Redgram

Morphology

Introduction

General description of the genus Cajanus

• Salient morphological features of the revised genus Cajanus are given below:
• "Perennial, rarely annual, erect bushes, 0.5 to 4 m, or creepers, or climbers, strong or weak. Pubescence various. Leaves pinnately, sometimes digitally trifoliate. Leaflets with vesicular glands below, membranaceous or rather thick. Stipellae present or absent. Flowers in axillary or terminal pedunculate or almost sessile racemes, yellow, or lined with red, or flag dorsally reddish, upto 3 cm long. Bracts small or large, caducous, bracteoles absent. Calyx teeth acute, acuminate or elongate-acuminate, two upper ones more or less connate. Corolla persistent or not, vexillum abovate-orbicular, reflexed, clawed, auriculate.
• Wings obliquely obviate, auriculate, keel rounded oblique, obtuse. Ovary subsessile, ovules (2-) to 10. Style thickened above the middle, upcurved, upper part glabrous or slightly hairy, not bearded. Stamens 9 connate, vexillar stamens free, anthers uniform. Fruit a pod, linear-oblong, apex obtuse or acute, compressed, bivalved, depressed between the seeds with transverse lines, more or less septet between the seeds. Seeds reniform to suborbicular, shiny, white, brown, grey, purple or black, variegated or not, strophiole conspicuous or vestigial".

Morphology of Cajanus cajan

• The morphological variation in pigeonpea (Cajanus cajan) is greatest in Asia especially in India, its place of origin. Several researchers working in India have described the morphology and studied the variation available within pigeonpea.
• On the basis of flower colour, seed numbers per pod, length of stipples, de Candolle distinguished two species under Cajanus viz., C.bicolor and C.flavus. Later workers reduced these two species to botanical varieties. Variety flavus (DC) is characterised by early maturity with shorter stature, yellow standard petals, green glabrous pods, lighter in colour when ripe, and usually 3 seeded. Variety bicolor (DC) is characterised by late maturity, large bushy stature, red or purple streaked standard petals, and hairy pods blotched with maroon, or dark coloured with 4 to 5 seeds, that are darker coloured or speckled when ripe.
• The above varietal distinctions appear to be of doubtful taxonomic validity since the two varieties are readily crossable, and a range of combinations of the above distinguishing features occurs in the present-day varieties.
• Based on the morphology, distinguished 86 different pigeonpea types from collections throughout India, while Mahta and Dave (1931) recognized 36 types from Madhya, Pradesh State alone.

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Classification

• The Cajanus cajan differs in plant character, pod character and maturity duration, etc., but most of the cultivated types belong to two categories.

Cajanus cajan var. Bicolor

• They are late maturing, plants grow very tall or probably they are tallest of both the types which are freely branched and bear flowers at the end of the branches. The pods are relatively longer and use to contain 4 to 5 seeds in them. Cajanus cajan var. Flavus

Cajanus cajan var. Flavus

• They have shorter duration and accordingly they fall in early maturing category of plants. Plants are shorter, bushy having flowers at several points along the branches. The pods are also shorter which bear two to three seeds in them.

Classification of pigeonpea based on growth habit

Classes	Basis of classification	Reference
Very erect (30") Erect (40") Semi-erect (50°) Spreading (60°)	Angle of branching	Mahta and Dave, 1931; Pathak, 1970
Erect (30-40°) Semi-erect (40-50°) Spreading (60 -70°)	Angle of branching	Baldev, 1988
Erect and compact Semi-spreading Spreading Trailing	Angle of branching	IBPGR/ICRISAT, 1981
Tall compact Tall open Medium-height compact Medium-height open Dwarf bushy	Plant height and angle of branching	Sharma et al., 1971
Compact Spreading Semi-spreading	Spreading Semi-spreading Angle of branching and number of branches	Remanandan et al., 1988

• In general, early-maturing types are relatively photoperiod insensitive compared to the medium and late-maturing varieties.
• Other factors such as soil moisture status and nutrition also influence mainly duration to some extent. Days to 50 per cent flowering by a variety vary from location to location and season to season.
• For instance, Hy 3C took 138 days to 50 per cent flowering at Palancheru, India whereas it took only 80 days in Puerto Rico and 64 days in Kenya.
• Similarly, depending on the genotype, days to 50 per cent flowering can range from about 60 to more than 200 in sowing made prior to the longest day at 17°N. Most photoperiod-sensitive cultivars flower more readily when sown after the longest day.
• Both low and high temperatures delay flowering in pigeonpea. Flowering of all maturity groups occurs sooner in moderate temperature (22° C - 30° C) even under relatively long days (12.5 - 16 h).
• Light affects inflorescence development and pod setting. Under dense crop canopies no pods are set. Bright, dry days are favourable for fertilization, while cloudy, damp weather results in excessive flower drop.

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Root system

• The root system in pigeonpea consists of a deep, strong, woody tap root with well developed lateral roots in the superficial layers of the soil. Under certain conditions the roots can go more than 2 m deep, but the most extensive development takes place in the upper 60 cm of the soil. Normally root depth ranges from 30 to 90 cm and is influenced by the date of sowing and the availability of moisture in the soil profile. Root growth continues during the reproductive phase and the total root length approximately doubles after the onset of flowering. The root system appears to be closely related to plant habit. Tall, compact varieties produce longer and more deeply penetrating roots, whereas spreading types produce shallower, more spreading, and denser root systems.
• In a dormant seed, the radical measures 0.2 cm below the cotyledonary node. Before the first pair of simple leaves unfolds, the radical grows to a sufficient length with a clear demarcation between the tap root and the hypocotyl region. The radical comes out through the hilum within 1.5 to 2.0 days. In about 5 days the radical attains a length of 4-6 cm. From the third day onwards, lateral roots make their acropetal appearance.
• The primary structure of both the tap root and secondary roots is tetrarch. Young roots posses parenchymatous pith that is disorganised at the onset of secondary growth. The epidermis of the young root is single-layered with a thin layer of cuticle. The wide cortex is parenchymatous, and consists of numerous rhomboidal crystals. The endodermal cells do not show casparian thickening on their radial walls. The pericycle is multilayered. Older roots with secondary growth appear more or less eccentric. Secondary growth occurs through the activity of a vascular combium. The development of this cambium is typically dicotyledonous. The cork consists of a few layers. In the secondary pholoem region, secretary ducts containing a tannin-like material are present, these ducts are also present in stems and other aerial organs.
• Mycorrhizae are often present in cortical cells of the roots, and occassionally mycorrhizal fruiting bodies can be observed. The first two leaves are simple, opposite, and caducous. They are narrowly ovate with a cordate to truncate base, and an acute to acuminate apex. The apices may have a small macro. The stipules are lanceolate and conspicuously forked. Rarely, the second and third nodes, show either a simple leaf or a compound leaf with only two leaflets.
• Subsequent leaves are compound, pinnately trifoliate, and arranged in a 2/5 type of spiral phyllotaxy. A pair of free lateral, lanceolate stipules is present at the pulvinate base of the petiole that bears the leaflets. In a fully developed leaf, petiole length ranges from 2.4 to 6.0 cm and is prominently grooved on the adaxial side. Lateral leaflets possess one stiped each, whereas the terminal leaflet has a pair of stiples. The leaflets are lanceolate or elliptic, with acute or obtuse apices. Terminal leaflets are mostly symmetrical, but the side leaflets are broader at the side furthest away from the terminal leafs. Terminal leaflets are usually bigger than lateral leaflets.
• Genotypic differences exist for leaf size and shape and are also influenced by the environment. Under extended day length conditions the leaflet size considerably increases. The leaf surface area varies from 13.0 to 93.5 cm2 in various genotypes, whereas in a minute leaf variant the total leaf surface for three leaflets only measures 6 cm2. The lengths of the petiole and rachis also vary greatly, but the petiolule length is not so variable. The stipellae vary from traces to 4 mm.
• In the midrib region of the leaf, the vascular tissue in the ventral half occurs in a continuous arched band with phloem on the outside and xylem inside. Two distinct strands mostly consisting of phloem are seen on the ventral side. The centre of the dorsal part of the midrib is occupied by fibres capped by collenchymatous cells.
• The leaf lamina comprises a distinct palisade layer, and in the lower part of the leaf a spongy mesophyll with large air filled intercellular spaces. There are far more stomata on the lower surface of the leaf than on the upper surface. Stomata are distributed between and over the minor veins, but not over the major veins. Mature stomata are either anomocytic, diacytic, or paracytic. Paracytic stomata are predominant.
• The venation pattern consists of the mid vein and conspicuously arranged secondaries which end at the leaf margins. The major veins from regular meshes, each of these are further divided several times with free vein endings. The vein ends have tracheids that are often forked.
• The petiole contains a number of distinct vascular strands above which lie fibre bands. Occassionally some of the xylem vessels of the petiole are filled with a darkly stained tanin-like substance. Pulvini are found at the bases of the petiole and the leaflets. These are responsible for leaf and petiole movements. Under drought stress conditions when the sunlight is intense the leaflets exhibit paraheliotropy i.e., they take up a position parallel to the incident light. Similarly, during the night the leaflets are folded vertically upwards into the "sleeo" position. Most of the pulvinus consists of cortical tissue. Changes in the turgor of these cortical cells are responsible for the movements of the pulvini.
• As the leaves approach senescence, an abscission zone develops at the junctions between the leaflets and the petiole, and between the petiole and the stem. The cells in the abscission zone show divisions parallel to the plane of abscission. The weakening of the walls of these cells results in an early separation of the abscission zone, and consequently the leaflet or petiole falls.

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Seedlings

• Except under cod conditions, laboratory studies have revealed a broad optimum temperature range (19-43^oC) for germination, with the most rapid seedling growth occurring between 29 and 36^oC. Germination is hypogeal, and the cotyledons remain underground. Under suitable field conditions the seedlings appear above the ground in about 5-6 days. On the second day, the tests splits open near the micropyle, and the tip of the radical elongates and emerges from the seed coat. On the third day the hypocotyl appears as an arch and continuous to grow upwards. The hypocotyl develops a light purple colour and becomes straight. The seedling epicotyl is light green, green, or purple in colour . The first pair of leaves are simple and opposite. The epicotyl elongates to 47 cm before the first isolate leaf emerges. The first pair of leaves generally drops off within 30 to 40 days, but they may remain longer.
• When the young plumes or axillary shoots are damaged, secondary shoots develop from the cotyledonary axils of the seeds, resulting in multiple shoots. This phenomenon helps the plant to overcome germination and establishment problems under harsh environmental conditions. The occurrence of secondary shoots is often mistaken for twin seedlings in pigeonpea. Variation in maturity, leaf size, flower, pod and seed size, and plant pigmentation between secondary shoots developed from the same seed.
• Large-seeded varieties produce bigger seedlings than those of the small seeded types, but these differences disappear as the plant grows.

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Stem

• Stem is ribbed, upto 15 cm diameter, show enormous secondary growth and become woody with age. In early maturing types stem girth seldom reaches 3 cm, whereas in late-maturing types it ranges from 4 to 10 cm at the base of the plants. Four different stem colour, dark purple, purple, sun red and green (the most common) are recognized.
• The internodes of the stem developed by elongation of the tissue between the leaf initials tin the apical meristem.
• The primary vascular tissue of the stem is organised in strands connecting the nodes. Each strand is associated with a ridge on the stem that is distinctly visible even in old, secondarily thickened stems. Collenchymatous bundle caps underlie the epidermis of the ridges.
• Photoperiod and temperature exert profound influence on days to 50 per cent flowering and maturity duration in pigeonpea, which is considered to be a quantitatively short-day plant. Genotypes differ in their response to photoperiod.

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Flowers

• The flowers are borne in short racemes. In the world germplasm collections the number of racemes per plant ranged from 6 to 915. Peduncles are (0 - 1.8 cm) long. Pedicels are thin, 7-15 mm long, downy and covered with hairs. Flowers (Figure 3.14 b,c) are predominantly yellow. Bracts are small with a thick medium nerve, triangular or ovate-acuminate scales, 1-4 mm long; their margins curve inwards to form a boat-like structure, and enclose 1-3 very young flower buds. The calyx tube is companulate with numerous glandular hairs with bulbous bases, the tube dorsally gibbous, about 5 mm long, with five subequal, narrowly triangular lobes 4-7 mm long. The smaller upper lobes are paired, free or partly connate, and the lower one is the longest.
• The corolla is highly zygomorphic, papilionaceous, and generally yellow in colour. The petals are imbricate in the bud. The standard petal (vexillum, flat) is erect and spreading, more or less orbicular, 14-22 mm long, 14-20 mm wide, base clawed, biauriculate, with two callosities. Both right and left handed flowers in pigeonpea with regard to contortion of the vexillum petal either to the right or to the left. Wing (aloe) petals are obviate with a straight appear margin, clawed base, asymmetrically biauriculate, 15-20 mm long, 6.7 mm wide, with a callosity. Keel petals are boatshaped, 14-17 mm long, 5-7 mm wide, clawed, entirely split dorsally, ventrally split near the base, left and right lengthwise furrowed, glabrous and more greenish than other petals.
• Stamens are 10, diadelphous (9+1), 15-18 mm long, with 4-7 mm free parts, flattening towards the base, tapering towards the top, geniculate near the base, the staminal sheath is about 12 mm long. Anthers are ellipsoid, about 1 mm long, dorsifixed and light or dark yellow in colour.
• Pigeonpea stamens exhibit dimorphism. Of the 10 stamens, four have short filaments and six, including, the odd posterior one, have long filaments. The odd stamesn has a groove for the passage of nectar that is secreted from the base of the filaments. The long stamens are antisepalous, and the short ones antipetalous. The anther lobes also exhibit dimorphism, those of the shorter stamens have blunt lobes, and the longer ones pointed lobes. The filaments of shorter stamens are thicker than those of the longer growth and development in short stamens is faster than the longer ones. The maturity of short stamens coincides with that of the stigma. The pollen produced by short stamens is used for self fertilization, whereas that produced by long stamens is used for outcrossing.
• The ovary is superior 5-8 mm long, sub-sessile, since it has a very short stalk, densely pubescent and glandular-punctuate, with 2-9 ovules. The stigma is capitate and glandular papillate. The style is long, filiform, upturned beyond the middle, 10-12 mm long and glabrous.

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