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Item Chemical manipulation of white clover (Trifolium repens L.) grown for seed production : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Seed Technology in the Plant Science Department of Massey University, Palmerston North, New Zealand(Massey University, 1992) Budhianto, BambangThe effects of chemical manipulation through the use of plant growth regulators on white clover (Trifolium repens L.) cv. Grasslands Pitau grown for seed were investigated in this study, using both sward and individual plant trials. A white clover seed crop was established in autumn 1988, certified breeders seed of cv. Grasslands Pitau being sown at 3 kg/ha in 45 cm rows. Three plant growth regulators, chlormequat chloride (1.5 and 3.0 kg a.i./ha), paclobutrazol (0.5 and 1.0 kg a.i./ha) and triapenthenol (0.5 and 1.0 kg a.i./ha) were applied at two growth stages; during reproductive initiation (11 October) or at the appearance of the first visible bud (8 November). A further plant growth regulator, daminozide (2.0 and 4.0 kg a.i./ha) was applied only in November. Chlormequat chloride, daminozide and triapenthenol did not significantly affect node production, inflorescence production or seed yield, although thousand seed weight (TSW) was reduced. Paclobutrazol significantly reduced petiole length and increased the number of nodes/m2, but did not affect dry matter production. The October application of paclobutrazol at 1.0 kg a.i./ha significantly increased potential harvestable seed yield by 71 % through increasing the number of inflorescences produced, but the 57 % increase following the November application at the same rate did not differ significantly from the control. Actual seed yield differences (+25 and 26 %) were also not significant. In the following season (1989/1990), three of the plant growth regulators (chlormequat chloride at 3.0 kg a.i./ha, paclobutrazol at 1.0 kg a.i./ha, triapenthenol at 1.0 kg a.i./ha) were applied using the same site as for the 1988/1989 trial (i.e. a second year crop), but avoiding plots previously sprayed with paclobutrazol to eliminate possible soil residual effects. Applications were either during early reproductive initiation (September), during peak reproductive initiation (October) or when reproductive buds/early flowers were first visible (November). Chlormequat chloride did not affect either vegetative or reproductive growth and development. Triapenthenol initially retarded growth (e.g. by reducing petiole length), but this effect was only transiatory, and was no longer evident 3 weeks after application. Although triapenthenol applied in November increased inflorescence number at peak flowering, seed yield was not increased. Triapenthenol applied in October did not affect inflorescence number at peak flowering, but reduced TSW. Paclobutrazol applied in September, October and November reduced petiole length and leaf size, but only application in November increased both node and stolon production. Application in October and November increased inflorescence numbers at peak flowering and harvest respectively, but seed yield was not increased. Data recorded from plots sprayed with paclobutrazol the previous season (1988/1989) provided no evidence of growth retardation through soil residual activity. In an attempt to clarify the effects of paclobutrazol on white clover growth and development, individual plants grown from seeds selected at random from a lot of certified breeders seed were established as spaced plants (80 x 80 cm) in the field in spring of 1990. Paclobutrazol was applied at 1.0 kg a.i./ha on 6 November 1990 (when more than 75 % of the plants were initiating reproductive buds at their terminal buds) or 23 November 1990 (when more than 50 % of the plant population had reproductive buds visible on their stolons). Petiole length and leaf size were initially reduced, but beginning two months after application, vigorous regrowth occurred, to the extent that paclobutrazol treated plants became as tall as the control plants. However, retardation effects occurred again at harvest. Total plant dry matter and root:shoot ratios were not affected by paclobutrazol. Chlorophyll content/unit leaf area and leaf thickness increased following paclobutrazol application, but increases were not correlated. Seed yield and yield components did not differ from that of the control plants, mainly because plant to plant variation was very large, irrespective of treatment. To attempt to reduce this source of variation, a further spaced plant trial was established in 1991/1992 using plants produced by clonal propagation from three distinct genotypes from within cv. Grasslands Pitau. Paclobutrazol was applied at the same rate and time as in the previous season, and while not affecting the number of nodes developed along stolons or inflorescence initiation at the stolon apices, it did significantly increase stolon production in all three genotypes through increasing secondary, tertiary and to a lesser extent quaternary branch numbers. However, not all these extra stolons were able to produce inflorescences, and this ability varied significantly with genotype. As a consequence, inflorescence number and potential harvestable seed yield were significandy increased only in one genotype following paclobutrazol application. However, paclobutrazol reduced seed abortion and increased seed weight in all three genotypes. In individual plants, inflorescence growth and development from emergence to the seed ripening stage occurred more quickly in paclobutrazol treated plants than untreated plants. A simulated sward trial was used in 1990/1991 to determine whether the previous failures to significantly increase actual seed yield were because paclobutrazol treated plots had ripened earlier than control plots, and as a consequence more seed had been shed by the time of harvest. However, no significant paclobutrazolXharvest time interactions for seed yield or seed yield components were recorded. These results suggest that paclobutrazol did not affect seed maturity in a sward situation. Irrespective of treatment, greatest seed yield came from harvesting 25 days after peak flowering, but this did not differ significantly from harvesting 35 days after peak flowering. Delaying harvest to 40 and 45 days after peak flowering significantly reduced seed yield. As in previous sward trials, paclobutrazol application significantly increased inflorescence numbers, but large (+56 %) differences in potential harvestable and actual seed yield were statistically not significant. In each case, high data variation (CV > 30 %) was recorded. Factors responsible for the failure of apparent biological increases to be statistically real are briefly discussed.Item Seed production studies in lucerne (Medicago sativa L.) cv. Grasslands Oranga : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science (Seed Technology) at Massey University, New Zealand(Massey University, 1993) Askarian, MohsenTwo years of field trials with lucerne (Medicago sativa L.), cv. Grasslands Oranga, were used to determine plant vegetative and reproductive responses to the effects of row spacing and sowing rate, application of two plant growth regulating chemicals, and weed control. For an autumn (March 15) sowing, seedling number per metre of row increased as sowing rate (1 to 12 kg/ha) and row spacing (15 to 60 cm) increased. However the number of seedlings was not directly proportional to the number of seeds sown, and percentage establishment six months after sowing was highest (73%) at the lowest sowing rate of 1 kg/ha. Overall mean establishment for all treatments was 57, 46, and 34% for 1, 6, and 18 months after sowing respectively. Dry matter production at 6 months after sowing was greatest at the 15 and 30 cm row spacings and 12 kg/ha sowing rate, but there were no significant differences in dry matter among treatments at later assessments. In the first year seed yield from the 15 cm row spacing was significantly lower than from the 30, 45 and 60 cm row spacings, while sowing rate had no effect on seed yield. In the second year, row spacings did not significantly affect seed yield, but the seed yield from the 1.0 kg sowing rate was significantly increased because harvestable racemes/m2 and thousand seed weight were significantly increased. Seed yield over the two years of the experiment was highest at the 1 kg/ha sowing rate and for the 30 and 45 cm row spacings. The average seed yield for all treatments was 127.2 and 186.9 kg/ha for the first and second year respectively. Neither row spacing nor sowing rate had any effect on the quality of harvested seed. There were no interactions between row spacing and sowing rate for plant establishment, dry matter production, or seed production. In the 1991/1992 season, the effect of two plant growth regulators, paclobutrazol at 1.0 kg a.i/ha (applied on 1 November or 1 December), and cycocel at 3.0 kg a.i/ha (applied on 1 December, 23 December, 1991 or 1 January 1992), on vegetative and reproductive growth was examined. Paclobutrazol applied during active vegetative growth (1 November) significantly altered vegetative shoot development by inhibiting apical dominance, thus inducing lateral branches which subsequently increased reproductive sites, and increased seed yield by 37%. This seed yield increase was due to an increased number of racemes/m2 (+36%) and pods per raceme (+72%). Paclobutrazol applied at first flower bud appearance (1 December) had no effect on seed yield or seed yield components because it did not alter shoot production or the number of racemes. Cycocel application did not retard plant height or increase racemes per unit area. However while application on 23 December (at first flowering) had no significant effect on seed yield, cycocel applied in early December (first flower bud appearance) or early January (at peak flowering) significantly decreased seed yield, because of a reduction in the number of flowers/m2 and/or harvestable racemes/m2. In the following season (1992/93), paclobutrazol at 0.5 kg a.i/ha and 1.0 kg a.i/ha was applied during active vegetative growth on 25 October 1992. Both rates significantly reduced plant height by 8 weeks after application, but this effect had disappeared by final harvest. As in the previous year, paclobutrazol at 1.0 kg a.i/ha significantly increased seed yield, but the increase (+153%) was much greater than in the previous year. This increase in seed yield was associated with an increase in the number of harvestable racemes/m2 (+126%), pods per raceme (+36%) and thousand seed weight (+11%). Paclobutrazol at 0.5 kg a.i/ha had no significant effect on seed yield. In 1992/1993 the effect of hand weeding and the application of three herbicides (hexazinone 1.0 kg a.i/ha, simazine 2.25 kg a.i/ha plus paraquat 0.6 kg a.i/ha) on seed yield in a second year crop was investigated. Hand removal of weeds, predominantly white clover but also Poa annua L. and broad leaved species increased seed yield from 0.7 to 21.3 g/m2, mainly because racemes increased from 89 to 1230/m2. Increases in pods per raceme and seeds per pod were also recorded. Hexazinone applied during active vegetative growth in early spring eliminated white clover from lucerne plots and increased seed yield to 14.3 g/m2. However this treatment did not control Rumex obtusifolius L. Simazine plus paraquat applied in winter before active spring growth controlled many annual weeds but, although initially checking white clover, did not control it. As a consequence, seed yield did not differ from that of the untreated control. Although hexazinone effectively removed white clover from a second year lucerne seed crop, it is recommended for use only on mature stands. Harvested lucerne seed viability did not differ among treatments, but hand weeding and herbicide treatments significantly reduced the percentage of hard seed.