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

The morphological and physiological adaptation of Lactuca sativaL. (‘Vegas’) to different irradiance levels andrates of nitrogen supply was analysed in such a way that effectsof irradiance were clearly distinguished from the effects ofnitrogen. Lettuce was grown in a glasshouse in aerated nutrientsolutions containing all necessary nutrients except nitrogen.Nitrogen was supplied in excess and at limiting rates in relationto plant growth to provide steady state nutrition. Shading theplants created the low irradiance level. The effects of nitrogensupply and irradiance on growth showed a marked interaction.Dry matter production decreased strongly with decreasing nitrogensupply at high irradiance, but decreased only slightly at lowirradiance. Nitrogen had no effect on radiation use efficiencyexcept for the lowest nitrogen treatment at high irradiance.The effect of nitrogen on growth was mainly mediated by itseffect on leaf area development and hence on light interception.Decreases in leaf area were due to decreases in specific leafarea and dry matter partitioning towards the leaves, while thedecrease in specific leaf area was the result of an increasein leaf dry matter percentage at low nitrogen supply. Dry matterand nitrogen partitioning, and nitrate concentration were closelyrelated to plant nitrogen concentration. Irradiance did notaffect these relationships. Irradiance influenced partitioningonly indirectly by affecting plant nitrogen concentration. Thedemand for organic nitrogen per unit leaf area was lower atlower irradiance. Organic nitrogen per unit leaf area appearedto be adjusted to the irradiance level, independently of thenitrogen supply, suggesting priority of nitrogen usage in photosynthesis.Copyright 2000 Annals of Botany Company Lactuca sativa L., lettuce, growth, irradiance, leaf area, nitrogen, radiation use efficiency, partitioning