The effects of length of growing season, with and without defoliation, on seed yield and hard-seededness in swards of subterranean clover

摘要

The effects of length of growing season and defoliation on seed yield and hard-seededness were nexamined in two strains of subterranean clover (Seaton Park, Midland B) grown in swards in the nfield. All plots were sown at the same time and the length of growing season was varied by altering nthe time of finish of the season (by withholding water). There were three length of growing season ntreatments: Ti (short), T, (intermediate) and T, (control). The defoliation treatments were Do, nuncut (control), and D,, defoliated at weekly intervals until the commencement of flowering. nReducing the length of growing season drastically reduced seed yield. Thus when the growing nseason was only 3 weeks shorter than the control (i.e. T, compared with T3), seed yields averaged nover strains and defoliations were reduced by at least one half. With a further reduction of 2 weeks nin the length of the season (Ti) seed yields were only about one-third of those obtained in the control n(T,). The reductions in seed yield were due to reductions in both the number of mature burrs nproduced and to a lesser extent in mean weight per seed. nAlthough defoliation increased seed yield in all growing seasons, the effect when measured on a nrelative scale was greater in Tz than in either T, or TB. But on an absolute scale the size of the nresponse was greater in T, and T, than in TI. The rate of breakdown of hard-seededness was faster nin Seaton Park than in Midland B, but it decreased in both strains with increasing length of growing nseason.