R.L. ISON, ANNE E.B. PARSON and B.C. JACOBS
School of Crop Sciences, University of Sydney, Australia.
Tetraploid (2) and diploid (4) accessions of Trifolium semipilosum, including the commercial cultivar Safari, were grown initially at 21/16°C and then in a range of temperature and daylength combinations to assess the extent of potentially exploitable variation present for seedling and plant growth. Trifolium repens cv. Haifa, which shares some of the range of adaptation of T. semipilosum, was included as a comparison species.
Seedling growth at 21/16°C was not related to initial seed weight; Haifa had the lowest and the tetraploids the highest seed weight. Plant weight was initially lowest for Haifa but the rate of accumulation of dry matter and root length extension was greater so that after 31 days, growth of Haifa was not limited by seed size. Net assimilation rate and relative growth rate of Haifa (15–31 days) were significantly greater than those of T. semipilosum accessions. This was not related to seed P and N concentrations; the former ranged from 0.5–0.75 % and were highest for the tetraploids which had greater seedling weights and root length (but not leaf area), than other T. semipilosum accessions.
Haifa white clover growth was superior to all T. semipilosum accessions in all temperature (15/10, 21/16, 24/19, 30/25°C, Experiment 2) and daylength treatments (15/10, 21/16, 27/22, 33/28°C × 10, 12, 14, 16, 24 h daylengths, Experiment 3). Daylength had little to no effect on growth whereas temperatures had differential effects depending on accession or character measured. Haifa growth varied least across the range of temperatures, confirming the relatively narrow, temperature determined niche to which T. semipilosum appears adapted. T. semipilosum dry matter production at low temperatures (15/10°C) was greatest for the tetraploid, thus warranting further field evaluation to determine whether the zone of adaptation of T. semipilosum might be extended. The commercial cultivar Safari was generally inferior to other accessions in these controlled experiments; this raises the possibility of re-examining a range of T. semipilosum germplasm in the field.