In this article we will discuss about the growth and development of maize.
Maize seed germinate within 4-5 days after sowing under warm, moist conditions. When temperature is less than optimum, 14 to 16 days may be necessary for emergence. The first two true leaves develop as soon as coleoptile emerges. The next leaves unfold out of the whorl at the rate of one leaf every three days, when growing conditions are ideal. At the age of two weeks, the seedling normally has 5 or 6 unfolded leaves.
Maize produces three types of roots. Seminal roots are 3-6 and persist. Secondary (adventitious) roots develop from a node just under soil surface. Brace, prop or aerial roots develop from lower two nodes from tasseling to start of grain filling and help to anchor the plant firmly.
Tassel initiation starts approximately when the fourth leaf becomes visible. This occurs about three weeks after emergence, when the plant is about 35-45 cm height. At this time, the growing point is only few cm above soil surface and leaves accounting for most of the height. About 7 to 10 days after tassel initiation, embryonic ear begins to form on the side of the growing point.
Shortly after flower initiation, the plants have their full complement of leaves and the stem starts rapid elongation. The leaf area increases from five to ten fold and stem length from 50 to 100 fold. The tassel at this stage is fully developed but still enclosed in the whorl made by the leaves.
About five weeks after tassel initiation, the top internodes elongate rapidly and tassels emerge and after a few days shed their pollen. Two or three days later, the silks emerge and elongate until they are pollinated.
Maize plant is monoceious and diclinous, with male and female inflorescences borne in separate inflorescences on the same plant. The male inflorescence is called tassel and the female one the ear. The ear is actually a modified spike produced from a short lateral branch in the axil of one of the largest foliage leaves about half way down the stem. Because of this separation of ear and tassel, plus the protandry of flowering, maize is primarily a cross pollinated crop.
The pistil of the female flower, known as silk, develops from the growing point of the flower. It elongates through the length of the husks propelled by growth of an intercalary main stem located at its base. Each silk continues to grow until it is pollinated and fertilisation takes place.
Growth Stages:
Several methods have been proposed to describe maize growth stages.
Larson and Hanway (1977) described five periods with unique characteristics:
1. Planting to emergence.
2. Emergence to tasseling.
3. Tasseling to silking.
4. Silking to physiological maturity.
5. Dry-down period.
The classical numerical system proposed by Hanway (1963) has 10 growth stages (Table 4.1).
TABLE 4.1: Growth stages of maize.
Dry Matter Production:
Maize is one of the first species shown to use the C4 photosynthetic pathway. Discovery of the C4 photosynthetic pathway explained its high leaf photosynthetic rates, low CO2 compensation points and absence of photosynthetic high saturation up to full sunlight (Fig. 4.1). Photosynthetic rate of maize, sugarcane and berumuda grass can reach 60 mg CO2 dm-2 hr-1, almost double that of soybean, cotton and alfalfa, which use C3 pathway.
Fig 4.1. Effect of light intensity on average leaf photosynthetic rates of different crops.
After emergence, dry matter production is initially slow but it accelerates when the plant is about 6 weeks old. For the next two months, dry matter accumulation is fairly uniform and by tasseling time, about half of the final amount has been produced. Maximum rate of dry matter production occurs during the period when the plants are tasseling and silking.
Dry matter accumulation of maize grown with good management in south Carolina indicated that the total shoot dry matter at physiological maturity was 31.8 Mg ha-1, including 16.3 Mg ha-1 of grain dry matter. Leaves accounted for 18 per cent of the aerial biomass, with 2.8 Mg ha-1 of leaves retained both below and above the ear.
Maximum accumulation in the stalk and tassel was 6.5 Mg ha-1 or 20 per cent of the aerial biomass. Daily dry matter growth rates show two distinct peaks. The first occurs during vegetative growth when the potential ovule number is being established.
During this period, maximum rates of dry matter accumulation for lower leaves, upper leaves and stalk and tassel fractions were about 100, 200 and 300 kg ha-1 day-1, respectively. The second peak, which occurs during grain fill, shows a peak growth rate of about 450 kg ha-1, which goes directly into the ear and shank.
Leaf Area Index:
Green leaf area, usually expressed as non-dimensional green leaf area index (LAI), is a major factor determining radiation interception, canopy photosynthesis and therefore, yield. Between LAI values of 3 and 4, horizontal leaves intercept about 90 per cent of the light while leaves 15° from vertical intercept only 75 to 80 per cent.
Eik and Hanway (1966) reported linear relationship between maize grain yield and LAI up to 3.3 at mid-silk. There is no evidence that leaf growth influences ear growth during the critical growth period. There is evidence that grain yields and grain number increase with LAI up to values ranging from 3 to 5.
Yield and Yield Components:
In temperate regions, maximum yield of 22 t ha-1 has been reported in Michigan and up to 10 t ha-1 at commercial level are common. In tropics, yields up to 12 t ha-1 have been reported at high altitudes due to long rainy season.
In lowland tropics, experimental yields range from 5 to 8 t ha-1 with good management. To obtain grain yield of 10 t ha-1, an accumulated temperature of 2,600°C, 60,349 sunshine hrs, 9320 m3 water, LAI 5.0, dry matter accumulation of 22,000 kg ha-1 and a net photosynthetic rate of 9.0 g m-1 are required.
Maize grain yield is the product of kernel number per unit area and kernel weight. Of these, kernel weight is the more stable and the large differences in yield are usually the result of fluctuations in kernel (grain) number. Grain number per unit area depends on events before and around flowering. Soil moisture stress, nitrogen deficiency and inadequate radiation at that time significantly reduce grain number.
There are many reasons for low maize yields in tropics including drought and nutrient stress, inadequate pest control and use of poorly adapted cultivars with low yield potential. Yield increase in developing countries will probably require simultaneous improvement of crop management and germplasm.
Considerable variation in grain yield is observed in India due to differences in cultivar, fertiliser used, rainfall, pests etc. Under irrigated conditions with recommended cultural practices, an average yield of 4 t ha-1 in Indo-Gangetic plains is not uncommon. In Peninsular region and at higher elevations, a mean yield of 5 to 71 ha-1 has frequently been obtained. Under low fertility and rainfed conditions with poor yielding varieties, about 1.0 to 2.0 t ha-1 can be obtained.