Ecophysiology of Ceiba glaziovii (Kuntze) K. Schum. Submitted to Shading

The Ceiba glaziovii is a forest species used for pharmacological purposes and in the recovery of degraded areas. Nevertheless, the availability of light may be somewhat limiting to its development and may directly influence its physiological and biochemical processes. Thus, the objective of this work was to evaluate the ecophysiological behavior of C. glaziovii plants under different shading levels. The design used was completely randomized, with 5 treatments and 8 repetitions. The treatments used were composed of different shading levels: 0% (full sun), 30%, 50%, 70% and 90% shade. For the analyzes, growth characteristics, gas exchange, chlorophyll indices and chlorophyll a fluorescence were evaluated. Data were subjected to analysis of variance by the F test, and in cases of significance a polynomial regression analysis was performed. The highest values for gas exchange, chlorophyll indexes and fluorescence were 90% shading. Shading is beneficial to C. glaziovii plants, providing positive effects on growth, gas exchange, chlorophyll indexes and chlorophyll a fluorescence.


Introduction
The Ceiba glaziovii (Kuntze) K. Schum. is a species belonging to the family Malvaceae, popularly known as barriguda or paineira, occurring in northeastern Brazil in hyperxerophilous caatingas in rugged environments and in arboreal caatinga (Araújo et al., 2019). It has fast growth, ornamental characteristics, and its bark and leaves are used in folk medicine to treat inflammation, heart problems and hypertension, and can be used in mixed plantations, especially for the recovery of degraded areas (Nascimento et al., 2012;Pereira Júnior et al., 2014).
Several abiotic factors can interfere with the growth and development of forest species, especially in the seedling phase, such as water availability, salinity and luminosity. In this context, light is one of the most important environmental factors and may interfere with physiological processes, and these responses may vary by species (Dutra et al., 2012).
The amount of light energy intercepted by leaves directly influences plant development, where the satisfactory growth of some species under higher or lower light intensity may be associated with the ability to quickly adjust their physiological behavior (Dapont et al., 2016). In addition to acting as the driving force of photosynthesis, light interferes with the structure and operation of the photosynthetic apparatus and consequently on plant yield (Li et al., 2014).
Given this, some studies show that under shading conditions forest species can behave in different ways, such as Caron et al. (2014) in Ilex paraguariensis A. St. Hil., where shading provides lower shoot biomass production, Queiroz and Firmino (2014) confirm that shading up to 30% has a positive influence on the initial development of Dipteryx alata Vog., and Reis et al. (2016) in plants of Copaifera langsdorffii Desf conclude that 50% shading provides better seedling quality and therefore better field survival. In this sense, studies that seek to know the physiological behavior of forest species under shading conditions is of great importance.
Therefore, the objective of the present work was to evaluate the ecophysiological behavior of C. glaziovii plants under different shading levels.

Material and Methods
The experiment was conducted in a greenhouse belonging to the Laboratory of Plant Ecology, Department of Plant Science and Environmental Sciences, Center for Agricultural Sciences, Federal University of Paraí ba, Campus II, Areia, Paraí ba, Brazil. During the experiment, the environment presented an average temperature of 27.9 °C and a relative humidity of 63.4%, whose measurements were made using a portable digital thermo-hygrometer (Minipa, model MT-241A).
The seeds of C. glaziovii were collected in matrices located in Campina Grande, Paraí ba State, Northeast Brazil. The seeds were planting in plastic pots with a capacity of 5 dm³ , and in substrate formulated of vegetal soil and sand (3:1). In Table 1 are the chemical attributes of the substrate used. Two seeds/pot were used and at 10 days after emergence (DAE) thinning occurred, selecting uniform individuals, with an average of 6 cm height. Then the plants were transferred to the different shading levels (treatments) and the evaluations started at 30 days after emergence (DAE). Each shading level within the greenhouse was determined from a digital luxmeter (Minipa, model MLM-1011), and the irradiance within the shaded screens was calculated in proportion to the full sun condition (0% shade) . Shading screens were purchased from local businesses.
The site of the experiment has high coverage and specific areas, so that the light was not influenced during the day. Daily, during the experimental period, the pots were weighed and rotated, maintaining the field capacity around 80%, according to the methodology proposed by Casaroli and Jong van Lier (2008).
For growth analyzes, plant height (PH) (cm), stem diameter (SD) (mm) and leaf number (NL) were measured at 30 and 60 DAE. At 60 DAE, the plants were collected from the pots and subjected to root separation. The roots were then rinsed under running water for cleaning and debris removal and thus the length of the main root (RL) (cm) was determined by a millimeter ruler and the diameter of the main root (RD) (mm) using a digital caliper. Subsequently, the volume of the root system (RV) (mL) was determined from the ISSN 2166-0379 2020 displacement of the water column in a graduated beaker, inserting the roots after washing, in a beaker with a known volume of water (50 mL) . From the difference, the direct response of the root system volume was obtained through the equivalence of units (1 mL = 1 cm 3 ).
For quantification of indexs chlorophyll a, b, total and a/b ratio at 60 DAE, a portable chlorophyllometer (ClorfiLOG®, model CFL 1030) was used in four leaves of the middle third of the plants.
For chlorophyll a fluorescence, initial fluorescence (F 0 ), maximum fluorescence (F m ), variable fluorescence (F m -F 0 ), maximum quantum yield of PSII (F v /F m ) and F v /F 0 ratio were evaluated at 60 DAE. For the measurements, healthy leaves were used in the middle third of the plants, being adapted to the dark by tweezers for a period of 30 minutes. For the analysis was used a portable modulated fluorometer (Sciences Inc.-model OS-30p, Hudson, USA).
Data were subjected to analysis of variance by the F test, and in cases of significance, a polynomial regression analysis was performed, with the adjustment of representative curves. To perform the statistical analyzes was using SAS® (Cody, 2015).

Results and Discussion
Shading levels positively influenced C. glaziovii growth ( Figure 1A, B and C). Plant height (PH) increased linearly at 60 DAE ( Figure 1A), the number of leaves (NL) followed the same trend, with the highest values obtained in shading of 90% ( Figure 1B). Stem diameter (SD) data best fitted a quadratic effect, with the largest increases observed in shading of 53.4% at 60 DAE when compared to the treatments with higher light intensity ( Figure 1C).
The better vegetative growth evidenced by these PH, NL and SD results can be attributed to the increase in the plants capacity to absorb water as the lowest evaporation under shading conditions, which consequently accelerates the vegetative growth rate (El-Bassiony et al., 2014). This growth pattern in C. glaziovii may promote its use in restoration of degraded areas, especially in advanced stages of succession, for under this condition to greater competition for light (Valadã o et al., 2014). The length (RL) and root diameter (RD) increased linearly as a function of shading levels, with increases of 33.7 and 25.3%, respectively, compared to plants conducted under full sun (Figure 2A and B). However, root volume (RV) was higher in shading of 70% ( Figure 2C).
These increases in RL, RD and RV indicate that C. glaziovii, when subjected to shading conditions, invests most of the growth in the shoots, a fact that can be observed in Figure 1, as well as in the root system, providing a larger biomass allocation. Thus, the deep root system improves the absorption of water and nutrients, increasing its survivability, especially during the dry season (Castro et al., 2014). ISSN 2166-0379 2020  Shading positively influenced C. glaziovii gas exchange ( Figure 3A, B, C and D). The CO 2 assimilation rate (A) increased linearly, reaching a rate of 8.7 µmol m -2 s -1 in 90% shading ( Figure 3A). The values for stomatal conductance (g s ) and transpiration (E) fit a quadratic effect, with the highest efficiencies in shading levels of 69.1 and 75.5%, respectively ( Figure  3B and C). The internal concentration of CO 2 (Ci) was reduced with increasing shading levels, 10.4% reduction in 90% shading ( Figure 3D).

Journal of Agricultural Studies
The increase in A is a strong indication that C. glaziovii is a species adapted to shaded environments, thus requiring a lower light intensity to reach the photosynthetic peak, and this behavior is also associated with increases in chlorophyll indices. In this sense, large amounts of light can cause physiological disturbances and may induce the photoinhibition process (Tang et al., 2015). Increases in g s may be associated with increases in A, due to the reduction in the amount of CO 2 in intercellular spaces, which may induce stomata opening. However, high shading levels provide decreases for this variable, probably due to the vapor pressure deficit along with a higher temperature (Saraiva et al., 2014).
The transpiration rate is directly influenced by g s , where the stomatal opening and closing mechanism controls the exchange of leaf gases with the environment (Sousa et al., 2016). ISSN 2166-0379 2020 Reductions in Ci can be attributed to higher photosynthetic rates, which is an indication that CO 2 is being fixed by Rubisco (Sun et al., 2014).

Journal of Agricultural Studies
Shading provided increases in instantaneous water use efficiency (WUE) and carboxylation efficiency (ICE), with the highest values recorded in shading of 90% ( Figure 3E and F). Leaf temperature (Tleaf) was reduced with increasing shading levels ( Figure 3G).
Increases in WUE and ICE may be associated with gains in A, showing a close correlation between these variables, because a higher CO 2 assimilation induces a higher Rubisco activity, providing greater CO 2 availability compared to O 2 (Nazar et al., 2015). Reductions in Tleaf can be explained by gains in g s and E causing leaf cooling, which is positive, since overheating of the photosynthetic apparatus can affect physiological and biochemical processes (Morais et al., 2017).  ISSN 2166-0379 2020 Chlorophyll indices were higher in the shade treatments ( Figure 4A, B and C). Up to the 72.3% level, shading was positive for chlorophyll a ( Figure 4A). Chlorophyll b and total indices increased linearly as a function of shading levels, with values in the order of 12.1 and 51.2 ICF, respectively, at 90% shading ( Figure 4B and C).
These additions can be explained by the fact that plants grown in shaded environments tend to synthesize a greater amount of chlorophyll per reaction center, thicker leaves and larger areas, possibly as an adaptation mechanism, favoring the capture of light (Taiz et al., 2017). The linear increase in chlorophyll b can be attributed to the fact that this pigment is degraded more slowly than chlorophyll a (Nery et al., 2016), contributing to a higher total chlorophyll index. Chlorophyll a fluorescence parameter was positively influenced by shading levels. The initial fluorescence (F 0 ) was reduced as the shading levels increased ( Figure 5A). Maximum fluorescence (F m ), variable fluorescence (F v ), maximum quantum yield of PSII (PQE) and F v /F 0 ratio increased in shaded environment ( Figure 5B, C, D and E).
F 0 is measured when all reaction centers are open, and reductions in this variable are a good indication that less energy is being lost in the form of heat, favoring the photochemical phase of photosynthesis (Fu et al., 2012). Increases in F m and F v suggest that photo-acclimation of ISSN 2166-0379 2020 plants occurred under shading as a way to maximize light uptake (Reed et al., 2012). The fact that these variables are indicative of the state in which the PSII reaction centers reached their maximum capacity, associated with the active potential energy in the PSII reinforces this statement (Sá et al., 2018).

Journal of Agricultural Studies
The positive results found in F v /F m indicate that in a shaded environment C. glaziovii plants tend to have a higher photochemical activity than PSII. This shows that under full sun conditions the plants are subjected to the photoinhibition process (Dąbrowski et al., 2015). The F v /F 0 ratio followed the same trend, which is a way of measuring the efficiency of PSII photochemical reactions (Zivcak et al., 2014).

Conclusions
Higher shading levels provide better growth, photosynthetic rate, chlorophyll indices and chlorophyll a fluorescence in C. glaziovii plants; Shading is beneficial for C. glaziovii plants and can provide higher quality seedlings.