Sowing Time of Sweet Corn in Summer Season in Northwestern Paraná , Brazil

The aims of study were to evaluate the effect of sowing time in summer season on agronomic characteristics of sweet corn grown in summer season in Northwest Paraná, Brazil. The experiments were conducted in 2011, and 2012, in both years, the sowing was carried out on ST1: 09/05; ST2: 09/19; ST3: 10/03; ST4: 10/17, and ST5: 10/31. The experimental design was a randomized block design in a 5 × 2 factorial scheme, consisting of sowing time and two corn hybrids, with four replicates. The evaluated variables were plant height, leaf area index and yield of commercial ears, in which the effects of sowing times were studied through regression analysis, and effects of hybrids and agricultural years were studied by F-test. Sowing times promoted reductions in phenology of hybrids, but did not provide variations in degree-days accumulation by hybrids. The maximum leaf area index (3.82) was estimated for 10/04. In year 2011, the maximum plant height (2.17 m) and yield (13.0 Mg ha) were estimated for 10/06, and 10/03, respectively. While, in year 2012, the maximum height (2.22 m) and yield (12.85 Mg ha) were obtained on 10/09 and 10/11, respectively. The hybrid RB6324 showed greater yield potential, independent of year.


Introduction
Sweet corn is characterized by presence of genes that block the conversion of sugars to starch in endosperm, resulting in accumulation of high sugar content (Okumura et al., 2014), and low starch content (Souza et al., 2013).The main food uses of sweet corn are canned and "in natura" consumption and, after harvesting, vegetative part can be used for silage production, destined for animal feed, directed perfectly at family farming (Pereira Filho & Teixeira, 2016).
Although sweet corn has increased interest in Brazil and World, little emphasis has been placed on its management (Souza et al., 2013;Okumura et al., 2014), with farmers being directed to use the recommendations for common maize (Williams II, 2008).However, agronomic practices differ markedly from those recommended for common maize, including stepped planting (Pereira Filho & Teixeira, 2016), shorter time interval for harvesting and reduction of grain filling period (Franco et al., 2016).Thus, the time that sweet corn is exposed to climatic factors is different from corn grain and, considering that Zea mays L., is strongly influenced by environmental factors (Tsimba, Edmeades, Millner, & Kemp, 2013a;Marques et al., 2015).
Moreover, in climate change scenarios (Liu, Hubbard, Lin, & Yang, 2013), the negative effects under sweet corn crop are mitigated by changes in sowing time (Waha et al., 2013), selecting periods in which current climatic conditions can be optimized (Silva, Ferreira, Andrade, & Araujo, 2010), reflecting directly on the incidence of The knowledge of information contributes to increase the sweet corn yield (Souza et al., 2013, Franco et al., 2016).It is extremely important to have seen the increase in world population and demand for food (Liu et al., 2013), providing options for agricultural diversification and strengthening of family agriculture (Pereira Filho & Teixeira, 2016), as well as climate change.The aims of study were to evaluate the effect of sowing time (ST) in summer season on agronomic characteristics of sweet corn grown in summer season in Northwest Paraná, Brazil.

Study Site
The experiments were conducted at Experimental Farm of State University of Maringá, located in Maringá city, Paraná State, Brazil (geographic coordinates: 23º20′48″ S and 52º04′17″ W, approximate altitude of 550 m).

Treatments, Experimental Design and Plot
The experiments was conducted in randomized complete block design, in a factorial 5 × 2 in two agricultural years (2011 and 2012), with four replications.The factors studied were: i) Factor A: five sowing time (ST1: 09/05; ST2: 09/19; ST3: 10/03; ST4: 10/17, and ST5: 10/31), and ii) Factor B: two sweet corn simple hybrids (Tropical Plus and RB6324).Each experimental plot was constituted of five rows of plants with 6.0 m in length, spaced by 0.9 m, totaling a total area of 27 m 2 and an experimental area of 13.5 m 2 (Marques et al., 2015).

Crop Management
The sowing density used was 5 plants m -1 in no-tillage system in order to reach a population density of approximately 55.500 plants ha -1 (Souza et al., 2013).Management of fertilizing was according to Okumura et al. (2014) and all other cultivation techniques used were those recommended for the corn crop, including irrigation management (Marques et al., 2015).

Evaluated Characteristics
The phytotechnical evaluations of growth (plant height, and leaf area index) were assessed in five randomly selected plants in each plot at VT (Ritchie, Hanway, & Benson, 1993).Plant height was measured on the stem length from the soil surface to the base of the male inflorescence (Moreira et al., 2018).The leaf area index (LAI) was obtained by measuring the length (L) and the width (W) of all the leaves in each plant.The leaf area (LA) was calculated by the following equation: LA = 0.75 × L × W, later, the LAI was calculated from the LA measures by the equation (Sangoi et al., 2007): where, S 1 and S 2 refer to the spacing in meter (m) between plants and between rows, respectively.
The sweet corn ears were harvested at R3 (Ritchie et al., 1993), in milky grain (Pereira et al., 2009).At this moment was assessed the yield of dehusked marketable ear (length greater than 15 cm, diameter greater than 3 cm and free from pests), according to Albuquerque, Von Pinho, Borges, Souza Filho, and Fiorini (2008).

Data Analysis
The experimental data from each one of the agricultural years were assessed using the Shapiro-Wilks and Levene tests (P ≤ 0.01) to verify the normality and homoscedasticity waste.Later, the data were subjected to an individual analysis of variance, and was verified whether the relationships between the residual mean squares were lower than 7:1.Finally, the data were submitted to analysis of variance with application of F-test (P ≤ 0.05), and then partitioning as necessary (Barbin, 2013).The sowing time effects were studied by polynomial regression analysis and the effects of the hybrids and years were studied by the F-test (P ≤ 0.05) of the analysis of variance.The statistical procedures were carried out by SISVAR statistical software (Ferreira, 2011). jas.ccsenet.

Climat
The  2), period which the lowest temperatures were recorded (Figure 1).Gaile (2012), studying the effect of sowing time on maize development obtained negative correlations between temperature and number of days until germination.

Joint Analysis of Variance of Experiments
The analysis of joint variance indicated a significant effect (P < 0.05) for sowing times and agricultural years in evaluated variables.While, hybrid factor was significant only for yield of commercial ears (Table 2).Note.* significant; ns not significant, in 5% probability level, by F-test.
For the interactions, analysis of variance showed dependence (P < 0.05) between the factors in times x hybrids interaction and in triple interaction for yield, showing that this variable is related to sowing times, hybrids, and years.However, for sowing times x years interaction the analysis showed independence (P > 0.05) among the factors for LAI variable.The analysis did not indicate a significant effect (P > 0.05) on hybrids × years interaction in any studied variable (Table 2).

Produc
The The year 2012 was better for sweet corn production, independent of hybrids adopted (Table 4).This result can be explained, among other factors discussed, by greater availability of radiation in 2012 compared to 2011 (Figure 1), because solar radiation is one of most influential climatic factors on maize crop (Brachtvogel et al., 2009), promoting excitation of chlorophyll molecules, initiating energy flow necessary for photosynthesis (Taiz & Zeiger, 2013).According to Fageria (1998), solar radiation incident on terrestrial atmosphere is main source of energy for physiological and biochemical processes that occur in plants.For these reasons, maize yield follows a relation directly proportional to radiation, not occurring water restrictions (Bergamaschi et al., 2006).

Conclusions
Sowing times influenced the growth and yield of sweet corn.The RB6324 hybrid showed higher yield of dehusked marketable ears compared to Tropical Plus hybrid.The use of irrigation was essential in all sowing times and years of study.

Table 2 .
Summary of the variance analysis of sweet corn phenotypic characteristics (plant height-PH, leaf area index-LAI and yield of marketable ear-Yield) in the summer season of 2011 and 2012, in Maringá city, Northwest Region of Paraná State, Brazil