Effectiveness of titanium treatment on photosynthesis and production in crop plants under stress conditions

In this review, we will discuss and evaluate the literature on how titanium (Ti) can limit the adverse effects of various biotic and abiotic stressors through different mechanisms involving morphological, physiological, and genetic changes. Photosynthesis is the main physiological process in plants profoundly affected by stress factors. Several studies have shown that Ti reduces the effects of stress by regulating the antioxidant defense system, thereby protecting the photosynthetic apparatus and maintaining photosynthetic efficiency. Photosynthesis-related processes including PSII quantum yield and electron transport rate were improved in stressed plants treated with Ti and the ruptured and missing thylakoid membranes and decreased starch granules were restored. Ti-treated plants showed increased expression of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit (rbcS) and Rubisco large subunit (rbcL) mRNA. Thus, one of the molecular mechanisms for Ti's action could be that ionic Ti treatment increases chlorophyll pigment content and electron transport rate, and elevates expression of Rubisco mRNA and protein, leading to greater activity, improved carboxylation and photosynthetic carbon reactions. Expression of the CaL Rubisco, CaS Rubisco, and Ca Chl a/b binding protein genes, the light harvesting-related genes including light-harvesting complex II (LHCII) b and LHCII II content on the thylakoid membranes were also significantly increased by Ti addition. Thus Ti appears to promote photosynthesis by enhancing effective light absorption by chloroplasts, protecting thylakoid membranes from ROS damage, regulating antioxidant systems, accelerating distribution of light energy from PSI to PSII through increasing LHCII, and increasing the conversion of light energy to electrical energy, water photolysis, and oxygen evolution. In this review we examined the current literature on Ti activity, especially as it affects photosynthesis, and identified areas where additional work was needed.