Regulation of Growth at Steady-state Nitrogen Nutrition in Lettuce (Lactuca sativa L.): Interactive Effects of Nitrogen and Irradiance

Abstract

The morphological and physiological adaptation of Lactuca sativa L. (‘Vegas’) to different irradiance levels and rates of nitrogen supply was analysed in such a way that effects of irradiance were clearly distinguished from the effects of nitrogen. Lettuce was grown in a glasshouse in aerated nutrient solutions containing all necessary nutrients except nitrogen. Nitrogen was supplied in excess and at limiting rates in relation to plant growth to provide steady state nutrition. Shading the plants created the low irradiance level. The effects of nitrogen supply and irradiance on growth showed a marked interaction. Dry matter production decreased strongly with decreasing nitrogen supply at high irradiance, but decreased only slightly at low irradiance. Nitrogen had no effect on radiation use efficiency except for the lowest nitrogen treatment at high irradiance. The effect of nitrogen on growth was mainly mediated by its effect on leaf area development and hence on light interception. Decreases in leaf area were due to decreases in specific leaf area and dry matter partitioning towards the leaves, while the decrease in specific leaf area was the result of an increase in leaf dry matter percentage at low nitrogen supply. Dry matter and nitrogen partitioning, and nitrate concentration were closely related to plant nitrogen concentration. Irradiance did not affect these relationships. Irradiance influenced partitioning only indirectly by affecting plant nitrogen concentration. The demand for organic nitrogen per unit leaf area was lower at lower irradiance. Organic nitrogen per unit leaf area appeared to be adjusted to the irradiance level, independently of the nitrogen supply, suggesting priority of nitrogen usage in photosynthesis.