Roots

Root structure has been shown to vary within the D . virgatus complex with a tap root recorded on some accessions (Gardiner, 1 992). Roots of the three cultivars released in Queensland have not been described. The roots of desmanthus have been reported to be xylopodic (store water) (Carvalho and Mattus, 1974) although there is little complementary evidence in literature.

2.2. 1 . 3 Nodulation

One of the most important features of legumes is the ability to fix atmospheric N via symbiotic association with Rhizobium bacteria (Beijerinck, 1 8 8 8 as cited by Allen and AlIen, 1 98 1 ). The formation of nitrogen fixing nodules on the roots is most common to the Fabaceae although there are a few exceptions. Not all members of the family form this association, however and not all strains of Rhizobium nodulate (AlIen and Allen, 1 98 1 ). The three legume sub-families vary in the ability to nodulate, nodulation being the rule in the Papilionoideae and Mimosaceae but less common in the Caesalpiniodeae (Faria et al., 1 9 89).

Figure 2. 1 Flowering branch and seed pod of Desmanthus virgatus L.

Source: Wrightson Seeds Ltd. , 1 994a

Nodules of temperate zone legumes tend to be annual (shed at end of season) whereas

those of tropical species tend to be perennial (Allen and Allen, 198 1 ). Rhizobium strains

are often only able to nodulate one or a few legume species. Cultures of specific inoculum for given legume species are therefore created and used to inoculate seed or

soil (Allen and Allen, 198 1). Inoculation is usually economic because the cost of

inoculant is generally less than one tenth the annual returns from increased production

(Frederick, 1 978).

Investigation into the identification of an effective Rhizobium strain for desmanthus has

paralleled its evaluation and release in Queensland. In a pot screening of 48 accessions

of D . covillei and 42 of D . virgatus) against 17 strains of Rhizobium for effectiveness of N-f1xation 23 accessions formed effective associations with 10 or more Rhizobium strains showing that Desmanthus does not generally have a high affinity for specilic Rhizobium strains (Date, 1 99 1 ). Similar promiscuity of rhiwbia was previously observed in D . virgatus (Davis, 1 982; Carvalho and Mattus, 1 974; Norris, 1 965) and D . Illinoensis and D . Leptobolus (Wilson, 1939a as cited by Allen and Allen, 1 98 1 ). Five Rhizobium strains (CB 1 397, CB 3 126, CB3 1 28, CB3 1 30 and CB3 1 32) have formed effective associations with most accessions. CB 1 397 (from neutral or alkaline soils) was recommended as the best overall strain for a wide range of accessions although CB3 1 26 (from acid soils) was recommended for D . virgatus CPI3835 1 and TQ90 indicating a possible strain / soil type interaction (Date, 199 1 ) . Since this trial CB3 l 26 has been released as the inoculum accompanying commercial supplies of desmanthus seed.

2.2.2 REPRODUCTIVE

2.2.2. 1 Flower Structure

The inflorescence of Desmanthus spp. contains 4 to 10 (advanced) or 20 to 5 0 (primitive) florets variously including sterile (basally arranged), male and perfect florets. These are found singly or paired in the axils of leaves. Andromonoecy is common and varies from an increase in the number of male florets to functionally male plants. Generally, florets are receptive for 1 to 2 days with sterile florets falling off in the fIrst morning. The clavate receptacle contains ovate, I -veined sessile, peltate bracts at the base of each flower (Luckow, 1 993).

The most comprehensive taxonomical description of D. virgatus inflorescence structure is provided by Luckow ( 1 993):

"One inflorescence per leaf axil, born on peduncles 0.6 to 4 cm long with a bract subtending each floret 1 to 2.2 mm long, 0.4 to 0.8 mm wide, deltate, pale green

with red tips, membranous with an opaque rnidvein, peltate and pedicellate at the center of the head, sessile and often fused into a whorl at the base of the head, strongly I -nerved, glabrous or minutely ciliolate. Floral buds obovate, apically

rounded. Heads 0.3 to 1.0 cm long containing 3 to 22 sterile, male and perfect flowers ( sterile or male flowers sometimes absent)."

The sterile florets of the Desmanthus genus are smaller than the perfect and male flowers and have (5-) 1 0 (- 1 2) white or pink staminodia. Differentiation of sterile florets is variable amongst Desmanthus species. In both male and perfect flowers the calyx is campanulate, 5-toothed and more or less regular. The corolla consists of 5 separate basally attenuate petals, typically I-nerved, white or pale green with white margins. Some have red anthocyanic blotches. The androecium contains 5 to 10 (sometimes variable) separate stamens located in 2 ranks of differing maturation times. The white fIlaments are constricted at the point of insertion with the anthers so the anther is caducous. Anthers are dorsifixed to nearly basifixed, oblong, the connective broad and darker than the thecae (Luckow, 1 993). Similar information is provided by Hacker (1990) and Alien and Alien (1981).

Floret structure of D. virgatus as described by Luckow ( 1 993):

"Sterile flowers 1 to 8: calyx 0.6 to 1 .2 mm long, 0.4 to 1 .0 mm wide, 5-10bed, widely obconic, pale green with white margins; staminodia 1 0, 1 .7 to 7.5 mm long and white. Male flowers usually absent, rarely 1 , born above the sterile florets, no ovary but with a similar androecium and perianth to perfect florets. Perfect florets 3 to 1 4; calyx 1 .5 to 3.0 mm long, obconic, the tube 1 . 5 to 2.6 mm long, 0.6 to 1 .4 mm wide, rimmed with free acute lobes 0.2 to 1 .0 mm long, pale green with white margins, glabrous; petals 204 to 4.0 mm long, 004 to 0. 8 mm wide, oblanceolate, green with red or purple tips, glabrous; stamens 10, rarely 5, 3.5 to 7.0 mm long, anthers eglandular; ovary 1 .5 to 204 mm long, linear, glabrous, style 1 .9 to 4.4 mm long, not exserted beyond the stamens."

Differences in flower structure occur among Queensland cultivars of D. virgatus. All cultivars are described (Cook et aI., 1 995) as having a solitary, axillary, pale green-cream head but numbers of perfect flowers vary: 'Marc ' 7 to 1 3 , 'Bayamo' 8 to 15, 'Uman' 9 to 20. Whereas the petals and sepals of 'Marc' have pale tips, those of 'Bayamo' and 'Uman ' have increasing proportions of red colouration at the tip.