It may be deduced from the above discussion that ALA could be a very promising plant growth regulating compound for increasing plant productivity and enhancing abiotic stress tolerance of crop plants since under certain conditions it has been found to mitigate the deleterious effects of various abiotic stress factors in numerous crop species (Fig. 12.5). The fact that it is the precursor of chlorophyll and all other porphyrins molecules, exogenous application of ALA enhances chlorophyll content of crop plants which translates into increased photosynthetic capacity thus yield. It is also documented that ALA application improves the quality of food crops by promoting the accumulation of antioxidant molecules and sugars and balancing the acidity within the fruits. Different plant species may vary in their responses to exogenous application of ALA and for any given species there is no consensus as to the optimal concentration or application method for ALA to maximize crop productivity under stressful environments. However, even though exogenous application of the lower concentrations of ALA proved to be beneficial in enhancing the plant growth and various other physiological and biochemical characteristics of plants exposed to abiotic stress factors, it should be noted that at higher concentrations, ALA itself may cause a high level of stress in plants acting as a herbicide. The effectiveness of exogenous applications of ALA on enhancing tolerance to various abiotic stress conditions may be resulted from boosted activities of enzymatic or nonenzymatic antioxidant system
providing significant protection to the membranes against harmful ROS within tissues.
The recently introduced PGR, however, still requires a lot of work to be carried out to elucidate the key regulatory points of biosynthesis, mechanism of action, and other specific and collaborative regulatory roles played by ALA since there are still large gaps in current knowledge at both theoretical and practical level. By looking at the results documented above, although it has become clear that ALA enhances the activities of the enzymatic and nonenzymatic antioxidants that scavenge the ROS and stabilizing the cell membranes, the mechanism(s) underlying this improvement remains quite unknown. For example, we have no information whether exogenous application of ALA could compensate for the imbalance in other plant growth substances normally caused by abiotic stress factors, interact and/or being regulated by the cross-talk in harmony with other established phytohormones and PGRs such as ABA or upregulate specific defense mechanisms against the stress factors. Such determinations will require extensive molecular and physiological examination of ALA treated and untreated plants subjected to stress conditions. Also, most of the currently known information comes from laboratory studies, therefore, extensive field studies are required to determine the optimal concentration and application method to exhibit the beneficial effects of ALA application in plant productivity and decreasing the sensitivity of crops species to abiotic stress factors. Additionally, considerable amount of data has accumulated on the effects of ALA applications on plants exposed to chilling and salinity stresses and partially to water stress but there is no information about how ALA application will affect the plants exposed to other abiotic stress factors such as high temperatures, ozone, or toxic metals. Thus, more work is necessary to determine the effectiveness of ALA applications on plants exposed to such stress conditions. The future applications of this PGR holds a great promise as a management tool for enhancing the productivity and protecting our agricultural crops against the aforesaid constrains ultimately aiding to increase potential crop yield in near future.
Acknowledgments This work was supported by a grant (project no: 1070611) from The Scientific and Technical Research Council of Turkey (TUBITAK) and the author is grateful for the financial support. He also thank Dr. Ferit Kocaginar for his critical review of the manuscript and Dr Yasemin Korkmaz for her technical help with the figures.
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