In this article we will discuss about the growth stages of groundnut plant.
Groundnut is self-pollinated, annual, herbaceous legume belonging to the subfamily Papilionaceae of the family Legnminosae. Plant is erect or prostrate, sparsely hairy and 15 to 60 cm tall. Arachis hypogaea describes the peculiar trait of the species, underground fruit formation (hypo means under and agaea means ground).
The special feature of groundnut is that the fruit begins as a fertilised flower above ground but pod and seed mature in the ground.
A system of staging the groundnut plant was proposed by Boote (1982). Vegetative stages are based on the number of nodes on the main stem with the cotyledonary node identified as node zero (Table 9.6). The next one is the node of the first true leaf, a trifoliate leaf and when the leaflets at this node have developed to the point that they are unfolded and flat and then the plant is at stage V-1.
Additional main stem nodes and thereafter V stages are counted as soon as the leaflets at those nodes are unrolled and flat. Reproductive stages are based on visually observable events relative to flowering, pegging and fruit growth and development.
Reproductive stage 1 (R-1) is defined as the point in plant development when the plant has its first bloom or open flower at any node. Stage R- 2 occurs when the plant has an elongated peg and so on. A field is considered in a specific stage when 50 per cent or more of the plants staged are at a given stage of growth.
The purpose of defining growth stages is to specifically identify the physiological and morphological degree of the plant development and to avoid the use of days after planting and calendar date in describing plant or crop development.
Dormacy and Germination:
The problem of seed dormancy in groundnut has two aspects. In the bunch varieties, it is one of inducing dormancy to prevent germination in the field itself if rains are received at maturity. The problem in spreading varieties is breaking dormancy to facilitate sowing next crop soon after the harvest of the previous crop. Dormancy period may extend from about 10 days to two and a half years.
The aqueous extract from the seeds of bunch groundnut can break the dormancy of spreading variety when the seed of the spreading variety is soaked in the extract. Treating the kernels with ethylene chlorhydrin (0.7%) can break the dormancy.
Storing the seed at 40°C for 12 days can also completely break the dormancy. Synthetic growth regulators like etheral, which releases ethylene effectively induces germination of freshly harvested dormant seeds within 24 hrs.
The first visible evidence of germination is emergence of the radical. Radical emergence occurs by 24 hrs or earlier for various Spanish types seed but requires 26 to 48 hrs in the Virginia types. During the first few days, developing seedling depend on food reserves in the cotyledons for energy.
After 5-10 days, depending on the type of groundnut and environmental conditions, the seedling becomes autotrophic. The optimum temperature for groundnut seed germination is 30°-35°C.
Dry Matter Production:
Production of leaves and increase in shoot weight can be considered as a measure of vegetative growth. The period of maximum growth will be between 55 and 95 days in bunch types and 70 to 125 days in spreading types. Studies at Tindivanam (TN) indicated that the rate of growth is rapid in the first two fortnights from flowering and peak growth during second fortnight in bunch types.
Growth rate is faster in bunch types than in spreading types and they attain higher dry matter earlier. In spreading types, growth continues for longer period. Maximum crop growth values ranging from 13 to 14 g m-2 day-1 have been reported under different environmental conditions for different cultivars.
The crop growth of legumes is, generally, less than that of cereals because both the capacity (leaf area) and intensity (rate of increase in dry weight per unit of leaf area) components are smaller. This may be associated with indeterminate fruiting habit, which compels the crop from an early stage to devote an increasing proportion of its assimilates to filling fruits rather than to making more leaves. Another reason is that legumes have a shorter vegetative period than cereals and dry matter in legumes may also be lost by respiration associated with dinitrogen fixation in root nodules.
Leaf Area Index:
The LAI varies with environmental conditions, cultural practices and stage of crop growth. The LAI should be related to growth stage when comparing different crop species or cultivars within species. For determinate crops, LAI measured at the beginning of reproductive growth stage is more meaningful whereas for indeterminate crops, the upper limit of LAI may be used as a criterion for comparison.
Maximum LAI values ranging from 3.3 to 7.0 have been reported for groundnut. The LAI reaches a maximum value 65 to 75 DAS in cultivars with 115 to 125 days duration. The LAI may continue to increase around 7.0 in some cultivars but light interception reaches a maximum at an LAI around 3.0.
Further increase may not have measurable effect on crop growth rate. The range in maximum apparent photosynthesis in groundnut is reported to be 16 to 65 mg CO2 dm-2 hr-1. This wide variability is mainly due to genotype differences and variation in environmental conditions.
Reproductive Growth:
Flower primordial initiation can be observed around 10 days after sowing and the time taken for primordial initiation to flower opening is 18-21 days. In bunch types, flowering commences from 20 to 30 days after sowing.
First flower opens, generally, 7-10 days later in spreading types than bunch. The number of flowers produced per plant may range from 40 to 250 in spreading types and from 98 to 140 in bunch types. Spanish and Valencia types have a shorter flowering phase than Virginia.
After fertilisation, cells just below the receptacle divide rapidly in a longitudinal plane. The proliferating tissue forms a peg (gynophore or carpophore). The peg grows upwards for about 0.5 cm before turning abruptly downwards. As it elongates, it carries within its tip the developing ovary. After penetrating into the soil, the tip turns into horizontal position. The terminal ovary and seeds then begin to swell.
Fertilised flowers range from 49 to 59 per cent in spreading types and 22 to 67 per cent in bunch types. After fertilisation, about 64 per cent of the potential fruits elongate as pegs. Pod set percentage (ratio of number of mature pods to total number of flowers) ranges from 8-17.
There is general agreement that a large number of early formed flowers develop into pods. Flowers that appear 10 days after sowing do not form mature pods. On an average, 40 to 60 per cent of the pegs alone form pods.
Harvest Index:
The harvest index (HI) varies between 0.35 and 0.50 in groundnut. Sequentially branched forms in which plant structure is smaller and formed over a shorter time and in which rate of leaf growth to total growth decline earlier tend to have a larger HI than the longer lived and more freely branched alternate forms in which vegetative growth is not arrested so soon by the internal regulation of partitioning. The shelling percentage is around 80 per cent for early maturing bunch types compared with 60-75 per cent for the spreading cultivars.
Yield Components:
Groundnut kernel yield is the product of pod number, number of kernels per pod and weight of individual kernel. Kernels per pod vary from 2 to 6, pods per plant from 50 to 105 and 100 kernel weights from 28 to 62 g. This variation is related to cultivar, spacing, fertiliser and climate. Yield components are most sensitive to environmental stress during flowering and kernel growth stages.
There is a high positive correlation between the number of mature pods and pod yield. In general, pod number per plant decrease with increase in plant density. Wider spacing increases the number of branches per plant and number of mature pods per plant. Literature on pod yields indicates maximum contribution to pod yield to from the number of pods per plant followed by 100 kernel weight.