The Effect of Putrescine on the Apoplast Ultrastructure in the Leaf Mesophyll of Mesembryanthemum crystallinum under Salinity Stress

Abstract

The effects of NaCl, putrescine (Put), and the combination of two agents on the contents of free polyamines (PA), peroxidase activity, and the ultrastructure of the mesophyll apoplast were studied in the third leaf pair of Mesembryanthemum crystallinum L. plants. The NaCl solution was added to soil daily for three days (100 ml of the 100 mM solution), and plants were sprayed with the 1 mM Put solution twice per day for a six-day period. The accumulation of Put and especially spermidine (Spd) by day 3 of salinization was followed by a dramatic drop in Put and Spd contents by day 6. In contrast, the activities of soluble and ion-bound peroxidases increased following a long lag-period. Treatments with Put and NaCl plus Put considerably enhanced this rise in two peroxidase activities. An electron microscopic examination of cell walls in the control and stressed plants demonstrated that a gap developed at the middle lamella with “pockets” filled with amorphous polysaccharides (AP), presumably pectins. At the maximum gap width, the pockets fused with the intercellular spaces, and, in this case, the intercellular spaces also contained AP. Following salinization, AP in the apoplast swelled and expanded. Apparently the genetic determination of high AP content in M. crystallinum plants is the basis for the ability of juvenile plants to bind Na+ and Cl– ions and excess PA and also to accumulate water. In the plants treated with NaCl plus Put, the number of pockets and their volume increased, and the surface of some cell walls became plated with suberin, thus providing an additional barrier for ion transport from the pockets and intercellular spaces into cells. The formation of suberin plates was correlated with the high activity of ion-bound peroxidase essential for suberin deposition. The authors presume that H2O2 results from PA oxidation and, in its turn, induces the activity of peroxidase involved in the suberin plate formation.