Toxic Effects of Li on Plant Metabolism
Li Toxicity Limits Plant Growth
Optimal plant growth and development is determined by numerous factors such as genetic factors, climatic conditions, soil fertility, and topography. As mentioned above, Li in low concentration is beneficial for plant growth, however, a high concentration of Li results in significant growth and yield reduction (Shahzad et al., 2017). Lithium reduces plant growth and development by altering or disrupting numerous physiological mechanisms in plants (Table 27.3, Shahzad et al., 2016). In tobacco, the Li application significantly reduces plant growth by inducing the development of necrotic spots on leaves and thus reduces total chlorophyll contents (Naranjo et al., 2003). Similar results are also reported in citrus plants (Aral and Vecchio Sadus, 2008). Li-induced growth reduction was also associated with Li-induced disrupted pollen development and altered rhythmic movement of petals (Birch, 1991; Zonia and Tupy, 1995a). In lettuce, a high concentration of Li resulted in a significant reduction in stem diameter, specific leaf area, root and shoot length (da Silva et al., 2019). In brassica, Li toxicity reduced seed germination and reduced chlorophyll contents and phenolic compound production (Li et al., 2009).
The growth of the aboveground plant part strongly depends on the below-ground plant part. Plant roots are the first organ that comes in contact with Li in soil and Li in excess alters root gra- vitropic growth of roots (Mulkey, 2005). Moreover, Li toxicity also induced necrosis in the root meristematic zone and the induction of necrotic spots in Li-treated plants seems to be mediated by ethylene (Naranjo et al., 2003). Tissue turgidity is also very important for plant growth and Magalhaes et al. (1990) reported that Li reduced dry weight of the roots in radish and leaves of watercress due to reduced water retention in roots. Likewise, in Arabidopsis, Li reduced relative water contents in plant roots by reducing either water retention in roots or by reducing water uptake (Duff et al., 2014). The role of aquaporins in response to Li should be examined to underpin the mechanisms behind above-mentioned responses to Li. In wheat, Li reduced net water content and dry weight; however, it was not clear whether tissue desiccation under Li stress was due to reduced water uptake or elevated transpiration rate (Kent, 1941).
Some other reported toxic effects of Li toxicity in plants include abnormal pollen development and inhibits pollen germination (Zonia and Tupy, 1995a), deformation of pollen, inducing symmetrical mitoses in the microspores (Zonia and Tupy, 1995b), inhibition of the rhythmic movements of pulvini and petals (Liskow, 1993; Birch, 2012). Moreover, inhibition of glycogen synthase kinase-3p has been proposed as a possible mechanism for the effect of lithium on development (Klein and Melton, 1996), and the most widely accepted action of lithium is the inhibition of inositol monophosphatases (Hanson, 1991; Berridge, 2009). Moreover, Li toxicity induces the depletion of cellular inositol and inhibits the inositol cycle and calcium signaling (Gillaspy et al., 1995; Saxena et al., 2013). In short, Li reduces plant growth and yield by reducing the development of various agronomical traits and altering numerous physiological processes.