Lazof, D.B., Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599-3290, United States
Bernstein, N., Inst. Soils, Water and Environ. Sci., The Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel

Most recent reviews of plant salinity response have included the concept of a nutritional disturbance as one likely mechanism by which shoot growth might be inhibited. None the less, few studies of dicotyledonous plants have presented data on nutrient transport into the most intensively growing shoot tissues. In this paper net nutrient deposition was followed for 3 d in 8 sequential, growing leaves of Lactuca sativa, which were grown either in conditions of moderate salinization, or in a growth-stimulating concentration of NaCl. The nutrient deposition was studied from 0.7 to 3.7 d following completion of stepwise salinization. This deposition was followed in immature leaves, which had attained only 1-2% of ultimate leaf mass by the completion of the study. In such young leaves development is still dominated by cell division. The transport of Ca2+ specifically to the youngest leaves was reduced by more than twice as much as was K+ transport. Transport of the other major divalent cationic nutrient, Mg2+2, was not decreased for these leaves. The factors of increase for Na+ and Cl- after 3.7 d after completion of salinization averaged 152 and 62% over control levels for the three youngest leaves (for Na+ and Cl-, respectively). Though significant, these increases were only 27 and 14% as great as increases in three leaf sets of more developed growing leaves. Decreases in net K+ deposition and leaf K+ concentration were not greater for the youngest than they were for the oldest leaves. Net S deposition was reduced 44% more in younger than older growing leaves, but for most leaves not beyond the level expected due to reduced sink strength. The reduction in net P deposition also seemed more related to reduced sink strength, but was reduced to approx. 50% in both younger and more developed growing leaves. While Fe concentration was not reduced by salinization at any developmental stage, Zn2+ net transport and Zn concentration were both reduced in the two youngest leaves (57 and 70%, respectively). Given the moderate treatment imposed (Na: Ca ratio of 22) the results suggest that Ca2+ transport to the youngest leaves is probably highly sensitive to salinization of the root medium and is perhaps a key physiological response in the inhibition of leaf growth.