Energy metabolism of Plantago lanceolata as dependent on the supply of mineral nutrients

Plantago lanceolata L., a grassland species from a relatively nutrient‐poor habitat, was grown in nutrient‐rich and in nutrient‐poor culture solutions. Half of the plants were trensferred from high to low or from low to high nutrient conditions. Shoot growth was immediately reduced upon transfer to low nutrient conditions, whilst it reacted more slowly upon transfer of plants to high nutrient conditions. Root growth was less dependent on the supply of nutrients, but it was slightly reduced upon transfer of plants to high nutrient conditions. Photosynthesis was largely independent of the nutrient supply, apart from an initial increase upon transfer of plants to low nutrient conditions. Photosynthesis decreased with age in all treatments, and this decrease was not due to mutual shading. The decrease of photosynthetic rate was not accompanied by a decreased relative growth rate: it was compensated by a more efficient root respiration, since the activity of the alternative nonphosphorylating pathway continuously decreased in plants grown in a high nutrient environment. It is concluded that the alternative pathway was of significance in removal of carbohydrates, which could not be utilized for growth, energy production, etc., due to a temporary or structural imbalance between assimilate production and requirement. The alternative pathway also appeared to allow P. lanceolata plants to adapt to a changed environment as regards mineral nutrition. The experimental value for root growth respiration of P. lanceolata grown under high nutrient conditions was compared with a theoretical value, calculated from the biochemical composition of plant dry matter and the known energy costings for biosynthetic and transport processes. A good correlation between the experimental and theoretical value of root growth respiration was found if it was assumed that ion uptake required c. 1.0 molecule of ATP per ion per membrane passage.