Effect of natural and artificial chilling and gibberellic acid (ga3) on bolting, flowering and seed production in carrot (daucus carota l.) cv 'names' at different agro-ecological sites in Kenya.
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These studies were carried out to determine whether some sites in Kenya highlands have sufficient natural chilling to induce satisfactory flowering and seed yields in carrot cv. `Nantes`.In the event of insufficient natural chilling, the first experiment examined the artificial chilling durations which could supplement the natural chilling while the second experiment investigated whether application of gibberellic acid (GA3) could replace or supplement natural chilling. The third experiment was aimed at establishing the age at which carrot is sensitive to vernalization and the critical temperature for vernalization of carrot roots. Progeny seeds harvested in the first and second experiments were evaluated at Juja for both used and root qualities. There were significant differences among chilling treatments and sites in the various parameters measured. Subsequent mean comparisons by LSD showed that percent flowering, number of umbels per plant, seedstalk heights,seed yields and germination percentage of the harvested seeds were lowest in the naturally chilled plants and increased with duration of artificial chilling at all sites. But in 1992/93, one hundred percent flowering was obtained in naturally, and artificially chilled plants at Kinangop. Days from planting to bolting, flowering and seed maturity decreased with increasing duration of artificial chilling of carrot roots at all sites, while in the naturally chilled plants the three events were much slower. The comparison among sites showed that plants at Marindas had the lowest number of umbels per plant while those at Juj a had shorter seedstalks with higher seed yields than those at the other sites. There were highly significant differences among GA3 treatments and sites in the various parameters studied. Comparison of means by LSD showed that percent flowering, umbels per plant, seedstalk heights, seed yields and germination percentage of the harvested seed increased significantly with increasing rates of GA3 applied to mother plants. Percent flowering was low at both Juja and Kinangop in the untreated plants in 1991/92, but in the following year one hundred percent flowering was obtained at Kinangop in both treated and untreated plants. Abortion of umbellets was observed at Juja in plants treated with 500 and 1000ppm of GA3.These plants had lower seed yields. When comparing sites, percent flowering, number of umbels per plant, seed yields and seedstalk heights were significantly higher at Kinangop than at Juja. Better flowering in 1992/93 at Kinangop in both experiments was attributed to higher rainfall in that year which ensured better plant growth than in the previous year. In both experiments, seeds produced at Juja ripened and dried faster, and had better germination percentage than those produced at other sites. Seed yields of as high as 3 to 9t ha-1 were recorded in both experiments in the best treatments. The progeny of different seed lots (harvested from both experiments) had good quality roots except for those harvested from Juja mother plants treated with 100 ppm of GA3 which produced a third albino roots. Most progeny roots had significantly longer tops than those raised from imported seed. A study in a controlled environment showed that imbibed germinating seeds of carrot cv. 'Nantes' may respond to chilling for flowering. Plants of ages 30 to 120 days also responded to chilling with one hundred percent flowering. Regression analysis showed that the critical temperature for bolting and flowering in carrot cv. 'Nantes' was about 6°C. The base temperatures for bolting and flowering were about -2.9 and -1. 7°C, respectively while the maximum temperatures were about 17 and 16°C, respectively. Linear relationships were found between the rate of vernalization and temperatures both below and above the optimum. Thermal times calculated from these relationships showed that carrot cv. 'Nantes' required a thermal time of vernalization of about 400°Cd for 100% flowering. Calculation of thermal times from the field temperature records showed that Molo, Marindas and Kinangop had above 400°Cd.