The photosynthetic apparatus ofConocarpus lancifoliusEngl. (Combretaceae) suffers damage in soil contaminated with heavy metals

Abstract

Conocarpus lancifolius Engl. (Combretaceae), a heat tolerant plant, has potential for use in the phytoremediation of polluted soil. We analyzed the physiological changes in C. lancifolius exposed to single and mixed heavy metals (HMs; cadmium, nickel, and lead). For 30 days under controlled growth conditions, we exposed some groups of plants to a single HM at concentrations of 25 or 50 μmol·L–1 and other groups were exposed to 25 μmol·L–1 of a mixture of HMs. Photosynthetic parameters such as electron transport rate, photosynthetic rate, chlorophyll fluorescence, chlorophyll content index, and photosynthetic pigments were measured. Chloroplast morphology was studied by transmission electron microscopy (TEM). In plants exposed to 25 μmol·L–1 of HMs, the photosynthetic parameters were unaffected, whereas at 50 μmol·L–1 of HMs, all parameters significantly decreased up to 20 days of exposure, followed by an increase up to 30 days, indicating a slow adaptation of plants to HM under stress. Compared with the single HMs, mixtures of HMs were more toxic at the same concentration. All parameters indicated damage to the photosynthetic apparatus due to stress from 25 μmol·L–1 of the HM mixtures and 50 μmol·L–1 of the single HMs. TEM analyses showed a dispersion of grana in the chloroplast of the affected C. lancifolius plants.

Synergistic and concentration-dependent toxicity of multiple heavy metals compared with single heavy metals in Conocarpus lancifolius

“…In a study of six HMs in maize, the order of toxicity was established to be Cd > Co > Hg > Mn > Pb > Cr (Ghani 2010 ). We have previously reported the effect of HMs on the C. lancifolius photosynthetic apparatus (Redha et al 2019 ).…”

“…The higher accumulation of highly toxic Pb and Cd shows that C. lancifolius preferentially accumulated these HMs. This is because C. lancifolius has a high growth rate with a large green crown (Redha et al 2019 ). Observations during this study revealed that C. lancifolius also has long hairy roots.…”

“…Soil salinity is a persistent issue in Kuwait and the salinity varies with the geomorphological landscape (Redha et al 2019 ; Bannari et al 2020 ). Soil salinity increases the availability and toxicity of Cd and decreases the soil microbial respiration rate, microbial biomass, and enzyme activity (Raiesi and Sadeghi 2019 ).…”

“…Plants were obtained from the Public Authority for Agriculture and Fish Resources (PAAFR; a government organization in the State of Kuwait). The growth and maintenance conditions of the plants were followed as reported earlier (Redha et al 2019 ). Local sandy soil and peat moss (3:1 v/v; 3.5 kg) were used for plant growth in plastic pots (9 × 6").…”

“…In 2009, we initiated a systematic study on C. lancifolius and discovered that the plant was resistant to drought conditions and high salinity, and it could endure high desert temperatures (Redha et al 2011 , 2019 ). These characteristics, fast growth rate, and its large biomass make this plant a good candidate for phytoremediation studies involving HM-contaminated soil.…”