Growth of Alfavaca-Cravo in Response to Different Levels of Shade and Tiririca Density

The species Ocimum gratissimum L. is widely utilized in food, cosmetics, and folk medicine, and is also an important source of essential oils. Understanding its behavior in response to environmental conditions is of paramount importance to improving crop management methods. In this context, the following study aimed to evaluate the effects of shade, and of competition with weeds (Cyperus rotundus L.), on the growth of Ocimum gratissimum L. The experimental design adopted was randomized blocks, in a 5 × 5 factorial scheme, with 5 levels of shading (48%, 75%, 77%, 83% and 90%) and 5 densities of Cyperus rotundus L. (0, 5, 10, 15 and 20 per pot), with 4 repetitions. The variables analyzed were main stem height (MSH), diameter of stem base (DSB), number of leaves on the principal branch (NL), number of ramifications (NR), chlorophyll index of leaves (CIL), foliar area (FA), dry mass of the aerial part of the medicinal species (DMAPm), dry mass of the aerial part of the weed species (DMAPw) and essential oil content (EOC). The results demonstrate that the Ocimum gratissimum L. plants presented compatible tolerance responses to up to 70% shading, and that competition with Cyperus rotundus L. was detrimental in a density above 13 plants per pot in interaction with shading. The highest dry mass production and, consequently, the highest oil yield, were obtained from the 48% shading treatment.


Introduction
The species Ocimum gratissimum L., commonly known as clove basil, African basil, or wild basil, is an aromatic subshrub native to Asia and Africa, and is subspontaneous in all Brazilian territory. It contains numerous compounds, primarily eugenol, that are likely responsible for its antimicrobial activity (Oliveira et al., 2016). When aiming to improve crop management, it is necessary to understand its behavior in response to environmental conditions. Secondary metabolites' regulatory mechanisms depend greatly on the genetic control inherent to each species, and on the external stimuli promoted by the environment, such as climatic and agronomic factors Environmental factors such as temperature, relative humidity, radiation, photoperiod and cultural practices influence the content and composition of essential oil in plants. Light, as a primary energy source for photosynthesis and morphogenetic phenomena, is a major factor in plant growth and development, which are further affected by the intensity, direction, duration, and quality of the light (Corrêa et al., 2012).
Similarly, competing with another species for light and other resources will activate a plant's defense mechanisms, thus altering the secondary metabolism. One such response is increasing allelochemical production and releasing them into the environment, which then affects the sensitive recipient plant's growth and development (Shah et al., 2016).
Nut grass (Cyperus rotundus L.) is a perennial weed plant species that is difficult to manage, and damages a variety of commercial crops year-round. Due to its wide adaptability to many agricultural environments, and its capacity for both sexual and asexual reproduction, nut grass is one of the top 10 weed species globally causing the most loss in the agricultural sector. This harm is directly caused by the competition for water and nutrients. Water availability influences all plant physiological processes, and therefore, water stress influences the allelochemical production (Rockenbach et al., 2018).
Currently, the majority of Brazilian medicinal plant studies are aimed at identifying their essential oil components. However, the crop's management is determinant to achieving both higher dry matter production rates and active ingredients of economic and pharmacological interest (Souza et al., 2014). Thus, this study's objective was to evaluate different shading levels' effects, in association with different Cyperus rotundus L. densities, on the growth of Ocimum gratissimum L.

Material and Methods
The Ocimum gratissimum L. seedlings were produced from the seeds of adult plants from the municipality of Campos dos Goytacazes-RJ. Exsiccate from the Ocimum gratissimum L. was deposited in the Darcy Ribeiro herbarium of the Universidade Estadual do Norte Fluminense (UENF), in Campos dos Goytacazes, under registration HUENF 10457.
The transplant to the 5.5 L capacity pot was performed 90 days after sowing. For greater yield of biomass and standardization of seedlings of Ocimum gratissimum L., flowering cuts were made, following the recommendations of Costa et al. (2007). Simultaneously, seedlings of Cyperus rotundus L. were harvested at UENF and standardized at approximately five centimeters in height.
The substrate used was composed of soil + sand + bovine manure, in a 1:1:1 proportion (v/v). The substrate's chemical characteristics were: pH in water = 6.0; P and K (mg/dm It is important to note that while conducting the experiment, additional fertilizer was not needed, nor were pest or disease control. This was due to the soil's physical-chemical characteristics: fertile (V% greater than 50), containing humidified organic matter, and free of toxic elements.
The experiment took place in a greenhouse in order to better control the experimental conditions. A portable Hygrometer-anemometer-luximeter (EMD THAL 300) was used to measure the shading levels. The quantity of light in every environment was determined on days with no clouds (average of three days with three measurements per day), at 12 noon. On these days, the average amount of light outside the greenhouse (11485 luxes) was considered to be 100% light (0% shading), which made it possible to estimate the levels of light intensity and shading (48%, 75%, 77%, 83% and 90%) for every treatment. In the 48% treatment, the Sombrite screens were not used, with shade provided only by the greenhouse's plastic covering. In the other treatments, the shading caused by the plastic together with the respective screen was considered.
Climatological data of temperature (T°) and relative air humidity (RH) were monitored during the experiment in two-hour intervals, using the Data Logger® device (model: RHT10, Extech brand) installed in the greenhouse. The temperature reached a maximum of 37.8 ºC, an average of 26.0 ºC, and a minimum of 19.4 ºC. The relative humidity was a maximum of 88.4%, an average of 69.1%, and a minimum of 38.5%.
At 120 days after transplant, the following variables were evaluated: main stem height (MSH), using a measuring tape, taking the distance between the neck and the apex of the plant, in centimeters; diameter of the stem base (DSB), with the aid of a digital caliper, taking the measurement at the base of the stem, in millimeters, 1 centimeter from the ground; number of leaves on the principal branch (NL); number of ramifications (NR); chlorophyll index of leaves (CIL), obtained with the aid of a CIL chlorophyllometer, Chlorophyll Meter SPAD-502 (Minolta®), through the average of three readings, in the third pair of leaves in the median region of each plant, indicating the presence of chlorophyll in SPAD values (Soil Plant Analysis Development); foliar area (AF), using a LI-3100 area meter from the company LI-COR, where the value is obtained at the moment the sheet passes through the device's sensor, in cm²; dry mass of the aerial part of the Ocimum gratissimum L. species (DMAPm) and of the Cyperus rotundus L. plants (DMAPw), in which leaves, stems and flowers of all plants were collected separately, packaged in paper bags properly identified, placed in a forced circulation oven at 40 ºC until they obtained constant weight and were later weighed on a digital scale (precision of 0.01 g); essential oil content (EOC) was extracted as done by Rechner et al. (2011) in which EOC% = mass of oil (g)/dry material (g) × 100. The oil was extracted by the hydrodistillation method in a Clevenger device, using dry matter of every medicinal plant (leaves + stem + flowers) in 1200 mL of water for 2 hours and 30 minutes.
The experimental data was initially submitted to preliminary tests for diagnosis of data normality, and of homoscedasticity of experimental errors, of Shapiro-Wilk and Bartlett, respectively. Once these assumptions were met, the data was then submitted to a variance analysis, according to the factorial model. When the significance of the shading × density interaction was detected, it was then studied. As these are two quantitative factors, a response surface model was investigated to assess the joint effect of the two factors on the (quantitative) response variable. When the interaction was not significant, a simple regression model was adopted for each factor in isolation (this being significant).
The choice of the best regression models, both the response surface and simple regression, were based on some criteria such as: analysis of variance regression; significance test of each of the regression model coefficients; model determination coefficient and the distribution of residues in a dispersion diagram; in addition, the parsimony criterion was also used. Once the regression models chosen, graphs were drawn up and critical points were also determined (maximum or minimum), using the mathematical knowledge of derivatives.

Results and Discussion
The results of analyzing shading effects variance (48%, 75%, 77%, 83% and 90%) and density of C. rotundus L. (0, 5, 10, 15 and 20 plants/pot) are presented in Table 1. It is observed that the main stem height (MSH), diameter of stem base (DSB), number of leaves (NL), number of ramifications (NR), chlorophyll index of leaves (CIL), and foliar area (FA) variables of Ocimum gratissimum L. were affected by the shade levels. There was an interaction between the shade levels and density of weed plants for the dry mass of the aerial part variable of the medicinal species (DMAPm). The weed's aerial part's dry mass (DMAPw) was influenced by the shade levels and density of weed plants, separately. However, the essential oil content (EOC) presented with no significant difference for the factors under study.   Note. ** = Significant at 1% probability level by F test; * = Significant at 5% probability level by the F test; ns = Not significant; CV: coefficient of variation (%).
The shading provided a significant increase in the NL ( Figure 1A) and CIL ( Figure 1B) variables, and for NL a maximum value of 28.21 leaves was found for a shading of 90%, indicating an increase of 39.52% in relation to the 48% shading, adjusting to the growing linear model. Such behavior suggests that in shading conditions, there is a need to acclimatize the plants to the environment, in order to obtain more efficient light absorption for photosynthetic processes (Martins et al., 2010).
The same was observed for the CIL variable (SPAD), in which there was a maximum value of 49.46 for the 90% shading, indicating an increase of 38.05% in relation to the 48% shading, adjusting to the growing linear model. The leaf green intensity was analyzed with CIL portable equipment, which evaluates the chlorophyll content of the plant in real time, since there is a significant correlation between the intensity of the green and the chlorophyll content in the leaf (Martuscello et al., 2009).      Vol. 13,No. 7; of the aerial pa  Vol. 13, No. 7;2021 The greater production of dry mass and, consequently, the highest yield of oil, was obtained in lower shading. In contrast, competition Cyperus rotundus L. proved to be harmful in interaction with shading for the dry mass of the aerial part of the medicinal species.