The Role of Nitrate in the Osmoregulation of Lettuce (Lactuca sativaL.) Grown at Different Light Intensities

Blom-Zandstra, M. and Lampe, J. E. M., 1985. The role of nitrate in the osmoregulation of lettuce (Lactuca sativa L.) grown at different light intensities.—J. exp. Bot. 36: 1043–1052. The effect of different light intensities on the nitrate accumulation vis-à-vis the concentration of other solutes in plant sap expressed from lettuce leaves was studied. After growing lettuce plants under constant environmental conditions for 52 d, they were transferred to different light intensities and harvested periodically. A quantitative analysis of components in solution in the expressed plant sap showed a decrease in nitrate concentration and an increase in the organic acids (mainly malate) and sugars (mainly glucose) with increasing light intensity. The light intensity only slightly increased the osmolarity of the expressed plant sap. The measured osmolarity corresponded very well with the value estimated from the quantitative analyses implying that all osmotically active compounds had been accounted for. The decrease in nitrate concentration in the expressed plant sap was fully compensated for by an increase in the dissociated organic acids that partly dissociate twofold to sustain electroneutrality and by an increase in both organic acids and sugars to maintain the osmolarity. The suggestion is supported that nitrate may serve as osmoticum at low light conditions to compensate for the shortage of carbohydrates resulting from suboptimal photosynthesis.