In this article we will discuss about the growth and development of sorghum.
Germination and Root Growth:
Adequate moisture and temperature are the two prerequisites for germination. Seeds of tropical crops such as sorghum may not germinate satisfactorily at temperatures below 20°C but will germinate well at temperatures as high as 40°C.
Optimum range for sorghum seed germination is 21° to 35°C. The lethal temperature for germination is 40° to 48°C. Sorghum seed germinate well between 25 and 50 per cent of field capacity in sandy loam soils.
Like other grasses, grain sorghum has two types of root systems. Seminal roots develop from the embryo below scutellar node, while adventitious roots are produced from the lower stem node or crown of the plant near the soil surface. The nodal or adventitious roots develop to become bulk of the sorghum root system.
Maximum root weight of sorghum occurs at about anthesis and roots can extend to a depth of more than 1.5 m at a rate of 2-5 cm day-1. Saintclair (1977) reported 84 per cent of roots in the top 25 cm. On weight basis, 70 per cent of sorghum roots are in 0 to 7.5 cm depth, 14 per cent in 7.5 to 15.0 cm depth and 98 per cent in the top 91 cm of the soil profile.
Stem, Leaf and Tillers:
Sorghum stems are solid, dry or juicy, starchy or sweet with a base dia of 0.5-3.0 cm. Plant height depends on number of nodes and length of internodes. In photoperiod sensitive cultivars, long photoperiods delay initiation of panicle leading to tall plants with numerous internodes.
Grain sorghum exhibits a relatively simple leaf area display. Under normal field conditions, most commercial cultivars exhibit a unimodal distribution of leaf size in which individual leaf areas increase from the ground upward to a maximum value and then decrease to the top of the stalk.
Grain sorghum cultivars vary in their tendency to produce tillers and environmental conditions can stimulate or retard their production. Defoliation or death of main stem reduces apical dominance and permits rapid tiller production.
Inflorescence:
The inflorescence is a compact to open panicle with primary branches arising from a central rachis. These give rise to secondary and sometimes tertiary braches, which carry racemes of spikelets. Anthesis begins near the tip of the panicle 0-3 days after emergence from the boot. Flowering proceeds basipetally for 4 -7 days.
Onset of reproductive phase commences with the initiation of panicle, which usually occurs 30-40 days after emergence but may vary according to genotype and environmental conditions. The grain attains maximum dry weight 25-35 days after blooming.
Growth Stages:
The life of grain sorghum plant is divided into ten stages. Stage one to three depends on recognition of leaf collar. A leaf is considered developed wherever the leaf collar (juncture of the leaf blade and leaf sheath) is visible without tearing the leaf blade or sheath.
The first leaf (lowest leaf) on a sorghum plant has a round trip while all others have pointed tips. Hence, if the lowest leaf has pointed tip, then one leaf has been lost from the plant. Stages of growth are outlined in Table 5.1.
TABLE 5.1: Growth stages of grain sorghum.
Dry Matter Production:
Sorghum is C4 plant with high assimilation capacity and potentially high rates for dry matter accumulation. Leaf photosynthetic rates around 72 mg CO2 dm-2 hr-1 have been reported under field conditions. In the first 30-35 days of plant growth, shoot dry matter consists primarily of leaves. The culm or stock then begins rapid growth and both leaf and stalk weights increase until they reach maximum values at about 60 to 65 days, respectively. Panicle weight increases rapidly from 50 to 60 days and after pollination, grain weight increases rapidly.
Dry matter accumulation rate of grain sorghum is nearly constant from about 40 days after planting until near physiological maturity. About 59 per cent of the total dry matter accumulates by flowering. Leaf blades reach their maximum weight, 7 days after the boot stage and maintain that weight until near maturity. Stalk weights reaches maximum, 7 days after half bloom and decline from then until maturity.
Leaf Area Index:
Grain yield of annual crops is usually related to duration of leaf area and establishment of a high leaf area index (LAI) as early as possible is important in order to obtain maximum yield. Large plant populations, narrow row spacing and large seeds can contribute to early leaf area development.
In cereals, most of the carbohydrate in the grain result from photosynthesis after heading, though reallocation of assimilates from the stem often accounts for 10 to 12 per cent of total grain weight. Due to the importance of postanthesis photosynthesis, the longer the leaf area is retained after heading, the more assimilates are available for grain growth.
Nitrogen deficiency and retranslocation of nitrogen from leaves to grain and leaf senescence reduce the duration of leaf area after heading. Maximum LAI in sorghum is usually achieved just before anthesis and an LAI around 5 is found in productive commercial fields. Fields in drier areas can have LAI values of 2 to 4. Highest crop growth rate reported for sorghum is 43.6 g m-2 day-1 at solar radiation of 26.5 MJ m-2 day-1 for the whole period.
Yield Components:
Grain yield of sorghum can be expressed by the equation:
Yield = No of plants (P) x Panicles or heads per plant (H) x Seeds per head (S) x Weight per seed (W).
High yields can be obtained only if all the yield components are at optimum level. This requires adequate plant population, adequate water and nutrients, optimum crop management and a cultivar of high yield potential. A greater number of seeds per plant has been recognised as the yield component that contributes most to hybrid vigor.