In this article we will discuss about the growth and development of wheat crops.
Wheat is typical of many annual grasses in its generalised growth and structure even with the distinct differences in growth habits within and across several species. Plant may reach even 1.75 m in height although, less than 1.25 m is common. Commercial semi-dwarf cultivars are usually less than 90 cm in height, depending on genetic makeup and management practices.
The first visible roots are the seminal or seedling roots that appear shortly after germination seed provide the young seedling with moisture and nutrient uptake during early growth stages. Additional roots called nodal, crown or adventitious arise from nodes within the crown, the crown being the meristematic region where the shoot and root meet. These roots may penetrate over 75 cm deep, with much penetration possible.
All leaves, adventitious roots and tillers arise from buds associated with crown or stem nodes. During germination and early seedling growth, the coleoptile elongates upward until it encounters sufficient light to cause growth to stop. The first leaf then emerges through the coleoptile.
In a normal sown wheat crop (mid-November) of 90-100 days to flowering in north India, the tiller number will be at its peak 50-55 days after sowing (DAS) and subsequently declines to a constant number. Shoot number continue to increase up to the end of flower initiation or beginning of ear growth. Varieties that flower late show profuse tillering.
Temperature appears to be the most important factor determining tillering behaviour. A positive relationship between time to flower and tillering, generally, holds good when varieties widely differing in flowering time are compared. However, there are differences in tillering as well as tiller survival between varieties of similar flowering duration.
Wheat stem elongates by a process called jointing, a joint being synonymous with node. Early leaves arise from lower nodes with unexpanded internodes. Once the plant shifts from vegetative to reproductive growth, nodes are differentiated. Maximum number of spikelets is set by late tillering stage, well before stem elongation and the maximum number of florets is set by the time the stem begins to elongate.
The inflorescence or head is pushed upwards as the stem internodes elongate, a process called jointing. The spike is enclosed in uppermost leaf sheath (the flag leaf sheath). The inflorescence is a spike, with spikelets born singly at nodes (joints) on alternate sides of a zig-zag flattened central axis called the rachis. Each spikelet may be composed of one to nine florets or flowers but two to five is normal. Florets are born on an axis termed rachilla.
Growth Stages:
The most common method for identifying growth stages in cereals is Feekes scale based on external appearance of the plant or plant organs, which was simplified by Tottman (1987). Waldren and Flowerday (1979) gave a growth description for winter wheat based on 10 point scale (Table 2.2).
Table 2.2: Growth stages of winter wheat
According to these authors, farmers do not easily distinguish the growth stages described by the Feekes scale because the scale is based on small morphological changes, which are not readily apparent, especially at the later stages. Their stages have less secondary and tertiary divisions than those of the Feekes scale and distinctly different periods of development, such as heading and flowering, appear in separate stages.
Crop Growth and Photosynthesis:
Most high yielding varieties have higher crop growth rate (CGR) but higher biomass could be due to longer crop duration. Dry matter accumulation varies between 3 and 5 g m-2 day-1 depending on the variety. Higher relative growth rate (RGR) is associated with high CGR. At congenial temperature of 27°C, RGR could be 0.4 g g-1 day-1 at Yacui Valley in Mexico.
The extinction coefficient (K) is greater for modern cultivars than traditional cultivars. Values of K calculated using photosynthetically active radiation (PAR) and green leaf area index (LAI) for pre-anthesis period averaged 0.63 to 0.72. Radiation use efficiency is significantly greater in modern cultivars at 2.4 g dm MJ-1.
Mathematically GY = RI x RUE x HI
where, GY = grain yield
RI = radiation interception
RUE = radiation use efficiency
HI = harvest index.
Partitioning Of Dry Matter:
Wheat, the determinate plant, diverts photosynthates to vegetative growth early in the season and to grain later. The HI of wheat reported in literature is around 0.34 in traditional cultivars and about 0.44 in improved new varieties.
This means that the recent gains in wheat yield with modern cultivars can be attributed to improved HI. Austin (1980) suggests that the HI of cereals might be increased from the current range of 0.4-0.5 to around 0.6 before diminishing return sets in.
Relationship between Growth and Yield:
Plant growth in cereals is directly related to grain yield. In a broad sense, grain yield is a function of the number of grains per unit area and the weight of individual grains. The number of ears per unit area and the ear size determine grains per unit area. After the ear size is formed, grain weight depends on translocation of carbohydrates, principally from leaves and ears.
Grain is directly dependent on the photosynthetic capacity of the crop after anthesis and sink capacity of the grain. The photosynthetic capacity after anthesis is mainly dependent on leaf area duration (LAD), which is a function of LAI at anthesis and leaf longevity. The sink capacity of grain is a product of the number of grains set and the growth characteristics of the individual grains.
In most of the studies, the indications are that the sink (number and size of grains) influences the production of photosynthates and its rate of translocation to storage organs. Evidently, efforts to bring about higher values of the sink would change its relationship with total dry matter (HI would become high).
This can probably be achieved when environmental conditions are not limiting the sink. Much of the ear differentiation and development would depend on the availability of carbohydrates in the early stages of plant growth when there is competition with strong sinks like tillers, leaf and stem growth.
Grain filling, though free from other competing sinks and little by radiation, is strongly dependent upon prevalent temperature. It is for these reasons that the HI is raised as the plant height is reduced.