Giberellic Acid in the Expansion of the Thermal Range of Lettuce Seeds

Lettuce presents an increasing demand, which makes necessary the expansion of the areas destined to its planting. Temperature is a limiting factor, because it acts directly on germination, where changes and adjustments in the hormonal balance, mainly of gibberellin (GA) and abscisic acid (ABA), occur. The objective of this study was to evaluate if the treatment of seeds with gibberellic acid is capable of mitigating the damages caused by high temperatures. The seeds were submitted to germination test, first count, percentage of abnormal seedlings and analysis of shoot and root growth. The treatments consisted of seven concentrations of gibberellic acid (control—not treated with GA, 25 mg L, 50 mg L, 75 mg L, 100 mg L, 125 mg L, 150 mg L) and three constant temperatures (25, 30 and 35 °C). The germination and first count, when submitted at 25 and 30 °C presented an increase up to 50 mg L of GA, while at 35 °C the growth was constant up to 125 mg L of GA. Treatment with GA increased root and shoot length. Giberelic acid mitigated the damage caused to lettuce seeds when subjected to temperatures above the optimum.


Introduction
The lettuce (Lactuca sativa L.) is a temperate, belonging to Asteraceae Family, which consists of the most consumed leaves and has an important role in the diet due to its taste, low cost and nutritional quality, as a source of vitamins, minerals and fiber (Santi et al., 2010).It is a short cycle crop, thus enabling, many harvests during the year.
Due to its increasing demand, it is necessary to expand the areas destined to its planting.However the climate conditions are a limiting factor, especially in relation to the temperature, being able to act directly in germination by affecting the dormancy and the viability of the seeds (Deng & Song, 2012).Since most of the cultivars used commercially have their germination suspended above 28 °C (Yoong et al., 2016).
When exposed to high temperatures the lettuce seeds may present a temporary inhibition (thermoinhibition) or complete in germination (thermodormity).The reasons of this inhibition are due to the stiffening of the endosperm, which presents itself as the main barrier of impediment to radicular protrusion and embryo growth (Nascimento et al., 2012).
During the germination, modifications and adjustments in hormonal balance occur, mainly of gibberellin (GA) and abscisic acid (ABA) (Nonogaki et al., 2010), with these exerting important and antagonistic roles in dormancy and germination (Flinkelstein et al., 2008).The GAs interfere in the regulation of division and cell elongation, causing greater embryo growth, besides inducing the synthesis of enzymes that act in the weakening of the micropillary endosperm (Yamaguchi, 2008).The ABA is indicated as the main inhibitor of germination, especially in the early stages of development (Taiz et al., 2017).
Studies have been conducted in order to establish the interactions between GAs and ABA with temperature in the induction and inhibition processes of seed germination in situ (Huarte & Benech-Arnold, 2010;Finch-Savage & Footitt, 2012).Yoshioka, Endo, and Satoh (1998) reported reduction in ABA content in lettuce seeds when they were soaked at optimal germination temperatures but increase at high levels when soaked at elevated temperatures.It is believed that the exougenous application of GA is responsable for the endogenous decrease of ABA by promoting its catabolism and consequently for the increasing of temperature tolerance for lettuce seeds (Gonai et al., 2004).
Based on the above consideration, the objective of this work was to investigate whether the conditioning of lettuce seeds cv.Grands Rapids with giberelic acid would be able to soften the damages caused by high temperatures both on its physiological potential and germination.

Method
The experiment was developed in the Laboratory of Seed Analysis of the Agricultural Sciences Center (CCA) of the Federal University of Ceará (UFC), Campus of Pici-Fortaleza, from April to June 2018 using seeds of crisp lettuce "Grand Rapids".
Before treating the seeds with gibberellic acid (GA3), they were submitted to determination of the water content as described by MAPA (2009).
For conditioning, the seeds were distributed in petri dishes on two sheets of Germitest® type paper moistened with gibberellic acid solutions (GA3) in the proportion of 2.5 times the weight of the dry substrate.Moinstening was conducted for 16 hours at a constant temperature of 15 °C, until seeds reached 40% water content.After this time, the seeds were placed in a dryer for four hours at a temperature of 40 °C, sufficient time to return to the water content it had before conditioning and then submitted to the germination and vigor tests.
For the conduction of germination test, four samples of 50 seeds were used, which were seeded in Gerbox® boxes (11 × 11 × 3.5 cm) containing two sheets of Germitest® type paper moistened with diestilled water in the proportion of 2.5 times the weight of the dried substrate.Then, the gerboxes were conditioned in BOD at constant temperatures of 25, 30 and 35 °C and photoperiod of 12 hours.For the evaluation of vigor in the fourth day after the germination test was installed, the first germination count occurs.On seventh day the final germination was observed, determining the seedlings' normal and abnormal percentage in relation to the number of seeds used in the sample, using as a standard of normal seedlings the classification established by MAPA ( 2009).
The analysis of seedling' shoots and roots growth was performed after the germination test was completed by seedlings per sample with were measured which the aid of a ruler graduated in cm.

Results and Discussion
The seeds presenteds 6% of water contente and after conditioning it was sought to maintain the same value by drying them, with the aim that only the exogenous application of gibberellic acid would interfere with the variables analyzed.
A significant effect was observed between the concentrations of gibberellic acid and the temperatures tested for the analyzed variables, except for the percentage of abnormal seedlings (Table 1).

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Reference
Figure 2