Germination of cocklebur seed and growth of their axial and cotyledonary tissues in response to C2H4, CO2 and/or O2 under water stress
Abstract. Germination modes of lower seeds of cocklebur (Xanthium pennsylvanicum Wallr.) under different water stresses, prepared with mannitol solution, were examined in relation to gaseous factors. As the concentration of mannitol increased, germination was increasingly inhibited at a mode which was drawn by two straight lines having different slopes and meeting at an angle. One is a sharp line occurring at the lower concentrations of mannitol; the other is a gentle line occurring at higher concentrations of mannitol. The former reflected the growth response of axial tissues to mild water stress, whereas the latter reflected the growth response of cotyledonary tissues to severe water stress. The germination potential of cocklebur seeds increased with increasing temperature. Thus, the seeds were more resistant to water stress at higher than al lower temperatures. This increased germination potential under water stress resulted from the greater growth potential of axial tissues, but not cotyledonary tissues, at higher temperature. Increased O2 levels improved both the reduced axial and cotyledonary growth under water stress. Carbon dioxide predominantly enhanced axial growth under water stress, whereas C2H4 exclusively enhanced cotyledonary growth. Thus, these gases were effective in potentiating germination under water stress. When combined with each other, these gases caused more pronounced growth of the axial and cotyledonary tissues, leading to germination under more severe water stresses. Maximal axial and cotyledonary growth under water stress occurred in the simultaneous presence of CO2, C2H4 and O2, which allowed the germination at higher mannitol concentrations above 0.6 kmol m−3 From these results, it was suggested that cocklebur seeds would override water stress by depending upon both the Corresponding axial growth and the C2H4-responding cotyledonary growth.