The grasslands and grassy understoreys are among the most threatened plant communities of eastern and southern Australia. The restoration of these communities depends on readily available supplies of high-quality seed from a diverse range of locally indigenous grasses and forbs. Rather than relying solely on seed collected from remnant vegetation, an increasing number of species (e.g. Bulbine bulbosa (R. Br.) Haw.) are being cultivated, often for the first time, in seed production areas (Delpratt, 1997).
©CAB International 2007. Seeds: Biology, Development and Ecology
The objective of the cultivation of plants for seed production has been defined as 'the maximization of the production of seed with suitable germination capacity' (Bowring et al., 1980). However, the structure of a species' inflorescence, the number and uniformity of inflorescences on an individual plant and the uniformity of flowering between plants can strongly influence the timing and uniformity of seed maturation within a seed crop and reduce the amount of seed that can be harvested economically. For instance, to minimize loss of seed from opening capsules, Desai et al. (1997) recommended that onion (Allium cepa L.) seed crops be harvested when 1-3% of the umbels in the field have mature seeds (i.e. when capsules are open showing black seeds). Seed heads are harvested intact, with a short (unspecified) length of scape attached. The intact umbels are then gradually dried for a period of days, after which the seeds are extracted by threshing.
For seed production, Copeland and McDonald (1995) described three general stages of seed development. Approximately 80% of seed growth occurs during the first stage, which begins with fertilization of the ovule, and is characterized by numerous cell divisions and differentiation of organs, acquisition of assimilates from the parent plant and a substantial increase in seed mass. The second stage involves the beginning of desiccation, the degeneration of the funicle and the consequent separation of the seed from the parent plant. At this stage, termed 'physiological maturity', the seed attains its maximum dry weight. The third stage involves further desiccation until the seed reaches a moisture content of usually 15-20%. If high-quality seed is to be harvested directly from the plant, the harvest process must be delayed sufficiently to allow the individual seed to reach harvest maturity, but not to the point where the mature seed is shed from the parent plant.
One consequence of differences in seed maturity at harvest is a lack of uniformity in seed size. All seed lots contain a range of seed sizes, and uniformity in seed size within a seed lot is considered to be one measure of seed quality (Thomson, 1979). Non-uniform seed lots are difficult to sow mechanically and small seed size may indicate a small embryo or reduced endosperm reserves. However, the causes of heterogeneity in seed size within a seed lot include differences in moisture content, genotypic differences between plants for seed size, seed position within a multiseeded fruit or complex inflorescence and seed maturity. The impact of small (relative to its cohorts) seed size on subsequent germination, seedling establishment and plant growth is complex and will be influenced by the factors that contributed to the size differences. However, it is a common practice to screen seeds during cleaning to improve the uniformity in seed size of the processed seed lot.
When harvesting seeds of a heterogeneous, undomesticated species, considerable variability can be expected in plant development and morphological factors that affect seed size. Many wild species have adaptations that spread the production of their seeds over time. Since only a small proportion of their potential seed production is mature at one time, they are difficult to harvest efficiently.
B. bulbosa is a perennial lily native to many grasslands and understorey communities in eastern and southern Australia. It has a basal tuft of succulent, linear leaves of 30 cm, growing from a compressed stem or bulb. In natural communities it produces leaves in autumn, followed in spring and early summer by one to numerous unbranched inflorescences. Although usually dormant from mid-summer to mid-autumn, in cultivation it will retain its leaves and extend flower and seed production if it has adequate water (Delpratt, 1997). The fruit is a globose to obovoid capsule (Conran and Walsh, 1994). Capsules generally contain 18 ovules and, in cultivation, mature capsules produce 1-13 black, angular seeds that are 2-3 mm long (author's observations). Capsules mature sequentially from the base of the inflorescence upwards. It is common to have dehiscing capsules, immature capsules, two or three open flowers and unopened buds in one inflorescence at the same time. In cultivation, single plants commonly produce ten or more inflorescences during spring, summer and autumn (author's observations).
Hand-harvesting of B. bulbosa every 1-3 days, and picking capsules just prior to opening, achieves a high recovery of mature seeds. However, this process is tedious and time-consuming. An alternative may be to harvest intact inflorescences, allow them to dry and then recover the seed. With the range of reproductive stages on a given inflorescence, it is not obvious when to harvest an inflorescence, or whether seeds will continue to develop and mature once an inflorescence is detached from the plant.
This experiment investigated the influence of harvest method of cultivated plants of B. bulbosa on total seed yield, seed size and germination capacity. The conventional hand-harvesting method was compared to storing of harvested inflorescences of various ages in a controlled environment.
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