Glycine betaine (GB) is an effective compatible solute that improves the tolerance in plants to various stresses. We investigated the effects of 2 mM GB applied to the roots of a tobacco (Nicotiana tabacum L.) cultivar on enhancing photosynthesis under low-temperature (LT) stress (5/5 °C, 12/12 h, 300 µmol m-2 s-1) and in the subsequent recovery (25/18 °C) from the stress. The net photosynthetic rate, intrinsic efficiency measured as the ratio of variable to maximum fluorescence, and actual efficiency of the photochemistry of photosystem 2 as well as the ATPase activity in the thylakoid membrane decreased, and a distinct K step in the fluorescence transient O-J-I-P appeared under cold stress. Exogenous GB alleviated the decrease in all these parameters. The LT-stress induced the accumulation of 33-66 kDa polypeptides and decreased the proportion of unsaturated fatty acids in the thylakoid membrane. In plants subjected to LT-stress, GB protected these polypeptides from damage and enhanced the proportion of unsaturated fatty acids. An increase in non-radiative energy dissipation (NPQ) may be involved in the improvement of the function of the thylakoid membrane by GB since exogenous GB protected violaxanthin de-epoxidase and enhanced NPQ. and C. Wang ... [et al.].
Experiments were performed to distinguish some of the proposed mechanisms by which thylakoid membranes regulate the performance of photosynthetic apparatus in relation to non-photochemical quenching, qN. Aliphatic diamines were used as uncouplers of transmembrane H+ gradient as they can be transported across the membrane at the expense of hydrogen cations. Diamines did not induce changes in low-temperature fluorescence emission but induced different changes in membrane ultrastructure. Positively charged peptides did not affect membrane ultrastructure but blocked qN. In addition, they caused an increase of low temperature fluorescence emission between 710 and 720 nm. For control peptide, the maximal fluorescence increase was found at 715 nm. Fragments of light-harvesting complex 2 in their phosphorylated and non-phosphorylated form shifted the position of this increase. We believe that peptides bind to membrane surface and reduce the mobility of membrane components whose migration is needed for observation of qN. Phosphorylated and non-phosphophorylated LHC2 fragments bind to different binding sites for corresponding forms of the protein. and D. Štys ... [et al.].