Effects of salinization on nutrient transport to lettuce leaves: consideration of leaf developmental stage

Most recent reviews of plant salinity response have included the concept of a nutritional disturbance as one likely mechanism by which shoot growth mi ght be inhibited. None the less, few studies of dicotyledonous plants have presented data on nutrient transport into the most intensively growing shoo t 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 co nditions of moderate salinization, or in a growth-stimulating concentration of NaCl. The nutrient deposition was studied from 0.7 to 3.7 d following c ompletion of stepwise salinization. This deposition was followed in immatur e leaves, which had attained only 1-2% of ultimate leaf mass by the complet ion of the study. In such young leaves development is still dominated by ce ll 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 ot her major divalent cationic nutrient, Mg2+, was not decreased for these lea ves. The factors of increase for Na+ and Cl- after 3.7 d after completion o f salinization averaged 152 and 62%, over control levels for the three youn gest leaves (for Nat and Cl-, respectively). Though significant, these incr eases 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 Ki conce ntration were not greater for the youngest than they were for the oldest le aves. 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 si nk 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 m ore developed growing leaves. While Fe concentration was not reduced by sal inization at any developmental stage, Zn2+ net transport and Zn concentrati on were both reduced in the two youngest leaves (57 and 70%, respectively). Given the moderate treatment imposed (Na:Ca ratio of 22) the results sugge st that Ca2+ transport to the youngest leaves is probably highly sensitive to salinization of the root medium and is perhaps a key physiological respo nse in the inhibition of leaf growth.