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Crop manipulation to improve hybrid radish seed yield and quality : A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University

Khan, Khan Mir
Date
2018
Type
Thesis
Fields of Research
ANZSRC::070302 Agronomy , ANZSRC::07 Agricultural and Veterinary Sciences
Abstract
Hybrid radish (Raphanus sativus L.) seed is grown in New Zealand for multiplication and re-export, and radish seed exports were worth $22.8 million in 2017. Seed yields vary with hybrid and season, but are reduced by lodging, which reduces seed filling, increases disease problems, and causes harvesting difficulties. In addition, the strong apical dominance in the radish plant reduces the ability of secondary stems to produce seeds. Crop manipulation, either by chemical or mechanical means, may be able to increase seed yield by preventing/ reducing lodging and reducing apical dominance. The hypothesis was that crop manipulation would increase the seed yield and quality of hybrid radish. Crop manipulation treatments included the application of the plant growth regulator gibberellic acid (GA₃), several plant growth retardants, and mechanical topping. In addition, the effect of lodging on radish seed yield was determined by mechanically supporting the crop. Lodging did not begin until after anthesis, but lodging reduced seed yield by around 21% through a reduction in all seed yield components. This followed a significant reduction in photosynthetically active radiation (PAR) intercepted by the lodged canopy. GA₃ was applied at 100 ppm at three growth stages: seedling stage (BBCH 19), green bud stage (BBCH 51) and yellow bud stage (BBCH 65) in three trials over two seasons, and all three treatments significantly outyielded the control by producing more branches per plant, pods per branch, seeds per pod and a higher 1000 seed weight. When results were averaged for the three trials, GA₃ applied at the green bud stage (BBCH 51) produced the greatest seed yield increase (+387 kg/ha). Topping at the green bud stage and yellow bud stages also significantly increased seed yield, with the response being greater for the former. As for GA₃, all the seed yield components were increased by topping. The three trial seed yield mean increase for topping at the green bud stage was +255 kg/ha. A combination of topping plus GA₃ at the green bud stage produced a greater seed yield then topping alone, but the increase (+ 315 kg/ha) was not as great as GA₃ alone. In further trials over two seasons, different plant growth regulators (PGRs) were applied at the green bud stage, viz. paclobutrazol at 100 and 200 ml a.i /ha, chlormequat chloride at 1.0 and 2.0 litres a.i/ha, trinexepac- ethyl at 150 and 300 ml a.i/ha, tebuconazole at 0.8 and 1.6 litres a.i/ha and mepiquat chloride at 305 g a.i/ha. The latter was applied in the seedling stage. Plant height and stem length were reduced by some treatments, but all the PGRs increased stem thickness. Seed yield was significantly increased by paclobutrazol (+23%), chlormequat chloride (+29%) and a mixture of chlormequat plus tebuconazole (+24%) but not by the other treatments. Once again the seed yield increases were explained by increases in all seed yield components Over the three seasons of trials, all treatments applied resulted in small but significant increases in seed germination, seed vigour and 1000 seed weight. However, no seed quality differences were found among pod positions on the raceme. In this study, there was a strong negative relationship between lodging and seed yield, and lodging and PAR. The majority of the treatments resulted in plants which were large did not lodger as severely and had more branches, pods and seeds per pod than the control. As a result, the percentage of PAR intercepted by the canopy was higher than that of the control, and more seeds were able to be supported through to harvest. Irrespective of treatment, however, between 20-25% of the seeds harvested were below the size standard required by the industry. The reason for this requires further investigation. A cost/ benefit analysis established that the greatest return to the grower came from the application of GA₃ at the green bud stage, being $670/ha greater than the next most profitable treatment which was chlormequat chloride plus tebuconazole. The former applied at BBCH 51 is therefore recommended as the best treatment to increase hybrid radish seed.
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