Study On The Defence Function Of Cotyledon In Seedling Stage Of Cruciferous Plants

Cruciferous plants, though more common in the north temperature zone, are widely distributed all over the world with 350 genus and 3,000 species.Many of them are important economic crops.Although as a kind of valuable vegetables to humans,the quantity and quality of Cruciferous vegetables often reduce by herbivory(especially insect pests) during growth. Seedling stage is the crucial one in the life cycle of plants which influences the seedling establishment and subsequent development.And cotyledon is a vital organism of seedling.So in this research,the defence function of cotyledon and the relationships between the defence and seedling attributes in two Cruciferous vegetables,Beassica pekinensis (Lour.)Rupr.and Raphanus sativus L.were discussed.The main results are as follows:1.The resistance of cotyledon and leaf in different periods(cotyledon germinated as the first day, 1st, 4th, 7th, 10th,13th day) of the two seedlings to herbivory were measured by detecting the different degrees(mass and area) of pests attack.The results indicated that the cotyledon and leaf of both two Cruciferous vegetables which were attacked by insect pests in early seedling stage resisted the herbivory with defense function of cotyledon and leaf. The attacking level of cotyledon were highest at the 4th day after germination, and lowest at the 10th day in B. pekinensis (Lour.) Rupr seedling. In R. sativus L. seedling, highest at the13th day and lowest at the 7th day. However,both the cotyledon of the two species got higher attacking level at the 4th day after germination.The attaking level of leves increased gently after the first leaf appeared.2.The content of phenolics were both lower at the 4th day after germination,it was highest at 10th day in B. pekinensis (Lour.) Rupr cotyledon,and was highest at 7th day in R. sativus L cotyledon. The content of phenolics decreased gently in both two leaves after the first leaf appeared at the 7th day.The content of phenolics varied contrarily with the attack level,suggested that the phenolics are the main compounds of chemical defence in the two Cruciferous vegetables.3.The relationships between the physiological and chemical attributes,included the mass,area, thickness,the contents of nitrogen,carbon and phenolics,with the attack index.of both leaf and cotyledon were analyzed.The result shows that,the content of phenolics in B. pekinensis (Lour.)Rupr leaf and cotyledon were significantly negatively correlated with attack index of area at 0.01 level,and the content of nitrogen were significantly positively correlated with the attack index of area at 0.05 level. In R. sativus L. leaf and cotyledon ,the content of phenolics were significantly negatively correlated with attack index of area at 0.05 level,but there was no significant correlation bewteen the content of nitrogen and the attack index of area. Furthermore,the mass and area were significantly negatively correlated with the attack index of mass at 0.01 level.The content of nitrogen were significantly negatively correlated with the content of phenolics at 0.05 level.