Chlorophyll a Fluorescence as a Probe of Heavy Metal Ion Toxicity in Plants

Chlorophyll (Chi) fluorescence has significantly contributed to our understanding of heavy metal ion stress in plants — providing both the mechanistic details and the extent of damage. Heavy metals inhibit multiple metabolic processes in plants and a variety of Chi a fluorescence measurements, such as steady state emission, ultrafast decay analysis, fast and slow induction have been used to obtain information on the nature of this inhibition including the alterations induced in the structure-function relationship of photosynthetic membranes. Almost all Chi a fluorescence induction parameters (minimal fluorescence (F_o), variable fluorescence (F_v), maximum fluorescence (F_M) and steady-state fluorescence (F_s)) are affected; the extent of variation is dependent on two important experimental conditions — the growth stage of plants at which they are exposed to metal(s) (in this chapter, metals and metal ions have been used interchangeably, although it is the ions that are considered to act) and the duration of metal ion exposure to plants. A significant suppression in decrease from F_M to F_s (Rfd) is attributed to the feed-back inhibition induced due to a slower Calvin-Benson cycle in heavy metal treated plants. In fact, fast and slow Chi a fluorescence induction studies have shown that the Calvin-Benson cycle, and not the thylakoid electron transport chain is the primary target of heavy metal toxicity. Being a non-invasive and rapid probe, Chi fluorescence can potentially be used for the early identification of metal ion toxicity and also in detecting cultivars with various degrees of metal ion tolerance. With advances in the resolution of measured Chi a fluorescence signal, and the development of Chi image analysis providing details at the level of single chloroplast, our understanding of the mechanistic details and the specificity of metal ion action on plants will further be advanced at a rapid rate.