EFFECTS OF PHOTOSELECTIVE SHADE NETS ON EARLY GROWTH OF SUGARCANE SEEDLINGS

Sugarcane has a great economic interest in Brazil, so it needs technologies to increase its yield strength, especially for seedling production. This study aimed to ascertain the influence of shading nets on early growth and development of pre-sprouted sugarcane seedlings. An experiment was conducted in a protected environment, in the Rural Federal University of Pernambuco (UFRPE), in Recife-PE, Brazil. The protected environments were covered with anti-UV low-density polyethylene plastic plus: Solpack red ultranet net, Solpack white net, Solpack freshnet net, and without shade net, and five replications each. The experiment was carried out in a completely randomized design, and multivariate principal component analysis (PCA) was used to verify associations among cultivation modules, micrometeorological factors, and seedling growth parameters. AntiUV low-density polyethylene + shade net improved seedling growth and development up to 27 days after planting. Higher substrate temperatures benefited early growth of seedlings. After 27 days of planting, sugarcane seedlings required higher air temperatures, as well as increased global and photosynthetically active solar radiation.


INTRODUCTION
Brazil is the world's largest sugarcane producer and export leader, responsible for over 50% of its derivatives that are traded around the world, mainly sugar and ethanol. Due to this, the search for new cultivation technologies has increased to provide greater profitability and production quality (Nascimento et al., 2015).
Pre-sprouted seedling (PSS) production is an alternative to conventional planting, which leads to an excessive number of stems and spread of diseases. PSS is a technology that allows greater control of seedling quality, vigor, and soundness (Xavier et al., 2016).
It is therefore of crucial importance to know the practices used from harvesting and propagative material storage to field transplanting of seedlings. About seedling growth, plants require certain shade conditions during the initial period of production to favor their development (Lee et al., 2007).
As micrometeorological conditions exert great influence on sugarcane seedling viability, photoselective shade nets enable changes in the environment that may influence sprouting and seedling growth Gama et al., 2017;Hirata & Hirata. 2015;Masabni et al., 2016). This has great importance since changes in solar radiation, photosynthetically active radiation, relative air humidity, soil and air temperatures can benefit or hinder seedling growth (Cantu et al., 2013;Klein et al., 2016;Silva et al., 2013).
Given the above, this study aimed to investigate the influence of shade nets on early growth and development of pre-sprouted sugarcane seedlings.

MATERIAL AND METHODS
The study was conducted between September and October 2016, in the campus of the Rural Federal University of Pernambuco (UFRPE), in Recife -PE, Brazil (8º 04' 03" S and 34º 55' 00" W). According to Köppen's classification, the climate is megathermal (As`), with a mean annual temperature of 25.2 °C (Pereira et al., 2002).
The protected environment consisted of an arched greenhouse, covered by low-density polyethylene (LDPE) with anti-UV additive, 50% shade net on the sides, and the following dimensions: 21-m length, 7-m width, 3-m ceiling height, 4.5-m total height, and 147-m² total area. Engenharia Agrícola, Jaboticabal, v.40, n.1, p.10-15, jan./feb. 2020 Seedlings were obtained from a pre-sprouted seedling (PSS) system, using a method adapted from Xavier et al. (2016). While these authors used three distinct environments for sprouting, early growth, and development of seedlings, we opted for a single environment.
Modifications consisted of planting mini-setts of sugarcane in 15-cell trays, conducting sprouting and seedling growth in the same protected environment, under shade nets. Five trays were used per plot, totaling 20 plots and 100 trays for sugarcane seedling production, using sugarcane cultivar RB92579 and coconut powder as substrate.
Seedling production was carried out under a subirrigation system equipped with a cultivation platform. It was composed of a water tank, a submerged pump, and an analog timer. The timer was set to turn the pump on at 7 am daily, pumping nutrient solution for 15 min to the highest part of the platform. This way the solution would rise through the substrate by capillarity until it reached the root system.
Energy availability was characterized in each environment and outdoors, using sensors. These enabled continuous micrometeorological measurements of global solar radiation (GSR; CMP3 Pyranometer LI200/R sensor; 400-1100 nm) and photosynthetically active radiation (PAR; LI190SB Quantum sensor; 400-700 nm). These sensors were connected to a Campbell® datalogger (model CR1000) for data acquisition. Then, the relationship between GSR and PAR was determined and expressed as PAR% for each crop module.
Substrate temperature (Tsubs, °C), air temperature (Tair, ° C), and relative air humidity (RH, %) were recorded in each cultivation module using HOBOware® minidataloggers. The sensors were installed at the geometric center of the cultivation modules, i.e., half-length, halfwidth, and 1.5 m above the ground. Measurements were taken daily at one-second intervals, with averages of 15-min intervals, until seedlings were ready for transplantation.
Non-destructive analyses were performed from 13 to 19 DAP, from 20 to 26 DAP, from 27 to 33 DAP, and from 34 to 40 DAP, when the following seedling biometric variables were recorded: Leaf area (LA) was determined by counting green leaf number and measuring them + 3. Leaves were measured lengthwise and crosswise in the middle portion, according to Hermann & Câmara (1999), and area determined as shown in the [eq. (1)]: Plant stem diameter (SD, mm) was measured using a digital caliper.
Stem length (SL, mm) was measured with the aid of a graduated ruler.
Plant height (PH, cm) was accounted for by direct measurement of 10 plants using a graduated ruler.
The experiment was carried out in a completely randomized design with four variation factors (anti-UV LDPE + red net; anti-UV LDPE + white net; anti-UV LDPE + freshnet; anti-UV LDPE solely) and five replications, totaling 20 plots. Each plot consisted of 75 seedlings, totaling 1,500 sugarcane seedlings.
Associations among cultivation modules, micrometeorological factors, and plant sprouting were evaluated by multivariate principal component analysis (PCA), based on the correlation matrix of variables. Before PCA, data were standardized given the large variation and order of magnitude of variables.

RESULTS AND DISCUSSION
According to the data covariance matrix, the first and second principal components (PCs) explained above 67.5% of the total variation in the four studied periods. This implies that data variability and associations among parameters can be described in only two main axes.
Anti-UV LDPE + shade net showed greater association with plant biometric parameters than those covered with anti-UV LDPE solely. It can be observed by their similar location in graphs for modules ( Figure 1A) and variables ( Figure 1B). Moreover, it can be confirmed due to negative correlation of GSR, PAR, and Tair with plant height (r = -0.68, -0.73, and -0.41, respectively) and stem length (r = -0.89, -0.92, and -0.66, respectively). Photoselective shade nets helped attenuate micrometeorological hazards and reduce extreme changes in the protected environments, which are unfavorable to seedling growth, thus allowing maintenance of growth factors within suitable ranges during early seedling development (Guiselini et al., 2013;Ferrari & Leal, 2015).
Net shading increased associations between cultivation modules and biometric measures. It can be seen by their similar location in graphs for modules ( Figure 1C) and variables ( Figure 1D). Anti-UV LDPE + red ultranet and anti-UV LDPE + white showed a higher association with LAI, PH, SL, and AGR. This may be related to larger Tsubs and RH accumulated in these modules.
Anti-UV LDPE + freshnet was more associated with AGRfw, SD, and AGRsd ( Figure 1D). This module presented a lower association with GSR, Tair, PAR, and had a mean Tair of 29.4 °C, which is the lowest among the studied modules. Therefore, micrometeorological components were more attenuated in this module. Freshnet nets have thermo-reflective properties, thus increasing GSR reflection and Tair alleviation (Araújo et al., 2016).
Both RH and Tsubs were positively correlated with biometric measures, among them PH (r = 0.72 and r = 0.31, respectively) and SL (r = 0.90 and r = 0.21, respectively) ( Figure 1D). Yet, Tair was negatively correlated with SD (r = -0.70), that is, the higher the Tair, the lower the SD. Guerra et al. (2014) observed that a Tair range between 29 and 23 °C increased SD of sugarcane seedlings.
From 27 to 33 DAP, white and freshnet shade nets had a greater association with RH and GSR/PAR ratio, as well as with PH, SL, SD, AGRph and AGRfw (Figure 2A and 2B). Moreover, during this period, sugarcane seedlings started to accumulate AGRsd and LAI in modules with anti-UV LDPE + red ultranet and anti-UV LDPE solely. Such change might be related to the development stage of sugarcane metabolism, as C4 plants are better adapted to light and drought stress (Kluge et al., 2014;Martinez et al., 2015). Figure 2B shows a negative correlation between Tsubs and biometric parameters, in other words, the higher the Tsubs, the smaller the seedling development after 27 DAP. LAI was positively correlated with GSR (r = 0.28), PAR (r = 0.27) and Tair (r = 0.30). This shows that, from 27 DAP onward, sugarcane seedlings required a greater intensity of GSR, PAR, and Tair to promote an increase in leaf area. Brunini & Turco et al. (2016) reported that smaller leaf areas end up harming plant development by reducing solar radiation interception, thereby affecting physiological processes and hence final production.
In anti-UV LDPE + white and anti-UV LDPE + freshnet modules, GSR and PAR transmittances into the environment were lower. This promoted higher SL and PH in seedlings under 50% shade net ( Figure 2C and 2D). It might have occurred as an adaptive mechanism to speed up plant growth under shaded conditions (Moraes- Neto et al., 2000).
Lastly, anti-UV LDPE-covered module had a greater association with GSR, PAR, and Tair ( Figure 2D), as well as with AGRsd and LAI. This confirms the behavior started at 27 DAP, since which seedlings required more GSR, PAR, and Tair to increase LAI (r = 0.68, 0.63, and 0.56, respectively) and AGRsd (r = 0.29, 0.33, and 0.42, respectively). Guiselini et al. (2013) reported that sugarcane seedlings had an increased LAI under greater availability of GSR (37.66 MJ m -² week -1 ) in the growing environment. Physiological changes have also been observed when GSR and PAR rates were reduced in sugarcane plants (Elli et al., 2016).

CONCLUSIONS
Until 27 days after planting, anti-UV low-density polyethylene + shade net improved growth and development of pre-sprouted sugarcane seedlings; From 27 days after planting onward, anti-UV lowdensity polyethylene + shade net inhibited growth and development of pre-sprouted sugarcane seedlings; Higher substrate temperatures benefited early growth of sugarcane seedlings; From 27 days after planting onward, sugarcane seedlings required more global solar radiation, photosynthetically active radiation, and air temperature.