Gibberellic Acid Utilization in Seeds and Plants of Beans : Effect on Growth and Seeds Physiological Quality

The growth regulators from gibberellins, when applied on plants, might improve physic and physiological features, stimulate cell division and elongation, this study aimed at evaluating the influence of gibberellic acid doses applied exogenously on bean crop through seed treatment, and analyzing its effects on morphological attributes of seedlings and physiological quality of the seeds produced. The experimental design was completely randomized design, with treatments corresponding to the doses: zero (distilled water only), 50, 100 and 200 mg L, arranged in four replicates. The use of gibberellic acid in bean seeds did not result in changes on the number of leaves, leaf area, dry mass of leaves and stem. Applications of foliar gibberellic acid potentiated stem height and root dry mass of seedlings.


Introduction
Beans (Phaseolus vulgaris L.) present great importance for Brazilian agriculture in the social and economic extent, where more than 3.2 million hectares were grown in the 2017/2018 agricultural year, in three cultivation periods (CONAB, 2018).It is characterized as one of the main leguminous, and daily relates to majority of Brazilians' diet due to its high protein and energetic value (Moreira et al., 2017).Brazil is the main producer and consumer in the world, with per capita annual consumption of 16 kg of grains (Barbosa et al., 2010).However, the national yield is low, being around 465 kg ha -1 in the 2016/2017 agricultural year (CONAB, 2017).It is due to low utilization of certified seeds and investments in genetic breedingfor specific micro regions, number of genotypes and precarious studies regarding management practices (Ribeiro et al., 2008).
Bean plants feature low first pod insertion height, which difficult mechanized harvest and increase losses (Marcos Filho et al., 2009;Silva et al., 2008).In order to minimize losses and increase yield per unit area, some researches are being carried out aiming at the development of highly yielding genotypes, which depend on genetic breeding, as well as the improvement of morphological and physiological traits such as harvesting rate and photosynthetic efficiency (Buzello, 2010;Carvalho et al., 2016).
The available genotypes present good amplitude regarding sowing timing, plant architecture and favorable vegetative growth for increased grain and seed yield, however, increments in several other desirable agronomic attributes are still sought (Souza et al., 2010;Szareski et al., 2018).In this relation, researchesapproaching the use of growth regulators have the purpose of modifying plants architecture and improving practices and management for crops of interest (Lana et al., 2009).Growth regulators modify the plant morphologically in a way that growth and development are either promoted or inhibited (Kappes et al., 2011).The regulators may be composed of indolebutyric acid, kinetin and gibberellic acid, which act to increase plant growth and development, increase cell wall extensibility, and absorb water and nutrients (Vieira & Castro, 2002;Taiz & Zeiger, 2013).
Giberellin sconstitute a group of plant hormones that act in the elongation and cell division (Buchanan, 2015), which are synthesized in the form of gibberellic acid (Carraher Jr. et al., 2010).This regulator's concentration and plant tissues sensitiveness to this compound determine plants response regarding growth and development (Espindula et al., 2010).Thus, the exogenous application of gibberellic acid in bean plants may directly influence plant growth, as well as its morphological and reserve attributes (Troyjack et al., 2017).Considering that growth regulators from gibberellins, when applied on plants, might improve physic and physiological features, stimulate cell division and elongation, this study aimed at evaluating the influence of gibberellic acid doses applied exogenously on bean crop through seed treatment, and analyzing its effects on morphological attributes of seedlings and physiological quality of the seeds produced.

Material and Methods
The trials were conducted in the Campus of the Federal University of Pelotas, under coordinates 31º52′ S and 52º21′ W. In order to determine the influence of exogenous gibberellic acid application (GA 3 ) on bean crop through seed treatment and foliar spraying, two experiments were performed: In experiment I, the influence of different GA 3 doses on morphological attributes of seedlings was verified.The application was carried out directly in the seeds with the aid of a graduated pipette.These seeds were placed in transparent polyethylene bags with capacity for 1.0 kg for homogenization and product adhesion.The doses used were zero (distilled water only), 50, 100 and 200 mg L -1 .After 5 minutes, 20 seeds were arranged to germinate in rolls formed by three sheets of germitest paper, moistened with distilled water in the proportion of 2.5 times the dry mass of the paper.After the linear arrangement of the seeds on the sheets, the rolls formed were directed to germination chamber type B.O.D. at constant temperature of 25 ºC.On the eighth day after sowing, the following characters were measured: number of leaves (results expressed in units), plant height (distance between base and stem apex, results in centimeters), leaf area (obtained through Licor meter LI-3100 ® , results expressed in square meters), dry mass of leaf, stem and root (obtained by drying the seedlings in oven with forced air ventilation at 70 ºC until constant mass, with subsequent mass measurement in grams).
In the experiment II, it was evaluated the influence of exogenous GA 3 application through foliar spraying on seeds physiological quality.The seeds were sown in polyethylene pots of 20 liters of capacity, with substrate composed of soil from the A1 horizon of a Planosol Haplic Eutrophic Solodic (Streck et al., 2008).The corrections and fertilization were performed according the technical regulations (CQFS RS/SC, 2004).Bean plants at V2 phenological stage were submitted to aerial application of gibberellic acid (GA 3 ) at doses zero (distilled water only), 50, 100 and 200 mg L -1 .Seeds were manually harvested with approximately 16% of humidity.After, they were dried in oven with forced air circulation at 35 °C, and dried in a steady dryer until homogeneous humidity of 12%.The seeds were manually harvested and stored in a cold room with average temperature of 15 °C and relative humidity of 60%.On this occasion, the following characters were measured: First Germination Counting (FGC): carried out along the germination test, being counted at five days after sowing, according to the Rules for Seed Analysis (MAPA, 2009).The results were expressed as percentage of normal seedlings.The germination test was performed through four samples of 50 seeds, arranged to germinate in rolls formed by three sheets of germitest paper, moistened with distilled water in the proportion 2.5 times the dry mass of the paper.After the linear arrangement of the seeds on the sheets, the rolls formed were later transferred to a germination chamber type B.O.D. at 25 °C.The evaluations were performed nine days after sowing, whit results expressed as percentage of normal seedlings (MAPA, 2009).
Aerial Part Length (APL) and Length of Primary Root of Seedlings (RL): verified from four samples of 10 seedlings, at the end of germination test.The length of aerial part was obtained by the distance between primary root base and aerial part apex, determining the length of primary root through the distance between the extremities of this structure, results in centimeters.
Root Dry Mass (RDM) and Dry Mass of Aerial Part (DMAP): Achieved through the mass of 10 seedlings at the end of germination test, these seedlings were packed in brown paper envelopes and subjected to forced ventilation drying under temperature of 70 °C for 72 hours, root and shoot dry mass were obtained separately for each structure, results in centimeters (g tissue -1 ).
The experimental design was completely randomized design, with treatments corresponding to the doses: zero (distilled water only), 50, 100 and 200 mg L -1 , arranged in four replicates.The data were submitted to the statistical model assumptions, and then, to analysis of variance by the F test at 5% of probability, separately for each experiment.When checking the significance, the traits were submitted to the diagnosis of the highest significant polynomial degree by the t test at 5% of probability.Furthermore, the linear regression was performed for each trait in order to identify the tendencies for the quantitative levels tested.

Results and Discussion
The analysis of variance revealed significance at 5% of probability for stem height and root dry mass of seedlings submitted to GA 3 applied on the seeds (Table 1), and root length, dry mass of aerial part and root dry mass of seedlings when GA 3 was applied in the plants (Table 2).The traits number of leaves, leaf area, leaf dry mass, stem dry mass, were not significantly influenced by the treatments with GA 3 applied via seed (Table 1).The length of aerial part, first germination counting, germination and emergence atfield presentedno significance for physiological quality of the seeds produced.
Table 1.Summary of the analysis of variance with mean squares for number of leaves (Nl), stem height (Sh), leaf area (La), leaf dry mass (DMl), stem dry mass (DMs) and root dry mass (DMr) of beans in response to different GA 3 doses applied in the seeds Note.Mean squares: * and ns: significant at 5% of probability and non-significant, respectively; CV: coefficient of variation; FV: factor of variation.
Table 2. Summary of the analysis of variance with mean squares for length of aerial part (Lap), root length (Rl), dry mass of aerial part (DMap), root dry mass (DMr), first germination counting (FCG), germination (G), and emergence at field (EF) in response to different GA 3 doses through leaf application in the matrix plant Note.
(1) Mean squares: * and ns: significant at 5% of probability and non-significant, respectively; CV: coefficient of variation; FV: factor of variation.
The trait stem height adjusted to the quadratic model with coefficient of determination of R 2 = 0.77 (Figure 1A).Increased stem height was observed due to increments of GA 3 doses, with maximum efficiency through the dose of 150 mg L -1 .However, the lower stem height was obtained in the absence of this compound application.The increase of stem height in response to higher GA 3 concentrations are due to the use of regulators that act as chemical signals through receptors, which trigger cellular changes, modifying the dimensions of plant organs, where gibberellic acid applied via seeds potentiated seedling performance, accelerating emergence speed and improving plant productive potential (Lima-Brito et al., 2006).
Research by Vasconcelos et al. (2015) determined that the effect of gibberellic acid application on seedling emergence were dependent on the use of higher concentrations, which provided increments in germination percentage, emergence speed index and averagetime for seed germination in relation to absence of gibberellic acid.The trait root dry mass (Figure 1b) adjusted to the quadratic polynomial model: Ŷ = 0.0723 + 0.0006x -3E-06x 2 , revealing coefficient of determination of R 2 : 0.76, being possible to increment this trait through the  The lowest magnitude of this trait is related to modificationsin the relation of plant drains during seed production period, which is related to growth, dry matter of seed and total in the plant (Benincasa, 2003).
The gibberellins are responsible for several physiological functions essentials for developing superior plants.They improve reserves mobilization in the seeds, improve germination and promote stem elongation in some species, may act on leaf expansion, floral induction and anthocyanin biosynthesis (Garcia-Martinez et al., 1987;Van Huizen et al., 1997).Studies demonstrate that exogenous GA 3 application in plants may inhibit enzymatic activities determinant for nitrogen metabolism, mainly through nitrate-reductase and glutamine synthetase (Goupil et al., 1997).
Due to the use of GA 3 applied directly to bean seeds, there are positive effects for seedling stem height, which is an extremely important feature for crop establishment.However, the application of GA 3 in the plants had no effects in the quality of seeds produced, but it increased aerial part dry mass, root length and dry mass of roots.