In search of female sterility causes in the tetraploid and pentaploid cytotype of Pilosella brzovecensis (Asteraceae)

Within the agamic Pilosella complex, apomixis (asexual reproduction through seed) involves apospory, parthenogenesis, and autonomous endosperm development. Observations of reproductive biology in P. brzovecensis throughout four growing seasons in the garden have shown that both tetraploid and pentaploid plants of this species do not produce viable seeds and reproduce exclusively vegetatively by underground stolons. The reasons for the seed development failure were unknown, therefore our research focused on the analysis of reproductive events in the ovules of this taxon. We found that apospory was initiated in the ovules of both cytotypes. Multiple aposporous initial (AI) cells differentiated in close proximity to the megaspore mother cell (MMC) and suppressed megasporogenesis at the stage of early prophase I. However, none of the AI cells was able to further develop into a multi-nucleate aposporous embryo sac (AES) due to the inhibition of mitotic divisions. It was unusual that callose was accumulated in the walls of AI cells and its synthesis was most likely associated with a response to the dysfunction of these cells. Callose is regarded as the isolating factor and its surprising deposition in the ovules of P. brzovecensis may signal disruption of reproductive processes that cause premature termination of the aposporous development pathway and ultimately lead to ovule sterility. The results of our embryological analysis may be the basis for undertaking advanced molecular studies aimed at fully understanding of the causes of female sterility in P. brzovecensis.

Maximising seed production potential in white clover: factors influencing seed set per floret

The causes of low seed set per floret in white clover (Trifolium repens L.) are reviewed. Three stages of flower head development are distinguished as important for a high level of seed set: a pre-fertilisation stage, a stage of anthesis leading to pollination, and a postfertilisation stage in which seed provisioning occurs. In sunny conditions the percentage seed set is limited at the pre-fertilisation stage by up to 20-30% ovule sterility. Relatively low light intensities during the postfertilisation stage can lead to abortion of a high proportion of fertilised ovules and developing seeds. Experimental results suggest that seed yields under optimal growing conditions can be limited solely by the level of pre-fertilisation ovule sterility and probably cannot be bettered, but further understanding of the seed-provisioning requirements for photosynthate could lead to improved management practices for seed production under conditions of lower light intensities. Keywords: abortion, light intensity, ovule, seed provisioning, sterility, white clover

Effect of canopystructure and light intensity on seed production in white clover

A field experiment was structure and light intensity on seed production A field experiment conducted during the summer of 1989-90 to determine to what extentand under what growing conditions the seed yield of white clover flower heads is influenced by canopy density and overcast weather conditions in plants of 'Grasslands Pitau'. Overcast weather was simulated by artificially shading the plants. The flower heads developed in a dense canopy produced 37% fewer seeds per head than those formed in an open canopy. Seed yield per flower head was also strongly affected by artificial shading. When plants were shaded only before pollination there was a 3 1% reduction in seed number per head; when shade was given only after pollination there was a reduction of 25%. It is argued that a major advantage of the practice of defoliation at the time of closing for seed production is enhancement of flower fertility; and that decreased seed set in duller, wetter summers is probably, at least in part; attributable to increased ovule sterility in the dense canopies formed under those conditions, Keywords: white clover, canopy, light intensity, seed yield