ABSTRACT
This study was aimed at studying the effect of gibberellin (GA3) on the growth of Eucalyptus plants. The experiment was conducted with 90-day-old seedlings on full sun bench at Goiás State University, Brazil, following the completely randomized design in a 5 × 2 factorial arrangement (five eucalyptus samples and two treatments with gibberellins). The application of gibberellin to eucalyptus is a promising practice, as it significantly affects the vegetative growth and increases the stem biomass accumulation.
Key words: Hormone, silviculture, early growth.
The ongoing increase in economic value and the scarcity of hardwoods have boosted the diversified use of eucalyptus (Souza et al., 2012). The productivity of plantations in Brazilian territory is superior to that of traditional countries such as Australia (center of the species origin), and its current situation provides opportunities for the consolidation of Brazil as one of the major powers in the world's forest-based industry. The forestry sector account for 3.5% of the country’s Gross Domestic Product (GDP) and generates 4.7 million direct and indirect jobs (Abraf, 2013).
Notwithstanding Brazil’s high potential in the forestry sector, the country can further expand production and transfer wealth to other segments of economy. And in order to remain competitive, carrying on this process in a consistent and sustainable manner, it requires the development of research aimed to increase productivity, which will open new markets, increase exports and simultaneously modernize and ensure high social and environmental standards to forestry activities (Ferreira et al., 2012).
Reducing the cutting age and increasing the biomass accumulation of eucalyptus forests will contribute to the sector development and to the growth of Brazil’s participation in the world market. However, the production of trees suitable for short-time harvesting will certainly depend on physiological changes in the species and/or adoption of new management techniques.
The use of plant growth regulators in agriculture has been an important management technique, as it has increased the productivity of several species; still its application is not yet a common practice in cultures that have not reached high technological level. There is great interest in understanding the interference of plant growth regulators in lignification, wood density, early growth and yield (Pereira et al., 2011), since their performance does not depend solely on chemical composition, but also on how they are "perceived" by target tissues. In other words, the same substance can cause different effects depending on the tissue or organ upon which it is acting, on hormone concentration, and on the time of development of the same tissue (Wei et al., 2012). The use of growth regulators to improve wood quality, induce flowering, slow down or accelerate the vegetative growth of several species has been a constant issue in scientific research (Doorn et al., 2011; Kiba and Sakakibara, 2010; Xiong et al., 2009; Pereira et al., 2011).
The current world scenario is characterized by increasing environmental pressure towards the exploitation of renewable energy sources rather than fossil fuels. And in this direction, scientific understanding of how physiological aspects guide the growth of the species will contribute to greater production of early growth plants. However, little is known about the biochemical action and physiology of growth regulators in eucalyptus. Therefore, the present study was designed to investigate the effect of gibberellin (GA3) on eucalyptus plant growth, with a view to reducing the cutting age and accelerating biomass accumulation.
Experimental design
The work was accomplished at Goiás State University, Ipameri Unit (17º43’19”S, 48º09’35”W, Alt. 773 m), located in the municipality of Ipameri, Goiás, a region with predominant Aw climate according to the Köppen classification. The experiment was carried out on a bench exposed to full sun, following the completely randomized design in a 5 × 2 factorial arrangement (five eucalyptus samples, including three clones from the crossing of Eucalyptus grandis x Eucalyptus urophylla “Eucalyptus urograndis GG100”, “Eucalyptus urograndis H13” and “Eucalyptus urograndis Super Clone”; one clone derived from the crossing between Eucalyptus urophylla x Eucalyptus camaldulensis “Eucalyptus urocan 58”, and one sample from seeds of Eucalyptus citriodora x two treatments with 100 and 0 mg L-1 of gibberellins) and three replications. The eucalyptus seedlings were irrigated daily to maintain soil moisture near field capacity. At 90 days after germination the seedlings were transplanted and cultivated in six-liter pots with soil, sand and manure at 3:1:0.5 proportion, respectively. After substrate analysis, pH correction to 6.0 and fertilization were carried out. The foliar applications of GA3 at 100 mg L-1 concentration and 50 ml per plant were accomplished on April 28 and 30 and May 28 and 30, 2012 at 100, 101, 129 and 130 days after germination. The plants that did not receive gibberellin were sprayed with deionized water in similar amount to that used for the regulator application. Maximum uniformity was sought in the application of gibberellin via spraying on all leaf area using a metering valve attached to a backpack sprayer. At 160 days after emergence the plants were analyzed.
Growth variables
The number of leaves and the stem length and diameter were measured between 8 and 10 am using a graduated ruler and calipers. The leaves, roots and stems were removed and put to dry in an oven at 72°C until steady dry mass state, and then weighed separately. Based on the dry mass data, the leaf mass ratio (LMR), root mass ratio (RMR), stem mass ratio (SMR), shoot/root system ratio (S/RS) and total biomass (TB) were calculated. Fully expanded leaves were used to obtain the specific leaf area (SLA) and the leaf area (LA). Six discs with 14-mm diameter and known area were collected from each fully expanded leaf and dried at 70°C for 72 h, after which mass and SLA were measured. The LA was obtained by measuring the length and diameter.
Photosynthetic pigments
In order to determine the total chlorophyll and carotenoid concentration values, leaf discs with known area were removed and placed in glass containing dimethyl sulfoxide (DMSO). Subsequently, extraction was accomplished in water bath at 65°C for one hour. Aliquots were extracted for spectrophotometric reading at 480, 649.1 and 665 nm. The contents of chlorophyll a (Cl a), chlorophyll b (Cl b) and carotenoids were determined according to the equation proposed by Wellburn (1994).
Statistical procedures
The experiment was set up following the completely randomized design in a 5 × 2 factorial arrangement with three replications, and the data was submitted to variance analysis and Newman Keuls test for multiple comparisons of treatment averages using SISVAR 5.3 software (Ferreira, 2011).
Statistical analysis showed significant differences between treatments and samples for all variables (Tables 1 and 2). Only for the reason of shoot/root system (S/RS) and because of the root mass was no interaction treatment x samples (Tables 3 and 4). The average leaf number values varied significantly among the samples, where the Eucalyptus citriodora species had the lowest number (55.16) compared to that of the other samples, which were similar. However, the Eucalyptus urocan 58 species presented a greater number of leaves (138.50) compared to the others.
There was statistical difference in the number of branches, where the E. citriodora sample had a smaller number (5.16) compared to the others, which had similar amounts. However, the Eucalyptus urograndis GG100 sample stood out (14.00) among the others, presenting the greatest numerical amount of branches. Regarding the plant height, all samples showed high growth, highlighting the E. urograndis GG100 hybrid, which differed significantly from the others, with 113.83-cm height. The samples treated with GA3 were statistically superior in height (104.46) compared to those not treated with gibberellin (Table 1).
The samples did not show any significant differences in the stem diameter (Table 1), however, there was a numerical variation of 1.82 mm between the E. urograndis Super Clone and the E. citriodora species samples. Regardless of the sample, the plants treated with GA3 showed longer diameter (8.51 mm), with 1.68-mm increase compared to untreated plants (6.83 mm). The leaf area varied among the different samples, highlighting larger leaf area in E. urograndis Super Clone (40.16) and smaller leaf area in E. citriodora (20.37). However, the treated and untreated plants had almost the same leaf area, and they did not differ statistically. Regarding the specific leaf area there was no difference between the samples and treatments with GA3 (Table 1).
The variance analysis and average test show interaction between the shoot/root system ratio (S/RS) factors, as shown in Table 2. In numerical values, the highest ratio was obtained in Eucalyptus citriodora (4.67) treated with GA3, and the lowest ratio was recorded in Eucalyptus urocan 58 (1.81), and even with this difference in average values, there was no statistical difference between samples. However, Eucalyptus urocan 58 had the lowest values among the samples treated with GA3.
The total biomass of samples differed statistically, and E. citriodora had the lowest total biomass (15.03). The leaf mass ratio was higher in plants not sprayed with GA3, however, the plants treated with GA3 showed higher stem mass ratio, with E. urograndis GG100 showing the highest numerical value. The root mass ratio showed slight variation between samples, with Eucalyptus urocan 58 presenting the highest numerical value in relation to the other samples, but without statistical difference between plants treated with GA3 and untreated ones.
The samples differed statistically in carotenoid concentration, where the E. urograndis. GG100 sample presented higher average (1.975) than the others. There was no significant difference between samples treated with g GA3 and untreated ones. When the sample treatment results are broken down, it is noted that regardless of the eucalyptus sample, the highest numerical values were obtained in plants treated with GA3 (Table 4). Different lowercase letters indicate significant differences between the samples; different capital letters indicate significant differences between treatments in each sample by Newman Keuls test at 5% significance.
Physiological analysis of the plants at an early stage helps to identify promising species with high productivity potential and contributes to the choice for suitable materials for the prevailing conditions (Peixoto et al., 2006). Originally the hybrid clones and E. citriodora showed marked differences in vegetative growth. The variations recorded in the samples growth and development are determined by genetic characteristics, however, the GA3 applications intensified the vegetative growth and promoted significant changes in the allocation of assimilates and biomass partition, as discussed below.
The application of GA3 did not cause significant changes in the vegetative growth of the studied samples. The similarity in the number of leaves, branches, leaf area, specific leaf area, biomass, leaf and root mass ratio, chlorophyll and total carotenoids between plants treated with GA3 and untreated ones indicate that the time between regulator application and evaluations was short for vegetative growth and changes in anatomical variables, such as specific leaf area. Despite the lack of statistical difference, the trend of specific leaf area increase in plants treated with GA3 is possibly related to higher leaf transmittance of solar radiation in the canopy. Increased leaf transmittance allows greater solar radiation distribution in the canopy, providing light energy to lower leaves, contributing to increased photosynthetic activity and biomass accumulation over time. Increases in the specific leaf area, and reductions in leaf thickness decrease absorption and increase leaf transmittance, changing the solar radiation distribution pattern in the canopy (Borges et al., 2014). Greater biomass in plants treated with GA3, even with no statistical difference, is an indication that a longer time interval between application and evaluation is necessary to record any difference.
The notable changes in vegetative growth are due to the differences among the samples used. E. urograndis Super Clone stood out among the samples for presenting high number of leaves, large leaf area and specific leaf area, and great biomass. However, E. urograndis GG100 was the sample with the most desirable features for forestry due to its higher biomass distribution to the stem resulting in greater height and large stem diameter. Larger stem diameter in E. urograndis GG100 clones has been observed in experiments and appears to be a significant variable of the sample (Pinto et al., 2011).
Greater plant height, stem diameter and stem mass ratio in plants treated with GA3 indicate that the growth regulator caused changes in biomass distribution with greater assimilates allocation to the stem to the detriment of the root system and mainly of the leaves. The high stem growth in height and diameter is of great interest to forestry as it directly affects the eucalyptus early growth and wood quality and productivity. The application of GA3 changes the vegetative growth, intensifying the development of shoots (Sponsel, 2006). The absence of difference in photosynthetic pigment concentration and leaf area is evidence that there was no change in the photosynthetic mechanism to maximize photosynthesis, the existing differences being related to changes in biomass distribution. These changes may be associated to the fact that hormones generally act upon cellular communication, since they are defined as “chemical messengers produced in a cell or tissue that modulate cellular processes in other cells, by interacting with specific proteins called receptors”, contributing to metabolic processes that allow the maintenance and growth of primary organs (Taiz and Zeiger, 2013).
Higher S/RS ratio in plants treated with gibberellin is because they allocate greater biomass to the stem, thus increasing the shoot mass. Lower RMR in treated plants is due to the same fact, as larger investment in the shoot took place at the expense of smaller biomass distribution to the root. Greater translocation of assimilates to the shoot can be beneficial under adequate soil fertility conditions because of higher stem and leaf development.
Biomass production is determined by the species genetic characteristics (Santana et al., 2002). The remarkable differences in vegetative growth indicate that the species have genetic variability. Regardless of the treatment with GA3, the E. citriodora species showed less vigorous vegetative growth than the hybrids. The E. urograndis GG100 clone showed high biomass distribution to the stem, proving to be a promising material for future studies. The application of GA3 in eucalyptus plants is a promising practice, as it significantly affects vegetative growth and enhances stem biomass accumulation (the element of economic interest).
1. The application of GA3 in Eucalyptus seedlings accelerates vegetative growth, changes biomass distribution and possibly interferes with the early growth and wood productivity.
2. E. citriodora was the sample with the least vigorous vegetative growth.
3. The E. urograndis GG 100 hybrid clone stood out from the others for showing the desirable features for forestry, such as greater stem mass ratio, height and stem diameter.
The authors have not declared any conflict of interest.
Coordination improvement of senior staff at AUXPE 2370/2014 project.
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