Stafford, Judith Anne
(1996)
The effects of prochloraz on the growth and yield of oilseed rape.
PhD thesis, University of Nottingham.
Abstract
Yield production was investigated in the winter oilseed rape variety Capricorn by comparing crops grown under standard husbandry conditions in three seasons (1991, 1992 and 1993 harvest years). Also investigated were the effects of the phytotonic imidazole fungicide prochloraz on the physiology of yield production. Prochloraz was applied in autumn, spring and summer in all possible combinations (eight treatments), except in the second season (1992), when the autumn application was omitted. Crop growth and development were studied in detail using stratified sampling in 20 cm layers. Detailed growth analysis between flowering and final harvest was restricted to untreated controls and plots receiving all three prochloraz applications. Solar radiation interception was measured using tube solarimeters arranged to correspond with layers of the profile obtained in sampling.
The qualitative pattern of growth and development was the same in all three seasons regardless of variations in environmental conditions, and could be divided into the four distinct but overlapping stages described in previous studies. Potential yield (pod number) was determined at flowering and was almost constant between seasons. It did not limit final yield and can never be fully realised. Final seed yield was dependent upon the amount of solar radiation intercepted in Stage IV (seed development), and was manifested in the extent of pod and seed losses and seed growth during this period. The efficiency of radiation use varied between seasons.
The main role of leaves was the development of the reproductive framework, and rapid leaf senescence occurred at flowering, particularly in Stage III (pod development). Losses of potential pods and seeds were continuous from flowering onwards but were severe in Stage III. Pod retention was determined by the availability of assimilates which depended on radiation interception and intra-plant competition for assimilates. Regulation of this was probably under hormonal control. Dry matter production was unaffected by the flower canopy except when radiation levels were unusually low during Stage III. Final yield was not affected by such effects. Seed number per pod is determined genetically, but modified during development by assimilate availability according to position in the canopy and the number of competing pods.
Seed growth occurred mainly in Stage IV and depended upon the extent of photosynthesis, largely in pods and branches. Seed yield was independent of growth up to flowering, and no remobilisation of dry matter occurred to support seed-filling. When open canopies were produced, leaf retention (largely at the base of the pod canopy), radiation interception by leaf, and therefore, assimilation by leaf, were all increased. Under such circumstances, assimilate production by leaf at the base of the pod canopy may have contributed up to 20% of the dry matter (seed) produced in Stage IV. Pod and seed retention were improved throughout, but particularly higher in the canopy, because seeds that were growing well were more likely to be retained.
Disease development in all treatments was monitored using detailed assessments throughout each season. The main fungicidal effects of prochloraz were on light leaf and pod spot and stem canker. Disease incidence was reduced in all seasons, but severity was reduced only in 1993. However, disease severities were generally very low, and these fungicidal effects probably had little or no effect on yield. Large losses of potential yield were caused by severe sclerotinia infection combined with high temperatures, a high soil moisture deficit, and possibly lodging (1992). Sclerotinia was controlled by iprodione in 1993. Disease data were used to form a model to estimate the expected yield losses caused by sclerotinia infection. Heavy infections of stem canker in 1993 did not seriously affect yield.
Prochloraz increased seed yield by up to 16% in 1991 through increased pod numbers largely in the upper and middle regions of the pod canopy. Seed number per pod was increased slightly, largely due to higher retention in lower pods, while 1000-seed weight was not affected. Effects were negative in 1992 and inconclusive in 1993. Prochloraz increased crop growth from March onwards in 1991, and the differences in green area and dry matter components were maintained to final harvest. There was no effect on harvest index. Leaf senescence was delayed by prochloraz so that during Stage IV (seed development), leaf area index in the pod canopy and just below was greater in treated plots, and the proportion of radiation intercepted by leaf was slightly increased. Total radiation interception was increased due to the increased green area index mainly due to increased pod and stem areas in the top and middle of the canopy. Prochloraz delayed crop senescence and therefore reduced the decline in efficiency in late Stage IV. Total assimilate production in Stage IV was increased partly because of continued assimilate production for longer in all organs including retained leaf. This enabled more pods to be supported throughout the canopy. By prolonging assimilatory activity in the organs at the base of the canopy, prochloraz probably modified the pattern of assimilate movement between layers of the canopy. Seed numbers per pod in lower pods and pods higher in the canopy would, therefore, have been maintained. Reasons for the failure of prochloraz to elicit a similar response in 1992 and 1993, and the nature of the phytotonic effect are discussed.
The findings are discussed in relation to the development of an oilseed rape ideotype for maximising yield production.
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