Photosynthetic responses of Eucalyptus seedlings submitted to chemical and mechanical stimulus

Respostas fotossintéticas em mudas de eucalipto submetidas a estímulo químico e mecânico

Abstract

This work aimed to investigate the photosynthetic responses of Eucalyptus urophylla x Eucalyptus grandis hybrids 1528 submitted to the application of jasmonic acid (JA) and stem bending. The experiment was conducted in a shade house using a completely randomized design with five replicates of 20 seedlings, with the following treatments: T1: control, T2: 8.0 µmol of JA and T3: 40 stem bending. The net assimilation rate of CO₂ (A), stomatal conductance (gs), internal CO₂ concentration (Ci), leaf transpiration rate (E), water use efficiency (WUE), intrinsic efficiency of use of Water (iWUE) and carboxylation efficiency (Fc) were measured with an IRGA. The dose of JA applied to the seedlings was insufficient to alter the photosynthetic responses in relation to the control treatment. Mechanical stimuli induced by stem bending in eucalyptus hybrid 1528 seedlings promoted an increase in the maximum photosynthetic capacity and maximum assimilation of CO₂, but the seedlings presented a reduction in the efficiency of the water use, that is, lower photosynthetic efficiency when compared to the seedlings submitted to other treatments. Seedlings of eucalyptus hybrid 1528 submitted to mechanical stimuli and jasmonic acid show greater respiration under low flow levels of photosynthetically active photons.

Keywords: Jasmonic acid; Stem bending; Quality of seedlings; Clone 1528

Resumo

Este trabalho objetivou investigar as respostas fotossintéticas em mudas de híbrido 1528 de Eucalyptus urophylla x Eucalyptus grandis submetidas à aplicação de ácido jasmônico (JA) e flexões caulinares. O experimento foi conduzido em casa de sombra utilizando delineamento inteiramente ao acaso com cinco repetições de 20 mudas, com os seguintes tratamentos: T1: controle, T2: 8,0 µmol de JA e T3: 40 flexões caulinares. A taxa de assimilação líquida de CO₂ (A), condutância estomática (gs), concentração interna de CO₂ (Ci), taxa de transpiração da folha (E), eficiência do uso de água (WUE), eficiência intrínseca do uso de água (iWUE) e eficiência de carboxilação (Fc) foram mensuradas com o auxílio do IRGA. A dose de JA aplicada nas mudas foi insuficiente para alterar as respostas fotossintéticas em relação ao tratamento-controle. Estímulos mecânicos induzidos através de flexões caulinares em mudas do híbrido 1528 de eucalipto promoveram incremento da capacidade fotossintética máxima e máxima assimilação de CO₂. Porém, as mudas apresentaram redução na eficiência do uso da água, ou seja, menor eficiência fotossintética quando comparado com as mudas submetidas aos demais tratamentos. Mudas do híbrido 1528 de eucalipto submetidas aos estímulos mecânico e ácido jasmônico apresentaram maior respiração sob níveis baixos de fluxo de fótons fotossinteticamente ativos.

Palavras-chave: Ácido jasmônico; Flexões caulinares; 1ualidade de mudas; Clone 1528

Introduction

Among woody species, Eucalyptus stands out for fast growth, adaptability and high productivity (MORA; GARCIA, 2000; GONÇALVES; SOUSA; VELINI, 2015). Brazilian total area with Eucalyptus have about 50% of hybrids (ASSOCIAÇÃO BRASILEIRA DE CELULOSE E PAPEL, 2008). Interspecific hybridization has become a fast and efficient manner of obtaining genetic gains in the improvement of Eucalyptus species, with direct and significant effects on the forest industry. The complementarity search for technological characteristics of wood, tolerance to biotic and abiotic stresses, as well as the manifestation of heterosis were verified in several hybrid crosses, as for example in the interspecific hybrid Eucalyptus urophylla x Eucalyptus grandis, called “urograndis” (ASSIS; MAFIA, 2007).

Eucalyptus Cl 1528, known as “super clone” according to Portal Florestal (2016) shows a genetic basis of Eucalyptus urophylla x Eucalyptus grandis with an annual mean increment of 50 m^-3 ha^-1 year^-1, basic wood density of 531 kg m^-3, total lignin content of 29.24%, holocellulose content of 68.14%, gravimetric yield of 50.41% and mechanical strength of 80.82%. The clone aims at good growth, which is an Eucalyptus grandis characteristic, as well as an increase in wood density and improvements in yield and physical properties of cellulose which is an Eucalyptus urophylla characteristic (CARVALHO; NAHUZ, 2001).

The natural environment in which plants develop is composed of a complex set of biotic and abiotic stresses. Consequently, productivity below ideal levels of plant responses to those stresses is dynamic and complex (CRAMER, 2010; SKIRYCZ; INZE, 2010). In addition, difficulties in establishing standards and recommendations for obtaining quality seedlings lead to post-planting mortality. Thus, the production of resistant seedlings capable of surviving and tolerating post-planting adversities, among them, water stress and wind gust stand out, is one of the possible alternatives to minimize losses.

Several authors indicated that some practices such as mechanical stimuli and plant growth regulators may be related to improvements of seedling quality from woody species (JACOBS; LANDIS, 2009; ORO et al., 2011; VOLKWEIS et al., 2014; CADORIN et al., 2015; DRANSKI; MALAVASI; MALAVASI, 2015). Thus, such practices could be included in nursery routine, favoring development after planting.

Plants show integrated or induced defenses, which encompass morphological, physiological and biochemical changes. The most common morphological alterations in response to mechanical and hormonal disturbances are leaf area reduction, stomatal closure and abscisic acid-mediated root system (LOPEZ; CHAUHAN; JOHANSEN, 2008; TAIZ; ZEIGER, 2013). Plant responses to stress include low leaf water potential (LIBERATO et al., 2006), reduction of quantum efficiency of photosystem II (SILVA et al., 2007) and relative leaf water content (SILVA et al., 2007; LOBATO et al., 2008), decreases in stomatal conductance and photosynthetic rate (POLIZEL et al., 2011) and adjustment in synthesis of sugars and proline (MCCORMICK; CRAMER; WATT, 2008; CAMPOS et al., 2011).

Mechanical stimuli have been applied to describe mechanically induced responses in plants using stem bending in seedlings (JAFFE, 1973). Current reactions to a significant number of woody species have been reported. Among them are cellular stretching reductions, with consequent reduction in the height of the seedlings and the increase in stem diameter, indicators of seedling quality (KERN et al., 2005; VOLKWEIS et al., 2014; CADORIN et al., 2015; DRANSKI; MALAVASI; MALAVASI, 2015).

However, mechanical stimuli usually used in forest seedlings hardening are difficult to operate in large scale nurseries justifying the search for other methods such as plant growth regulators. These compounds act as resistance inducers, and some are classified as plant hormones. Plants produce a wide variety of plant hormones including auxins, gibberellins, abscisic acid, cytokinins and ethylene (GONÇALVES; SOUSA; VELINI, 2015).

In this context, plant markers such as brassinosteroids, polyamines, salicic acid and jasmonic acid have been mentioned as a new class of plant growth regulators related to several morphological and physiological effects on plant development. Among those effects are defense, cell division, reproduction, growth and development (FRANCISCO et al., 2008; BORCIONI; NEGRELLE, 2012; TAIZ et al., 2017).

Jasmonic acid (JA) and its jasmonate derivatives are endogenous regulators of plant growth originating from linolenic acid, which occur in several plant species (FRANK; ROSS, 1991; KERBAUY, 2012). The exogenous application of jasmonates in plants produces a wide variety of effects, including inhibition of root stretching, chlorophyll degradation and stomatal closure under stress conditions (SANCHEZ, 2008).

In addition, JA is involved in regulating defense responses and signaling of plants faced abiotic stresses (LORENZO; SOLANO, 2005; YASUDA et al., 2008). The signaling pathways of these plant hormones are not isolated, but interconnected by a complex network involving several defense responses and development processes (MARTÍN, 2009). According to Deuner et al. (2015) induction or application of jasmonic acid provides resistance in plants under stress, resulting in better development and crop production.

Although stress induction by mechanical and chemical stimuli has the potential to represent an immediate advantage, since they may stimulate plant defense system. Kerbauy (2012) has indicated that stress situations can also directly affect assimilation of CO₂ and consequently net photosynthetic rate. A direct influence that stress can cause in plant photosynthesis is through the impact on the stomatal opening (MARENCO; LOPES, 2009). As the stomata close in response to stress, the resistance to CO₂ diffusion within the leaves increases.

Stress can also affect photosynthesis through a number of other mechanisms: by affecting the metabolic intermediates level, by inhibiting the photosynthetic electron transport system, and by changing the respiratory rate. Additionally, mechanical perturbations may decrease leaf temperature, as they reduce the resistance of the boundary layer promoted by the vapor pressure, affecting the diffusion of gases and water vapor (CHAVARRIA; SANTOS, 2012). On the other hand, JA application can compromise photosynthesis, reducing the expression of genes located in the nucleus and in the chloroplasts, besides being potential to degrade chlorophyll in leaves (SANCHEZ, 2008).

Understanding the behavior of defense and protection of plants is essential to obtain organisms resistant to several stresses (SOARES; MACHADO, 2007). In this perspective, studies are need to understand the physiological responses of plants as a function of stress induction promoted by the application of mechanical and hormonal disturbances, in order to better understand the regulatory role of these defense inducers in the adaptation mechanisms of plants, as well as the possibility of estimating the management reflexes on plant metabolism.

Therefore, this study aimed to investigate the photosynthetic responses in hybrid seedlings of Eucalyptus urophylla vs Eucalyptus grandis subjected to the application of jasmonic acid and stem bendings.

Material and methods

The experiment was conducted from May to October 2016, in a shade house located at 24°33’S and 54°2’ W with and average altitude 414 m. The climate of the region, according to Köppen is Cfa subtropical, with average temperature of the coldest month lower than 18°C and the warmest month above 22°C, with infrequent frost and tendency of concentration of rainfall in the summer but with no defined dry season. The annual precipitation is around 1,600 to 1,800 mm (CAVIGLIONE et al., 2000).

We used seedlings of Eucalyptus clone 1528 (Eucalyptus urophylla vs Eucalyptus grandis) propagated in 50 cm³ conical plugs filled with commercial substrate (Plantmax®) made of pinus bark with the addition of 300 g m^-3 of controlled-release fertilizer (Basacote® Plus 6M).

Shade house consisted of a galvanized iron structure with arc-shaped roof of 7 x 30 m and 3.5 m of high. The ceiling is covered with a low-density polyethylene (LDPE) and anti-UV 150 µm thick plastic film. The sides were closed with a 30% white shading screen.

The experimental design was a completely randomized with five replicates of twenty seedlings each submitted to the following treatments for 4 consecutive weeks: T1: control with deionized water + nonionic surfactant applied weekly; T2: 8.0 µmol L^-1 jasmonic acid (JA) + deionized water + nonionic surfactant applied weekly; T3: 40 stem bendings (thigmomorphogenesis) + deionized water and nonionic surfactant (AGRAL®) applied weekly.

The application of the chemical stimulus occurred at weekly intervals by means of leaf pulverizations of jasmonic acid in the concentration according to each treatment, with a total solution volume equivalent to 7 mL per seedling until runoff. The solutions consisted of jasmonic acid, deionized water and nonionic surfactant, applied with a manual sprayer, working at a pressure of 0.3 MPa.

The stem bending treatments comprised daily mechanical stimuli using a structure composed of a 25 mm diameter PVC bar arranged horizontally and fixed in a metallic structure with bearings. The seedlings were vertically bended not more than 45º by passing the bar in the upper third of the foliage, always at the same time, at a speed 0.10 m sec^-1 (VOLKWEIS et al., 2014). During treatment application, irrigation was provided twice a day by aspersion until substrate saturation.

After two weeks of the treatments application, gas exchange response curves with an IRGA model LI-6400XT (Licor Inc. Lincoln, Nebraska, USA) were determined as a function of the photosynthetic active photon flux density (PPFD) by reduction from 2,000 µmol m^-2 s^-1 to 0.1 µmol m^-2 s^-1 and CO₂ set at 380 µmol m^-2 s^-1 using 25 seedlings per treatment between 9 a.m. and 11 a.m. We sampled fully developed leaves from the middle crown.

As a function of PPFD, the net assimilation rate of CO₂ (A), stomatal conductance (gs), internal CO₂ concentration (Ci) and leaf transpiration rate (E) were determined according to (ZHANG et al., 2001), where WUE corresponds to the efficiency of water use, iWUE is the intrinsic efficiency of water use and Fc is the carboxylation efficiency.

The CO₂ liquid assimilation response curves as a function of PPFD were preferentially adjusted to the equation: A= Amax [1-e-k(Q-Ic)], allowing to estimate the maximum net assimilation (A _max) (PRADO; MORAES, 1997). Data of A originated from PPFD below 200 µmol m^-2 s^-1 were used to calculate the apparent quantum efficiency (Ф), adjusting linear equation from 0 to 200 µmol m^-2 s^-1 PPFD. Light compensation point (() at the point of intersection of the line with the X-axis was also determined.

Data was tested for normality of the distribution of residues, variance homogeneity and submitted to ANOVA followed by regression analysis at 5% error probability. For A as a function of PPFD, we used equation proposed by Prado and Moraes (1997) while other variables had their models adjusted by significant equations, based on the highest coefficient of determination. The apparent quantum efficiency and light compensation point was submitted by Tukey test at 5% error probability.

In parallel, was quantified the increment of the number of leaves, being obtained by counting all the leaves. The increments were calculated by the difference between the values before and after the imposition of the treatments, that is, between a time interval of four weeks.

Results and discussion

Stem bending promoted an increase in the maximum photosynthetic capacity of eucalyptus seedlings (Figure 1 a), with the maximum CO₂ assimilation (A_max) of 14.80 µmol of O₂ m^-2 s^-1, whereas the control and the treatment that received jasmonic acid (JA) resulted in A _max of 9.65 and 9.98 µmol of CO₂ m^-2 s^-1, respectively (p-valor = 0.0138).

This photosynthetic capacity promoted by the mechanical disturbance enabled the control by feedback of the stomatal opening (Figure 1 b), increasing in approximately 50% the capacity of gas and water exchanges between plant-environment in relation to control and JA seedlings. Therefore, CO₂ input in the sub stomatal chamber in seedlings subjected to stem bending was increased (Figure 1 c). However, greater stomatal conductance promotes greater water loss to the atmosphere than CO₂ diffusion. Therefore, transpiration showed high values for seedlings submitted to stem bending (Figure 1 d), which caused greater leaf cooling (Figure 1 e).

The above transpiration event relates to the difference in leaf area between treatments, since mechanically disturbed seedlings had lower leaf area (p<0.05) in relation to the control and JA seedlings. The reduction in leaf area of seedlings perturbed by stem bending in relation to the control and JA was 14.68% and 21.55%, respectively. Plants with smaller leaf area and adequate water status of the substrate (condition during this study) tend to increase stomatal conductance consequently favoring greater transpiration. Thus, this sequence of events favors CO₂ diffusion to the intercellular spaces of the leaf mesophyll, in contrast to the cost of water loss in the form of vapor.

Koo et al. (2009) observed in Arabidopsis that induction of damage in leaves with a hemostat led to a marked decrease in the expression of JA response genes during a period in which JA levels remain elevated. This observation suggests that the damaged leaves become desensitized to the JA-Ile (Jasmonoyl-L-isoleucine) signal. However, several authors have reported that JA responses typically depend on large-scale changes in gene expression, noting that in non-stressed cells with low JA levels, JA-responsive gene expressions are repressed by jasmonate zim (JAZ) proteins (CHINI et al., 2007; THINES et al., 2007; YAN et al., 2007). Therefore, when taking into account that plant cells use additional mechanisms to attenuate stress responses, possibly the applied dose of JA in this experiment was not enough for the plant to present responsive expressions of genes in relation to the control treatment. For Chung and Howe (2009), JA repressors that are stabilized against hormone-induced degradation may be involved in this form of negative feedback control.

Different results as a function of the application of the JA derivative were reported by Heijari et al. (2005) who investigated the methyl jasmonate (MJ) potential in inducing defense reactions in two-year-old Pinus sylvestris L. seedlings. The authors found that the highest dose (100 mmol L^-1) reduced (P = 0.027) net photosynthesis in relation to control seedlings. However, the same authors did not observe effect on stomatal conductance between treatments. Suhita et al. (2003) observed that stomatal opening in Nicotiana glauca Graham plants was suppressed by the presence of MJ.

Results divergent from our experiment were reported by Gonçalves, Sousa and Velini, (2015) when studying hybrid eucalyptus seedlings (Eucalyptus grandis vs. Eucalyptus urophylla) under water stress. The above mentioned authors reported a transpiration reduction of control treatment in relation to JA and MJ treatments, showing that the application of those compounds had influenced perception and signaling resulting in greater efficiency of stomatal closure and transpiration rate. Pereira et al. (2006) evaluating the possible stress effects on Eucalyptus urograndis seedlings detected an inverse relation between transpiration and stomatal closure.

Higher transpiration values allow cooling of leaf temperature reducing potential damages to the photosynthetic apparatus and losses in the photosynthetic effectiveness (MILBURN, 1979; ENDRES et al., 2010; TAIZ; ZEIGER, 2013). Results from Lima et al. (2014) showed that water stress induced stomatal closure resulting in leaf heating in seedlings of Handroanthus impetiginosus (Mart. ex DC.) Mattos. In addition, Endres et al. (2010) reported that leaf temperature increase in response to stress could be explained by the latent heat loss reduction through transpiration. JA seedlings initially reduced values of PPFD in relation to the control seedlings under low radiation. This tendency may indicate difficulties of the photosynthetic apparatus in the conversion of light energy into ATP under external JA since part of the unused energy is lost as heat.

Figure 1
Net assimilation rate (A), internal CO2 concentration (B), stomatal conductance (C), transpiration rate (D) and leaf temperature (E) in Eucalyptus seedlings Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) subjected to pulverization of jasmonic acid (8.0 μmol L^-1) and thigmomorphogenesis (40 bendings) as a function of the photosynthetic active photon flux density.
Figura 1
Taxa de assimilação líquida (A), condutância estomática (B), concentração interna de CO₂ (C), taxa de transpiração (D) e temperatura foliar (E) em mudas de eucalipto Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) submetidas à pulverização de ácido jasmônico (8,0 µmol L^-1) e tigmomorfogênese (40 flexões caulinares) em função da densidade de fluxo de fótons fotossinteticamente ativos.

Regarding photosynthetic efficiency, in spite of expressing high values of net CO₂ assimilation, stem bending reduced water efficiency by 31.85% when compared to other treatments (Figure 2 a). Mechanically disturbed seedlings assimilate fewer CO₂ molecules per molecule of transpiration of water when compared to control and JA seedlings (Figure 2 b). Consequently, due to the lower efficiency in water use, eucalyptus seedlings subjected to stem bending needed to open their stomatals wide in order to assimilate the same amount of CO₂ than the other seedlings.

Figure 2
Efficiency of water use (A/E) (A), intrinsic water use efficiency (A/gs) (B) and carboxylation efficiency (A/Ci) (C) of seedlings of eucalyptus Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) subjected to pulverization of jasmonic acid and thigmomorphogenesis as a function of photosynthetic active photon flux density.
Figura 2
Eficiência do uso da água (A/E) (A), eficiência intrínseca do uso da água (A/gs) (B) e eficiência de carboxilação (A/Ci) (C) de mudas de eucalipto Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) submetidas à pulverização de ácido jasmônico e tigmomorfogênese (40 flexões caulinares) em função da densidade de fluxo de fótons fotossinteticamente ativos.

Under adequate water availability, mechanically bended seedlings showed greater carbon sequestration, whereas, in situations of restricted water availability seeddling treated with jasmonic acid and control seedlings could improve utilization of water by controlling the stomatal opening.

Chamber substomatic CO₂ concentration in seedlings submitted to stem bending showed higher values when compared to JA and control seedlings (Figure 2 c). Despite the greater CO₂ availability in the substomatal chamber (Ci), there was also an increase in Calvin cycle in relation to the other treatments, collaborating in photosynthetic efficiency. Such trend can be explained by leaf nitrogen evaluated through SPAD readings from mechanically disturbed seedlings (p < 0.05). SPAD units were 28.82, 28.00 and 32.52 for control, JA and mechanically perturbed seedlings. It is important to highlight that from 30 to 50% of plant nitrogen is for rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) synthesis (SINCLAIR; HORIE, 1989; MARSCHNER, 2011). Therefore, seedlings submitted to stem bending showed higher content of the above enzyme per unit leaf area.

Attaran et al. (2014) analyzing the effect of JA on the photosynthetic efficiency in leaves of Arabidopsis thaliana (L.) Heynh.did not observe a decrease of photosynthesis.

Based on the fact that high CO₂ concentrations can overcome stomatal limitations in photosynthesis, Attaran et al. (2014) tested the exposure of plants treated with JA under elevated CO₂ levels and verified an induction of photosystem II (PSII) decrease in early morning. However, after the supplementation of plants with high CO₂ during a period of 2 hours (starting 1 h before dawn), such PSII effect was alleviated. Those results help to explain the responses found in this study, since seedlings under mechanical stress showed a smaller amount of leaf area, but with higher SPAD unit in relation to the control and JA seedlings.

Figure 3
Net assimilation rate of CO2 in seedlings of Eucalyptus Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) submitted to jasmonic acid pulverization and thigmomorphogenesis as a function of the photosynthetic active photon flux density between 0 and 200 μmol photons m-2 s-1.
Figura 3
Taxa de assimilação líquida de CO2 em mudas de eucalipto Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) submetidas à pulverização de ácido jasmônico e tigmomorfogênese (40 flexões caulinares) em função da densidade de fluxo de fótons fotossinteticamente ativos entre 0 e 200 μmol photons m-2 s-1.

Hristova and Popova (2002) found a different result when studying barley seedlings (Hordeum vulgare L.) sprayed with MJ and observed a decrease in chlorophyll content, in the net photosynthesis and leaf transpiration rate in relation to the control treatment.

Using low PPFD values (0 µmol m^-2 s^-1 to 200 µmol m^-2 s^-1), linear equations were set (Figure 3) which indicated the apparent quantum efficiency Ф promoted by the treatments (Table 1). In other words, the inverse relationship of the constant of the linear equation shows the capacity of application and use of reducing power and ATP in the Calvin cycle (MACHADO et al., 2005). It mean that 17.95 µmol photons^-1 being required for fixation of 1 µmol CO₂ for seedlings submitted to stem bendings, 22.32 µmol photons^-1 µmol CO₂^-1 in the seedlings submitted to JA and 22.72 µmol photons^-1 µmol CO₂^-1 in control seedlings respectively, evidencing an increase in the photophosphorylation efficiency promoted by mechanical disturbances.

Results from this study are different from those of Ballaré (2009) and Meldau, Erb and Baldwin (2012). Both papers reported that JA signaling by the plant effectively decouples when photosynthesis increases, and that JA acts as a signal to re-orient the biosynthetic capacity of the plant defense metabolism. Recent studies of nitrogen flow dynamics in response to stress induced by herbivorous insects support this idea (ULLMANN-ZEUNERT et al., 2013). Other studies have shown that (induced stress) performed with herbivory simulation can redirect the carbon allocation from the leaves to the roots (SCHWACHTJE et al., 2006; FERRIERI et al., 2013; DRANSKI et al., 2016). Carbon partitioning translocating to underground tissues may represent a plant strategy to protect energy consumption and better tolerate abiotic stress (SCHWACHTJE et al., 2006). Perhaps the amount of JA to which the plants have been submitted in this work were not sufficient to express photosynthetic reactions other than the control treatment, or even, such absence of difference in behavior is related to the genetic factor of the hybrid.

Several authors have reported that JA and its derivatives exert distinct biological activities when applied to plant tissues (BLECHERT et al., 1999; MIERSCH et al., 1999; STINTZI et al., 2001; RIBOT et al., 2008; WANG et al., 2008). JA applied exogenously does not target a single specific cell type. It is often applied to non-physiological concentrations and is subject to unknown metabolic paths, necessitating additional approaches to determine the specific in vivo role of JA and its derivatives.

Table 1
Apparent quantum efficiency (Ф) and light compensation point (Γ) obtained from net assimilation rate (A) values between 0 μmol m^-2 s^-1 and 200 μmol m^-2 s^-1 density of photosynthetically active photons in Eucalyptus Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) seedlings as a function of treatments.
Tabela 1
Eficiência quântica aparente (Ф) e ponto de compensação luminoso (Γ) obtidos a partir dos valores da taxa de assimilação líquida (A) entre 0 μmol m^-2 s^-1 até 200 μmol m^-2 s^-1 de densidade de fótons fotossinteticamente ativos, em mudas de eucalipto Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) em função dos tratamentos.

Treatment	Ф (µmol CO2 µmol photons)	Γ(µmol m-2 s-1)
Control	0.0438 b	5.0192
Jasmonic Acid	0.0447 ab	23.4366
Thigmomorphogenesis	0.0555 a	35.8772

• Different letters represent difference by the Tukey test at 5% of error probability.

It is well established that JA levels vary widely depending on the stage of development and the biotic and abiotic conditions in which plants are submitted (CREELMAN; MULLET, 1997; WASTERNACK, 2007). Our results lead us to believe that 8.0 µmol L^-1 of exogenously applied JA was not detrimental to the organism physiology, since the responses were similar to that of the control treatment. However, mechanical stimuli (stem bendings) simulating a stress induction were enough to expose a higher photosynthetic rate, greater stomatal opening, resulting in greater CO₂ assimilation, probably in response to a smaller leaf area and higher concentration of the SPAD unit.

Authors have been reporting that mechanical damage results in an increase in the rapid JA accumulation of up to 10-fold (within 5 minutes) after tissue injury (CHUNG et al., 2008; GLAUSER et al., 2008; KOO et al., 2009). However, JA level can be altered by mechanically induced damage leading to an increased JA concentration in the plant, it seems likely that the jasmonic acid dose used in this work was not sufficient for Eucalyptus seedlings to present responses similar to those caused by mechanical stress. Thus, it is evident the need to quantify the JA concentration after the mechanical stress induction by stem bending.

Despite higher Ф values, mechanically stressed seedlings showed the highest light compensation point ((), followed by JA seedlings (Table 1). On the contrary, control seedlings showed the lowest ( values. These proportions, although not show statistical differences, indicate the tendency that in spite of the photosynthetic increments promoted by the mechanical disturbance and jasmonic acid, they show greater respiration in the dark, due to the need of energy expenditure to recover any lesions at the cellular level caused by the stem bendings and increment of the secondary metabolism for structuring intermediate compounds promoted by the jasmonic acid.

Conclusion

JA dose applied to Eucalyptus Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) seedlings was insufficient to change the photosynthetic responses in relation to the control treatment.

Mechanical stress increased maximum photosynthetic capacity and maximal CO₂ assimilation. Therefore, they showed higher transpiration rate and lower leaf temperature in Eucalyptus Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) seedlings.

Induced mechanical stimuli in Eucalyptus Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) seedlings, despite high values of photosynthesis, resulted in a reduction in water use efficiency, that is, lower photosynthetic efficiency when compared to other treatments.

Eucalyptus Cl 1528 (Eucalyptus urophylla x Eucalyptus grandis) seedlings submitted to mechanical stimuli and JA showed greater respiration in the dark.

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Publication Dates

• Publication in this collection
  30 Sept 2019
• Date of issue
  Apr-Jun 2019

History

• Received
  04 Jan 2018
• Accepted
  24 Apr 2018
• Published
  30 June 2019