New Zealand is the world’s major exporter of white clover seed. This export has been to more than 40 countries with Australia, USA and European countries being the major markets. According to the Statistics New Zealand (2012) clover is an important contributor to seed export earnings (Figure 2.3). A dominant fraction (98%) of clover export earnings was contributed by white clover. New Zealand exported 2524.68 metric tons (Mt) of white clover seed worth NZ$13.80 m during 2011. Most (96%) of the white clover seed was certified (Figure 2.4). The variety Grasslands Huia has remained a prominent contributor to white clover seed exports. Mather et al. (1996) reported that the world consumes 9000–10,000 Mt of white clover seed annually. Western Europe and USA are the main consuming regions. New Zealand has an edge in research, production and marketing of white clover seed. This dominance could be maintained by breeding new cultivars capable of meeting future demands and ensuring consistent and reliable seed supply. Along with other measures, research is required to improve seed yield at low cost (Rolston et al., 1990).
Figure 2.3. Share of different plant species in New Zealand seed export earnings from 1988 to 2011.
Figure 2.4. Shares of different clover types in New Zealand seed export earnings from 1988 to 2011. lucerne 0.03 % timothy grass 0.003 % beets 0.2 % herbaceous plants 0.5 % fescue 2.0 % other forages 6.5 % Miscellaneous 7.5 % clover 22 % rye grass 32 % vegetable 29 % alsike serradella, uncertified miscellaneous cluster and white clover mixtures white clover, uncertified white clover, Huia, certified white clover, Tahora, certified white clover, Kopu, certified white clover, Pitau, certified white clover, miscellaneous, certified
Rolston and Clifford (1989) showed that from the mid-1940s to 1989 the area under seed production of white clover had reduced from 18,000 hectares to 15,000 hectares. However, the national average seed yield increased from 80 kg/hectare to 230 kg/ hectare due to effective in research seed production. Reduction in white cover seed production area has been successfully compensated by research causing an increase in yield per unit area. However, it appeared that New Zealand’s white clover seed production was declining at the rate of 168 tons per year. The viability of the New Zealand herbage seed industry is crucial for the delivery to farmers of improved traits, either of a direct (in genetics of plant) or indirect (endophyte, rhizobia) nature.
Woodfield and Caradus (1994) evaluated a collection of 110 white clover ecotypes and cultivars originating from 24 countries, representing more than six decades of plant improvement. They found a 6% genetic gain per decade for clover herbage yield and clover content. However, no significant difference was found for the number of flower heads per square meter. This shows that improvement in white clover had occurred without reducing flower head production. This also indicates that there has probably been less focus on improvement of seed production traits.
High seed production potential is a very important factor in commercial success and survival of a variety (Marshall et al., 1995; Abberton and Marshall, 2005). It ensures appropriate seed supply and production costs. Inadequate seed yield of T. ambiguumis one of the major hindrances to the commercial success of this persistent, high quality forage, and drought and pest resistant clover species (Williams and Nichols, 2011). Low seed production of many white clover cultivars like Tillman (USA), Siral (Australia) and many European cultivars has resulted in their commercial failure (Woodfield et al., 2004). Ayres et al. (2002) described the need for development of ‘NuSiral’ from ‘Siral’ white clover. ‘Siral’ was developed from an introduction from Algeria as a variety with broad adaptation, strong winter growth in mild climates and efficient recovery of vegetative growth after summer-autumn moisture stress. However, it lacked a distinct and intense flowering phase and had a tendency to retain the leafy phase during the reproductive period. This resulted in poor seed production, which made it commercially unviable. It required three additional cycles of selection on a phenotypic basis, primarily for increased seed yield and disease resistance to create the commercially viable ‘Grasslands Nusiral’ cultivar. Seed production attributes should be focused on during
the early stages of cultivar development for commercial success (Jahufer and Gawler, 2000).
White clover seed production is an output of complex interactions among a wide range of environmental and genetic components (Thomas, 1987). There is a substantial gap between potential and actual seed yield (Pasumarty et al., 1993a). Moreover, adaptation and usage of white clover can be increased by improving its gene pool through wide hybridization with its wild relatives (Williams and Nichols, 2011). New Zealand’s edge in white clover seed production can be maintained or enhanced by improving seed yield per unit area and producing improved cultivars.
2.2
WHITE CLOVER SEED PRODUCTION
Zohary and Heller (1984) described most species of Trifolium, including white clover,
T. uniflorum, as allogamous (cross pollinated) having insects as the main pollinators.
White clovers can propagate sexually by seeds and vegetatively by stolons (Williams, 1987b). Thomas (1980, 1987) explained reproductive development of white clover. The production of floral primordia initiates at terminal apical meristems of elongating lateral stolons. Only lateral and axillary positions of the stolon apex produce floral primordia. Each new inflorescence emerges a few days after the appearance of its subtending leaf. Low temperatures of autumn, winter and/or spring are the most influential factors for floral initiation for most cultivars studied in New Zealand. Long days with high temperature also increase inflorescence initiation for many white clover cultivars. Observations based on ‘Grasslands Huia’ under Palmerston North conditions, showed that inflorescences emerged out of the stipular sheath after seven or eight leaf primordia had formed at the same stem apex. Adaptation to different latitudes also influences flowering. Cultivars from low latitudes e.g., Tamar, Spanish and Louisiana ecotypes, categorized as ‘Mediterranean’, initiated flowering in response to cool conditions and terminated it with the start of warm days. By contrast, cultivars from high latitudes e.g., Kent wild white, Ladino and Russian lines, categorized as ‘summer growing, from high Latitude; did not initiate flowering in response to cool conditions until June or July. This initiation trend extended until December and peaked during November and December, with no further progress in January. These genotypes are called short-long- days plants, for responding positively to both short and long days for floral initiation. Favourable environmental conditions for flowering promote reproductive bud
formation. The ratio of flowering to non-flowering nodes is dependent on genotype as well as environment. The developmental sequence of floral parts is sepals, stamens, petals and lastly the carpel. An inflorescence, normally consisting of 20-40 white or pinkish white florets arranged in a raceme, is born on a peduncle originating from the axil of a leaf. The peduncle is usually longer then the adjacent leaf petiole. Each floret is a typical papilionaceous flower. The calyx consists of five sepals. It encircles the corolla of five petals identified as a larger posterior petal, the vexillum (standard petal) enclosing two lateral wing petals (alae) which further surround two anterior keel petals. The white clover flower is hermaphrodite with 10 stamens and a single carpel. Nectary is located at the base of the staminal tube. Anthers dehisce before full elongation of the petals. Flowers are self-incompatible or pseudo-self-compatible and they are cross pollinated mainly by bees. Seeds become mature after 4-6 weeks from pollination. Mature seed is small in size i.e., 1 mm wide and 0.5 to 0.7 mm thick, cordate (heart) shape and yellow to brown in color. A significant proportion of seed has an impermeable seed coat (hard seed) and can survive in the soil for many years.
Seed production of white clover is highly variable and often low (Marshall, 1995). However, considerable natural variation exists in white clover for this trait. Jahufer and Gawler (2000) evaluated 36 wild accessions, collected from Morocco and Tunisia, along with three varieties and 14 commercial cultivars of white clover for seed yield components in Australia. The populations showed significant variation for all seed components. They found strong phenotypic and genotypic correlations between the total number of ripe inflorescences and potential seed yield. Florets per ripe inflorescence were also moderately associated with potential seed yield. Finne et al. (2000) studied vegetative and seed yield traits of eleven Norwegian white clover cultivars. The genotypic coefficient of variation for seed yield was considerably higher than those of the studied vegetative traits. This suggested that reproductive characters are genetically more variable than vegetative attributes. Except ‘Milkanova’, all cultivars showed significant within population genotypic variation for all traits. However, even this cultivar possessed significant (P < 0.001) genotypic variation within a population for seed yield along with winter survival and spring growth. The test of heterogeneity of genotypic variance showed that seed yield was the most variable trait within the populations. Williams et al. (1998) studied a wide range of commercial varieties of
different leaf sizes under field conditions of UK. They observed significant variation for various seed yield components.
New Zealand has an edge in knowledge, skill and the favourable environmental conditions in the Canterbury area for white clover seed production. According to Woodfield et al. (2004), in New Zealand white clover inflorescence density regularly exceeds 800 inflorescences per square meter. The 1000 seeds weight of 0.7 gram (g) and seed yield of 0.25g per inflorescence is achievable. A conservative estimate for potential seed yield is 2000 kg/ha, whereas, the best commercial cultivar averages around 1000 kg/ha. The researchers selected high seed producing plants from a range of pre-released cultivars at Lincoln, New Zealand. These cultivars were polycrossed to produce F2. This F2 population was screened to isolate 4 groups of plants i.e., High
inflorescence density (HID), Low inflorescence density (LID), High seed yield per inflorescence (HYI) and Low seed yield per inflorescence (LYI). The groups were selected such that the HID and LID had a four-fold difference of seed yield and inflorescence density but were not different from each other for seed yield per inflorescence. Similarly, the HYI and LYI had four-fold differences between them for seed yield per inflorescence but had the same inflorescence density. These four groups were crossed with each other in various combinations. It was observed that an increase of 33% in inflorescence density resulted in 34% increase in seed yield from LID to HID. There was no change in yield per inflorescence associated with this improvement. Similarly, an improvement of 24% in yield per inflorescence caused a 17% increase in seed yield from LYI to HYI without affecting inflorescence density. This showed that traits of inflorescence density and seed yield per inflorescence are under different genetic control and can be manipulated independently. Leaf size was negatively associated with increase in inflorescence density but was not affected by seed yield per inflorescence.