Kinetin and spermine mediated induction of salt tolerance in wheat plants: Leaf area, photosynthesis and chloroplast ultrastructure of flag leaf at ear emergence

The antagonistic effects of kinetin and spermine on stress imposed by seawater on leaf area, pigment, Hill reaction, 14CO2 fixation and chloroplast ultrastructure of wheat flag leaf were investigated. Irrigation of wheat plants by seawater at 25% caused marked decrease in leaf area, pigment content, Hill activity and photosynthetic efficiency of wheat flag leaf at ear emergence. Grain priming with kinetin, spermine or their interaction alleviated the adverse effect of seawater stress by stimulating leaf area expansion, pigment production as well as photosynthetic activity. From transmission electron microscopy micrographs, a continuous “end-to-end” distribution of regular (oval or elliptical) chloroplasts around the cell's periphery was observed in flag leaf mesophyll cells of control wheat plants. Conversely for seawater-stressed plants, the irregular spherical chloroplasts appeared “bulbous” and discrete, the cells also displayed extensive but thin peripheral cytoplasmic regions devoid of chloroplasts. Grain presoaking in 0.1 mM kinetin caused the chloroplast of stressed wheat plants to be more regular, with organized membrane system, large starch grains and projections in the form of tails. Furthermore, ultrastructure analysis cleared that grain priming with spermine, either alone or in combination with kinetin, caused the chloroplast in flag leaf mesophyll cells of stressed wheat plants to be more regular in shape with more starch grains. The changes in pigment content and photosynthetic activity of flag leaf appeared to depend mainly on chloroplast ultrastructure and its numbers, showing a positive correlation between chloroplasts number and pigment content.