a measure of the water status of the plant under water deficit conditions. After induction of water stress, CRAF showed a reduction of 80% and reached 38% on the sixth day, while irrigated plants always remained above 80% (Figure 4). However, it is important to highlight that, from the 4th day after the beginning of the water deficit, the effects on the crop are detected.

Dry shoot biomass production (MSPA) is an excellent physiological and morphological parameter to evaluate plant development and its efficiency in accumulating biomass during the development period. With the incidence of water deficit in soybeans, it was identified that from the fourth day onwards there is a reduction in mass production by the plants (Figure 5). This reduction in mass production is due to a reduction in the leaf area index and a consequent reduction in photosynthetic activity.

Regarding the minimum water potential in the leaf (ᴪmin), in the test carried out in the reproductive period, there was a significant difference between the stress levels in the reproductive period, for days 2, 4, 5, 6 (Figure 6). For all days evaluated, the irrigated treatment showed a higher minimum leaf water potential. When evaluating the response of soybeans to water conditions during the reproductive period, it is expected that the minimum water potential in the soybean leaf (ᴪmin), without irrigation and with irrigation, will present negative values ​​(between -0,5 to -2,5 Mpa), indicating that the crop is under soil water limitations. In the development work during the reproductive period, it was found that the non-irrigated (stressed) treatment presented values ​​of (ᴪmin) around -3,5 Mpa on the 6th day under water deficit, indicating severe stress on the plant.

Final considerations

The evaluated ecophysiological parameters responded significantly and positively under conditions of water stress during the reproductive period, highlighting the negative effects when compared to adequate conditions of water availability.

This study demonstrated that the resilience of soybean cultivars in the face of adverse development conditions is decisive for maintaining productivity rates, in addition to highlighting the benefits of advances in plant genetic improvement.

By Anna Elisa Petersen Gatelli, UFRGS; Elizandro Fochesatto, UNIARP; João Paulo Vanin, SLC Agrícola SA; André Luis Vian, UFRGS; Christian Bredemeier, UFRGS