Biomonitoring of HgCl2 with Dracaena sanderiana under Two Different Irradiance Levels: Effects on the Photosynthetic Apparatus

Abstract

Heavy metals such as Hg are phytotoxic. Interference of heavy metals with photosynthesis has been a topic of intensive research during the last few decades. Some plants can be used in biomonitoring and the remediation of such pollutants. It is important to assess the biomonitoring potential in ornamental plants as they do not enter into the food chain. This experiment was conducted to examine the effect of HgCl2 on the photosynthetic apparatus in Dracaena sanderiana to assess its potential to use in biomonitoring. D. sanderiana cuttings were subjected to heavy metal stress (100 μM HgCl2), along with control (distilled water) under two contrasting irradiance levels (High light-: 22,500±2,000 lux and Low light-: 750±200 lux). OJIP chlorophyll fluorescence measurements were conducted using a fluorometer (FluorPen, FP-110). The measurements were recorded four times a day (9 am, 12 noon, 3 and 6 pm) continuously for four days after imposing the treatments. Specific energy fluxes per reaction center, flux ratios and Performance Index (PI) were recorded. There was a significant reduction in PI, maximum quantum yield of primary photochemistry (φPo) and quantum yield of electron transport (φEo) with the time in HgCl2 treatment compared to the control. This indicates the negative effects of Hg+ on the photosystem II. However, the visual rating of the foliage quality was at a satisfactory level. Moreover, there was a significant increase/progress/growth in effective antenna size (ABS/RC) and maximum quantum yield of nonphotochemical de-excitation (φDo) in HgCl2 with time. Except in electron transport efficiency (ψo), a clear unimodal circadian variation was observed in all OJIP parameters under high light condition. Photosynthetic apparatus of D. sanderiana is affected by the Hg+. However, the plant was able to cope up with the heavy metal stress during the experimental period indicating that, it can successfully be employed in short-term biomonitoring of mercury. In the experiment, the temperature effect could not be eliminated, thus future experiments for studying that might be crucial.