Effects of Sowing Date on Physiological Characteristics , Yield and Yield Components for Different Maize ( Zea mays L . ) Hybrids

In order to determine the effects of sowing date on physiological characteristics, yield and yield components of six sweet maize hybrids, an experiment was arranged in split plot based complete randomized block design with three replications. Sowing date in two levels (15 June and 1 July) and sweet maize hybrids in six variants (‘Chase’, ‘Temptation’, ‘Challenger’, ‘Basin’, ‘Obsession’ and ‘Ksc403su’) were the treatments. Results of ANOVA revealed significant difference of sowing date and sweet maize hybrids on the number of days for tassel emergence, number of days to anthesis, plant height, cob height, stem diameter, plant dry weight, cob dry weight, number of grain rows, length flowers, number of leaf above the cob, raffinose content of grain and grain yield. Mean comparisons showed that the highest grain yield was obtained for ‘Basin’ variety and it was obtained from the crop established on the 15 of June as planting date. In temperate regions, maize potential productivity seems to be more limited by the amount of solar radiation available around silking (determinant of grain set) than during grain filling (determinant of grain weight).


Introduction
Fresh sweet corn (Zea mays L. var.saccharata) has been consumed broadly by boiling or by grilling since past times.Production and use of sweet corn has expanded rapidly in recent years.Sweet corn varies from other corns (field maize, popcorn and ornamental) since the grains have great sugar content in the milk on early dough stage.It is consumed in the immature stage of the crop.The taste of sweet corn kernels is 25-30% sweeter than normal corn.At optimum market maturity, sweet corn will contain 5 to 6% sugar, 10 to 11% starch, 3% water-soluble polysaccharides and 70% water.Sweet corn also has medium levels of protein, vitamin A (yellow varieties) and potassium (Najeeb et al., 2011).It can be consumed as fresh, frozen or conserved and also used particularly as garniture in the salads.Corn (Zea mays L.) is one of the most important cereal crops grown principally during the summer in Iran.Is one of the important cereal crops in the world after wheat and rice (Gerpacio and Pingali, 2007;Golbashy et al., 2010).The International Maize and Wheat Improvement Centre (CIMMYT) accompany with National Agricultural Research Systems (NARS) routinely conducted maize regional trials with the objective of evaluating, selecting high yielding and stable genotypes in a wide range of environments (Banziger and De Meyer, 2002).Grzesiak (2001) reported remarkable genotypic variability among corn varieties for several characteristics.Ihsan et al. (2005) also demonstrated considerable genetic differences for morphological variables for corn genotypes.This mutability is a clue to crop improvement (Welsh, 1981).
Environmental variations related with different sowing dates have an altering effect on the growth and development of corn plants.Each corn hybrid has a desirable planting date, and the larger the deflection from this favorite (early or late planting), the larger the yield loss (Sárvári and Futó, 2000;Berzsenyi and Lap, 2001).Sowing date was introduced to affect the growth and yield of corn significantly.To date, compete for corn growers is finding the thin window between cultivation too early and cultivation too late (Nielson et al., 2002).Either early cultivation or late cultivation can result in lower yield since the probability exists that unfair climatic conditions can happen after cultivation or during the growing season.Norwood (2001) suggested that farmers should plant on more than one sowing date in order to protect against unforeseen seasons.Short season hybrids can be cultivated early without harmful effects on their upmost yield potential.It can also minimize the risk of obtaining immature cobs and grains or sustaining early frost loss (Hicks et al., 1993).
accumulation in kernels and comes to an end with physiological maturation.Kernel weight is also determined in this stage.However, an important effect of temperature is that higher temperature (especially at night) shortens grain filling period and increases grain filling rate, while lower temperature have an inverse effect (Jones et al., 1981).Therefore, the present work was carried out to study the effect of sowing date and cultivar on grain yield of sweet corn.

Materials and Methods
The experiment was accomplished at Agricultural Research Center of Karaj, Iran.Soil preparation operations included plowing by moldboard plow, completing it by disc and leveling.Before carrying out the experiment, the soil was sampled from the depth of 0-30 cm.The experiment was carried out on clayloam soil (Table 1).The results of soil analysis indicated that the absorbable phosphorus and carbon were 81.15 ppm and 81% respectively, and its pH was 7.61.The Meteorological information of Karaj is shown in Table 2.The previous crop was wheat.The soil was fertilized by 350 kg Urea/ha (1/3 during sowing, 1/3 at six leaf stage and the remaining during tassel emergence as top-dressing), 180 kg K2O/ha, and 150 kg P2O5/ha after leveling and before making the furrows.Six varieties namely 'Chase', 'Temptation', 'Challenger', 'Basin', 'Obsession' and 'Ksc403su' were sown in two dates: 15 June and 1 July 2014.The experiment was arranged in split plot based complete randomized block design with three replications.Main factors were sowing date at two levels (15 June and 1 July) and sub plots were the six cultivars.Spacing of 75 cm row to row and 18 cm plant to plant spacing was maintained and two seeds were sown at the depth of 5 cm; at four-leaf stage, one plant with the best development, was kept and the other was eliminated.Plot size was 6 × 3.6 m, out of which 5 × 2.4 was used to assess final harvest.The furrow irrigation was applied twice a week.Data were collected when each cultivar, for both planting dates, was judged to be at optimum fresh market maturity.
Measurements were made immediately after harvest.All morphological and yield component traits were measured on 10 randomly selected plants of each plot.Ten ears per replication were randomly selected and husked for the length, width and ear tip fill measurements.The number of days until 50% crop tasseling, 50% silking and 5% pollination were recorded.Sweet corn growths in height and leaf number, as a function of thermal time, were determined for each plot.To evaluate the effect of planting date on sweet corn establishment, growth and yield, the additional variables measured after crop emergence, near silk emergence and at harvest were subjected to ANOVA and means separation.Yield was measured in 3 m 2 for each treatment.Fresh ears were immediately husked with a husking bed (Sweet Corn Husker; A&K Development Co., Eugene, OR) and kernels were removed from the cob with an industry-grade corn cutter.Husked mass and kernel mass were recorded and adjusted to 15% moisture level.Ten ears per replication were randomly selected and husked for the length, width and ear tip fill measurements.The data statistical analysis was done by SAS statistical software (SAS, 2002) and the comparison of mean was also done by LSD test at 5% probability level.
The environmental and agronomic respond of corn hybrids recognize their adaptability and influence improvements in corn production through agronomy and breeding.Newly improved varieties usually need to be examined at several sowing dates or locations and for many years before being counseled for a given location.The basic environmental effects and genotype environment interaction have been introduced as the most important sources of alteration for the measured yield of crops (Dehghani et al., 2006;Yan et al., 2007;Sabaghnia and Sabaghpour, 2008).The yield of maize in Iran is very little (Xue et al., 2002).Maize undergoes three stages from pollen dissemination to physiological maturation (Johnson and Tanner, 1972;Cox et al., 1998).The first stage is known as lag phase (slow growth), the cells start to divide (Duvick, 1951;Abdul et al., 2001).At the end of this stage, grain weight slightly increases since endosperm cells play an important role in sink capacity (Outtar et al., 1987;Ahmad et al., 2001).The second stage is known as grain linear filling (log phase), the stage of sharp increase in grain dry weight due to the conversion of sugar to starch in endosperm, which commences 2-3 weeks after tassel emergence.Over 90% of grain dry weight is realized in this stage (Johnson and Tanner, 1972;Cox et al., 1998).During stage, grains grow with a speed of 2-3% of final yield per day.
The most important effective factors on grain yield are application of optimal maize hybrids and suitable sowing dates.The research works focused more on breeding aspects rather than crop management (Xue et al., 2002).Under the optimum planting time for maize conditions, would be from the last week of September to the end of October in Khartoum area.High grain yield (2,952 kg ha -1 ) was obtained during this period (Imam, 1966).The period from November to February is the best time for the highest dry matter production in the Khartoum area.It was also reported that the mean daily temperature is the major environmental factor that affects the crop development and yield (Elkarouri and Mansi, 1980;Begna et al., 2000).Grain yield maize was reduced when sowing time was delayed to the end of October (Mc Cormick, 1974).Delaying sowing date to mid-December reduced the individual 1,000 kernel weight (Cirilo and Andrade, 1996); the authors also indicated that maize varieties differed in their growth characters in Gainesville Florida (El-Koomy, 2005;Gardner et al., 1990).It has been shown that July 15 is an optimal sowing date for maize in Peshawar (Ahmad et al., 2001).In India, Sadek et al. (1994) and Zaki et al. (1999) reported that maize cultivars differed in yield and its components in the same region.
Variation in biological yield of corn varieties at different planting dates was associated with differences in the amount of intercepted radiation.Shorter cultivars had greater assimilated allocation to the grain than the taller cultivars (Benga et al., 2000).Grain growth rate is directly associated with dominant temperature and largely independent of dry matter accumulation in final crop (Duncan et al., 1965).If kernel growth outruns dry matter accumulation in the final crop, its required dry matter will be supplied and remobilized from stalks, leaves and cob covers (mostly from stalks).The third stage is accompanied with a decrease in dry matter had similar grain yield, grain glucose, grain fructose, grain saccharose, grain raffinose, number of grains row and number of leaves at the two sowing dates.All varieties had similar grain yield except 'Obsession' and 'KSC403su' (Table 4).
Applying the optimum sowing date for maize cultivars has a positive effect on a grain yield and physiological index in maize.The study revealed that both sowing date and cultivar had significant effect on grain yield in applied sweet maize varieties under the field conditions.Similar results have been obtained where seeding dates and varieties significantly influenced on physiological characteristics, yield and component yield (Quayyum and Raquibullah, 1987;Abdul Rahman et al., 2001;Nielson et al., 2002).In the current study sowing date × cultivar interaction significant affected days to anthesis, leaves above the cob, length of male flowers, grain yield, grain glucose, grain raffinose.Plants at optimum sowing date (15 June) performed higher yield; 'Basin' produced a higher grain yield (about 9.77 t/ha), while 'KSC403su' produced the lowest quantity (7.25 t/ha).Plants at optimum sowing date (15 June) performed the high grain raffinose; 'Chase' produced a higher grain raffinose (about 0.71 mg/gr), while 'Challenger' produced the lowest (0.00 mg/gr).This result is in agreement with findings of Otegui et al. (1995), saying that optimum planting date resulted in higher grain yield compared with early and late planting dates because of higher cob numbers and greater kernel numbers per plant.
In conclusions, overall, the best planting date for the six sweet corn varieties tested was 15 June and the height yield was obtained for the variety 'Basin'.

Table 2 .
Meteorological information of the study area

Table 3 .
Analysis of variance for physiological characteristics, yield and component yield traits in the sowing date and variety treatments

Table 4 .
Mean comparison of physiological characteristics, yield and component yield traits in interaction effect of sowing date and variety treatments Means followed by same letters in each column have not significant difference at 5% probability