Studies Of Chemical Ripening Of Rapeseed Siliques And Its Physiological Mechanism

Rapeseed(Brassica napus L.),one of the most important oilseed crops,is planted widely in the world among which China is the earliest and largest,country for rapeseed mass production. In 2004,the planted area in China was 7.27×10~4 hm~2,and the mean yield was 1813 kg/hm~2, along with the total production to reach 13.18 million tons,which were nearly equal to one third of the world's planted area and oilseed production.However,the continuous decrease of sown area has threatened our oil self-supported ability of oilseeds in recent years along with the loss of advanced planting technologies and the high cost of highly intensified labours.Therefore,it is urgent to study on the mechanization of rapeseed harvesting technology.However,the natural maturation periods of rapeseed siliques are inconsistent,which results in the large loss of the yield during harvesting.To save this loss,the chemical ripening technology can improve the homogeneity of rapeseed maturation effectively,but unsuitable chemical ripening methods may arouse some negative effects.Since the study of chemical ripening becomes an important issue for the harvesting,the works in this dissertation are concentrated on investigations of the genotype difference of various rapeseed maturations,the choice of chemical ripeners and dosages,and various physiological and biochemical fundamental mechanisms of siliques's ripening and the effects of ripeners on the yield and quality in consideration of different stages of silique's growth and inconsistent maturation periods,from which some conclusions are drawn:1.The maturation and senescence of different siliques have a relationship with different physiological-biochemical characteristics and endogenous hormone regulations.The efficiency of solar energy utilization,transpiration rate(Tr) and stomatal conductance(Gs) of the middle and late ripening types of Rapeseeds as well hybrid Rapeseeds are big.The heat dissipation due to photosynthesis is small.The net photosynthetic rate(Pn) and water use efficiency(WUE) are high.IAA content is big,and ABA is small.In comparison,these are in contrary to the early ripening Rapeseeds.2.The ripening effects and mechanisms vary obviously for different ripeners.The effect of touching ripens such as diquat is good on ripening and fast dehydration,but bad on the yield and oil content,which not only degrades chlorophyll seriously,but also reduces Pn.At the same time, The ripening accelerates aging as the content of ABA rises significantly.In comparison,though the ripening effect and dehydration rate of systematic ripeners,such as roundup,are slow and long,negative effects on the yield,quality and Pn are weak without changing of the ratio of ABA over IAA in siliques.3.The chemical ripening is effective in improving the percentage of thresh at first,but decreases the yield and quality of rapeseeds,The percentage of thresh at first by using diquat ripening could reach 50%.But,the high dosage of used diquat ripening can reduce the yield and the oil content obviously,which is nevertheless avoided by lowering the dosage below 0.3%. Though the percentage of thresh at first by using roundup ripening is only as high as 15%,the negative effect on the yield and the oil content is weak.The ripening at the right time could prevent effectively against the deterioration in rapeseed's quality and yield,which nevertheless is observed during early ripening,Therefore,it is necessary to control ripener's dosage and ripening time strictly under the condition that the yield and quality would not decrease significantly.According to our experimental data,it is recommended to select either 0.1%～0.3%diquat or 0.5%～0.8%roundup for ripening in 2～3 d or 8-10 d before siliques's maturation with the addition of 0.5%plant spray for the best.4.The chemical ripening has a significant effect on siliques' chlorophyll content, fluorescence,photosynthesis efficiency and protective enzyme.The siliques' chlorophyll content,Pn,and the PSⅡelectron transfer chain(ETR) decrease obviously when using diquat ripening,which affects the photosynthesis seriously.At the same time,it shows the acceleration of silique's membrane liquid peroxidation,the damage of membrane permeability and endogenous protective systems,the increase of the MDA content and SOD,POD and CAT activities,the instability of membrane system,and the fastened aging of siliques.However,the suitable dosage of roundup for ripening can retard siliques' chlorophyll degradation,and promote electron transmission speed of photosynthesis by maintaining the integrity of ultrastructure for a balanced membrane System.5.Our results show that the ripener composed of both single ripener and growth regulator of plants ethephon complementarily promotes the rate of ethylene and ABA production and inhibits IAA formation,which accelerates rapeseed's aging.But the effect would be different for the mixture of other ripeners.6.It's harmless for human being to eat this kind of chemical ripened products.The pesticide residues are within the limit of our experimental detection for the harvested rapeseeds by using diquat and glyphosate at right dosages and conditions mentioned above.On the basis of our findings,it is suggested that the maximum limit of residue(MRL) for diquat in rapeseed grains is below 2 mg/kg and that of glyphosate below 15 mg/kg for the safety.Therefore,to apply chemical ripening is beneficial to promote siliques' maturation in unison,which is a practical technique standing from a scientific viewpoint.This technique is recommended for the mechanization harvesting of rapeseed in a large area.