The effect of seed weight and coleoptile tiller development on seedling vigour in tall fescue, Festuca arundinaceaSchreb.
SummaryTwo characters, previously shown to influence the rate of seedling growth in tall fescue, were studied in detail in three distinct populations as part of a programme to improve seedling vigour in the species. The effect of seed weight was found to be confined to the very early stages of growth and was apparent in seedling dry matter yield but not in tiller number. The development of a tiller in the axil of the coleoptile, which varied significantly between populations, had a far greater and more permanent effect both in terms of dry matter yield and tiller number. The increase in tiller number attributable to the coleoptile tiller and its derivatives increased geometrically in step with the total tiller number, so that the intitial difference of 28% in the first harvest remained practically unchanged in the subsequent three harvests. Similarly, the percentage difference in dry matter yield between plants with and without a coleoptile tiller remained more or less constant at about 22% from the second harvest onwards.It is concluded that the development of a coleoptile tiller may prove to be a useful selection criterion in the breeding of this species.
INHERITANCE OF SEED WEIGHT AND ASSOCIATED TRAITS IN BROMEGRASS, BROMUS INERMIS LEYSS.
Among 71 S0 and S2 clones of diverse origin in a space-planted, replicated polycross nursery the range in mean seed weight was 0.71 to 1.30 grams per 300 seeds. A correlation of 0.25 was noted between seed weight and open-pollinated seed set for these clones. Their polycross progenies exhibited a range of only 0.74 to 0.94 grams per 300 seeds in a solid-planted test. The progeny-parent regression value was non-significant (b = 0.09) while the correlation (r = 0.56) was significant at the 1 per cent level.Inbred segregates of 10 representative clones were evaluated in a replicated test for seed weight, fertility, seed yield and bloom date. In each family, segregates showed significant differences in performance for the former three traits which is indicative of genetic segregation. These three traits all exhibited substantial inbreeding depression, averaging 21, 20 and 53 per cent for seed weight, fertility and seed yield, respectively, from S0 to S1 for seven families, and 19, 24 and 66 per cent, respecti...
Studies on the Expansion of the Leaf Surface I. THE INFLUENCE OF TEMPERATURE
SUMMARY An investigation was made of the expansion of the leaf surface of cucumber at temperatures of 12o, 18°, 24o, and 30o C. with two levels of visible radiation (40 and 80 cal. cm.-2 day-1). The relative rate of expansion of the leaf surface increased with temperature up to 24o but was lower at 30o than at 24o. It was slightly greater with the higher than the lower level of radiation at the lower temperatures only. This rate was the resultant of the rate of unfolding of new leaves and the rate of expansion of the component leaves. The rate of leaf pro duction increased with increasing temperature up to 240 and was constant there- , after, but leaves unfolded from the terminal bud more rapidly with increase in temperature over the entire range. The rate of expansion of individual leaves was greatest at 240, being less at both lower and higher temperatures. Differences in this rate between temperatures increased in the order: cotyledon, leaf 1, leaf 2. Leaf production and unfolding was greater with the higher level of radiation but the expansion of individual leaves was not influenced. These results suggested the following interpretation of the expansion of the leaf surface. Its potential rate is set by the rate of unfolding of leaves from the terminal bud, which depends mainly on the temperature and the rate of assimila tion by the upper leaves and the terminal bud, the demand for assimilate exceed ing the supply in this region. The demand for mineral substrates by the terminal bud is low and not influenced by a wide variation in potential supply. After un folding from the terminal bud, the leaf provides most of its own supply of carbo hydrate by assimilation and this can be met at a low level of radiation. Surplus assimilate is diverted to the roots and stems which respond much more to in creased radiation than does the leaf surface. The demand for mineral substrates by expanding leaves, however, is high—the greater the number expanding at any time the more likely is the demand by any one leaf to exceed the supply. This leads to a reduction in the number of cell divisions and, consequently, a reduced rate of expansion and a smaller leaf. The optimum level of any environmental factor is that at which the most effective compromise between these conflicting processes is reached.