Versão on line CAN PHYTOHORMONES STIMULATE INITIAL GROWTH OF BRAZILIAN SAVANNA TREES ?

H ern g er ia n a ABSTRACT – The initial growth of Cerrado tree species is slow, which impairs large scale seedling production for planting. The effect of plant hormones is well known for crop species, in this study we aimed to test the effect of two commercial biostimulants on the initial growth of six native tree species widely distributed and abundant in the Cerrado. We applied nine treatments using foliar spray: T0 – control (water); commercial Progibb ®


INTRODUCTION
The need for restoration of degraded areas in both rural and urban regions has largely increased in recent decades.In Brazil, both state and federal level legislation are clear about the rules to restore degraded areas after illegal deforestation or infrastructure construction (e.g.Federal Law 12.651/ 2012; state law of the Federal District 14.783/1993).
Planting seedlings of native tree species is the most commonly applied technique for restoration of degraded areas, which generates a commercial demand for seedling production in nurseries.The initial growth of these seedlings is important for economic reasons, as faster initial growth rates allows faster seedling production and selling; and for ecological reasons, since taller seedlings are more likely to survive in field conditions, especially when competing with exotic fast-growing grasses (Giotto et al., 2009).
For these reasons, even in regions dominated by savanna ecosystems, most seedlings produced in nurseries are from forest fast-growingtree species.
Species from forest ecosystems (mostly riparian forests) within savanna biomes have higher growth rates than savanna trees from the same regions (Hoffman & Franco, 2003).These forest species are native from the region and therefore, legally, they can be used in restoration projects.However, forest tree species are ecologically not appropriated to be used for the restoration of savanna ecosystems, especially because they are mostly fire-sensitive and likely to die after fire events, which are common in savanna ecosystems (Hoffmann et al, 2000).
In contrast, tree species from savanna ecosystems usually present slow shoot growth (Caldas et al., 2009;Scalon et al., 2009), associated with a large root system development (Ratnam et al., 2011).These characteristics, associated with low natural tree densities and variable seed production over the years (Macedo et al., 2009), constrain production of seedlings in commercial nurseries within the Brazilian savanna, the Cerrado.
In this context, the use of phytohormones can help to overcome the slow initial growth of savanna tree seedling (Scalon et al., 2009).
Phytohormones are chemicals naturally produced by plants; they regulate plant growth and development and can be artificially synthetized (Vieira & Castro, 2001).Gibberellic acid, auxin and cytokinin are among the most commonly used and studied plant hormones (Taiz & Zeiger, 2006).
Gibberellic acid is mostly produced in young plant shoot tissues and in maturating seeds (Paulilo et al., 2010).In seedlings, gibberellic acid stimulates plant shoot elongation, commonly increasing internode length, foliar area and plant dry mass (Vieira & Castro, 2001;Taiz & Zaiger, 2006;Dantas et al., 2012).Auxin is naturally produced by apical shoot meristems and regulates phototropism, geotropism, fruit development as well as secondary root and shoot growth.Cytokinins are produced mostly in plant roots and transported via xylem to the whole plant.They regulate senescence in plant tissues, stimulate shoot dominance, flower production and seed germination (Taiz & Zeiger, 2006).These three phytohormones are commercially produced, for instance, Progibb ® is a commercial gibberellic acid compound, whereas Stimulate ® combines gibberellic acid, auxin and cytokinin.
Studies have recently tested the effects of phytohormones in the initial growth of tree species from savanna ecosystems within the Cerrado and the results appear to be speciesspecific, differing even among species of the same family.Hancornia speciosa (Apocynaceae) presented a 30 cm increase in shoot growth in 105 days after GA 3 application (Caldas et al., 2009).Dimorphandra mollis (Fabaceae) seedlings also had 60% increase in growth rates after the application of Stimulate ® in seeds (20 mg.L -1 in 500 g of seeds), compared to untreated seeds (Canesin et al., 2012).In contrast,
Additionally, the application of Stimulate

RESULTS AND DISCUSSION
All species had slow seedling growth (Table 1), which is a characteristic of savanna species, including Cerrado (Moreira et Klink, 2000;Caldas et al., 2009).No significant effect of the treatments was found on seedling initial growth in any of the analyzed parameters (Table 2).In general, the seedlings treated with different phytohormone concentrations had similar growth rates and biomass allocation patterns to the untreated (control) seedlings.The effect of.
phytohormones in different concentrations varied among different species inconsistently.
Additionally, there was a considerable individual variation in seedling growth (Figure 1), which can be related to individual genetic variations and not to the treatments.For most species, the phytohormone concentrations tested did not prevent growth and Progibb ® tended to increase seedling growth of Anacardium humile and Jacaranda cuspidifolia, whereas Stimulate ® tended to improve seedling growth of Hymenaea stignocarpa and Copaifera langsdorfii.We therefore state that these phytohormones, probably in higher concentrations and/or in repeated applications, could be beneficial for the initial growth of these species.
In contrast, Stimulate ® can be toxic to the growth of Jacaranda cuspidifolia, since the plants in treatment T 5 and T 7 (Stimulate ® 6 mL.L -1 and Stimulate ® 12 mL.L -1 ) had significantly lower relative growth rates than control plants (Kruskal-Wallis H = 25.5, DF = 9, p = 0.002).
Among control plants, Hymenaea stignocarpa seedlings presented the higher elongation rates and this species is classified as slow/moderate growth rate (Table 1).This can be attributed to the species larger seeds compared to the other five studied species (Hoffman & Franco, 2003;Fagundes et al, 2013).
By day 30 after phytohormone application, the seedlings growing rate declined for all species and treatments, which suggests that hormones reapplication could benefit plant growth as it was described for other Cerrado native species (Canesin et al., 2012).Since this also occurred in untreated control seedlings, this slowdown in growth may be due to restricted space of polyethylene bags used for the seedlings development or to photoperiod changes.
Photoperiod decreases has been described to affect Cerrado seedling growth in nursery (Costa et al., 2011) and in soil conditions in tropical areas (Marimon et al., 2008).
The slow growth of native tree species seedlings of Cerrado is one of the barriers to the use of these species in ecological restoration projects.The use of phytohormones to accelerate the growth of these species may be an alternative to overcome this barrier (Caldas et al., 2009).
The fruits were collected at the Contagem Biological Reserve, Distrito Federal, in Central Brazil (15°41'00''S e 47°50'00''W) from June to October 2015 (according to fruit production phenology).The seeds were cleaned and stored in kraft paper bags at room temperature until the sowing, at the following wet season, in December 2015.The seeds were then disposed in plastic trays filled with yellow-red latossols for germination tests and seedlings were transferred to black polyethylene bags (10 cm diameter and 25 cm depth) containing yellow-red latossols and sand (mixture 3:1).Next polyethylene bags were disposed randomly in a nursery covered with a shade cloth (50% sun light) and watered seedling twice a day for 70 days.After this nursery-growing period, five Heringeriana 11(2): 1-12.2017.
day -1 ) and y k e y k-1 are the height or the diameter accumulated from time t k to t k-1 .At the end of the experiment (111 growing days) all seedlings were uprooted and separated into root, leaves and stem parts.Leaf area was measured with a flatbed scanner to assess the photosynthetic capacity of each plant (Varma & Osuri, 2013).After that, all plant tissues were oven-dried separately at 70 o C for 72 hours and then each plant part was weighed in a digital scale.Differences across treatments were compared within each species considering plant relative growth (height and diameter), shoot and root dry mass and leaf area using non-parametric analysis of variance.Means were compared using Mann-Whitney test (α = 0.05), all analyses were performed in R program version 3.0.2(R Development Core Team 2016).

Table 2 .
General data of the different parameters evaluated in the growth of the six tree species from Cerrado submitted to different types of plant hormone and their respective doses.