3.1 Plant morphology
As described in Section 1, Lupinus is a genus with diverse species. Some of the species are annual plants (e.g. those Old World species of agricultural significance), while most species are herbaceous perennial plants and a few are shrubs. Plant height of various species ranges from 0.2 -1.5 metres with some shrubs reaching 2.5 metres. Only a brief description of the morphology and anatomy of lupin is presented here with an emphasis on herbaceous annual species. An example of different parts of a lupin plant is shown in Figure 1.
Figure 1. An illustration of different parts of L. perennis (modified from: USDA-NRCS PLANTS Database / Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. Vol. 2: 348) (Britton & Brown 1913).
Lupins generally have a taproot system. Root morphology varies widely between species, ranging from a dominant taproot with relatively few lateral roots to a highly developed lateral root system (Clements et al. 1993). For lupin species with a tap root system, the main root reaches the depth of 1-2 metres. Lupin roots, especially the main axis, bear nodules formed by Bradyrhizobium for nitrogen fixation. In addition, morphological adaptations occur in many plants for increased nutrient uptake. For instance, proteoid roots, also known as cluster roots, can form in response to phosphorus or iron deficiency (Gardner et al. 1982; Gilbert et al. 2000; White &emp; Robson 1989a). Root morphologies may reflect differences in the adaptation of lupine species to different soil types. In the case of domesticated genotypes of L. angustifolius, which are well suited to deep sandy soils, the plants have a dominant taproot and a high number of primary lateral roots, but relatively few secondary or tertiary lateral roots, with no proteoid root formation (Clements et al. 1993).
Lupin stems vary among species and are fascicular for herbaceous species and arborescent (treelike) for shrub species (Kurlovich et al. 2002b). The cross-section of lupin stem is commonly terete shape. Annual lupin species differ from each other by the shape of the cross-section of their stems and by size (Petrova 2002). The surface of lupin stems is either pubescent with various degree of density or naked with a waxen tinge.
Lupins have a characteristic palmate leaf shape with leaf blades divided into various numbers of leaflets. The shape of leaflets varies largely among different species, including oval oblong, ovate oblong, obovate, narrow linear, calceolate and more (Kurlovich et al. 2002b). The surface of leaflets is in most cases covered by silver three-celled hairs with various densities (Petrova 2002). Leaves are soft green or greyish green and connected to stems by long petioles (leafstalks) with elongated stipules.
3.2 Reproductive morphology
In the majority of lupin species, the main stem and lateral branches terminate into racemes of the apical truss type (Figure 1). This type of inflorescence has an ascending flowering order and flowers are produced in dense or open whorls on an erect spike with the bottom flowers blossoming first. The flower is hermaphroditic. It is zygomorphous (bi-laterally symmetrical) with a typical pea flower shape 1-2 cm long, consisting of five joined sepals, five petals, an ovary with a pistil and ten stamens (Figure 2). The petals are not all joined and are of different shapes and sizes. The uppermost petal is called the standard (also called the vexillum or flag) and the two partly joined petals at the side are the wings. Within the wings are two partly joined petals forming a boat-shaped keel (carina). Inside the keel are the long, narrow and pod-shaped ovary and ten concrescent stamens arranged in two circles of five each. The ovary usually contains two or more ovules.
Figure 2. Structure of a lupin flower (L. perennis) – half flower (© D G Mackean)
The lupin pod is orbicular or flattened in a cross-section view and straight or curved longitudinally. The pod surface is rough and pod colour varies from cream, brown to black. Some species have easy shattering pods while others have non-shattering or weakly shattering pods.
Lupin seeds are very diverse in size, shape and colour and their surface can be smooth or rough. The seed stalk hangs over the micropyle. Within the seed, the bent embryo is at the top of the cotyledon where nutrients are stored. Primary true leaves are opposite, while other leaves cannot be seen until germination.