Effects of Potassium Deficiency on Cotyledon Photosynthesis and Seedling Development in Cucumis sativus L.

Seedlings of Cucumis sativus L. grew much less vigorously whencultured in potassium-deficient nutrient solution than in fullnutrients. The poorer growth was attributed to reduced CO₂ fixationby the cotyledons, which form the bulk of the photosyntheticsurface at this stage, and to a much lower level of export ofphotosynthetic products from the cotyledons. The magnitude ofthese effects increased as the seedlings aged and they werea major factor in the poorer development of the plumule in potassium-deficientplants. Electron microscopic examination of the cotyledons shows thatthe lower ¹⁴CO₂ fixation in potassiumstarved seedlings is associatedwith poorly-defined granal stacks and a proliferation of intergranalthylakoids. In plants grown in full nutrients the incorporationof radiocarbon into a cell wall fraction and into other insolublecomponents of the cell increased with cotyledon age whereasin potassium-deficient seedlings there was an increase in thepercentage of the total radiocarbon in the soluble fraction.This suggests either that the incorporation of photosyntheticproducts into cellular components was inhibited or that theability of the cotyledon to translocate photosynthetic productsto the rest of the seedling was reduced in plants grown in potassium-deficientconditions.