Evaluating the combined effects of pretilachlor and UV-B on two Azolla species

• Comparative toxicity of natural and man-made stressor were analysed in Azolla spp.
• Azolla spp. showed appreciable damage caused by pretilachlor and UV-B.
• Inhibitory mechanism showed that pretilachlor is more intense than enhanced UV-B.
• A. microphylla exhibited greater resistance compared to A. pinnata.

The present study assessed the comparative responses of two agronomic species of Azolla (A.microphylla and A. pinnata) exposed to man-made and natural stressors by evaluating biomass accumulation, pigments (chlorophyll a and b and carotenoid contents), photosynthetic activity and nitrogen metabolism. The study was carried out in field where two species of Azolla were cultured and treated with various concentrations (5, 10 and 20 μg ml− 1) of herbicide; pretilachlor [2-chloro-2,6-diethyl-N-(2-propoxyethyl) acetanilide] and enhanced levels (UV-B1: ambient + 2.2 kJ m− 2 day− 1 and UV-B2: ambient + 4.4 kJ m− 2 day− 1) of UV-B, alone as well as in combination. Biomass accumulation, photosynthetic pigments; chlorophyll a, b and carotenoids, photosynthetic oxygen yield and photosynthetic electron transport activities i.e. photosystem II (PS II) and photosystem I (PS I) in both the species declined with the increasing doses of pretilachlor and UV-B radiation, which further declined when applied in combination. The lower doses (5 and 10 μg ml− 1) of pretilachlor and UV-B (UV-B1 and UV-B2) alone, damaged mainly the oxidation side of PS II, whereas higher dose (20 μg ml− 1) of pretilachlor alone and in combination with UV-B1 and UV-B2 caused damage to PS II reaction centre and beyond this towards the reduction side. A significant enhancement in respiration was also noticed in fronds of both the Azolla species following pretilachlor and UV-B treatment, hence indicating strong damaging effect. The nitrate assimilating enzymes – nitrate reductase and nitrite reductase and ammonium assimilating enzymes – glutamine synthetase and glutamate synthase were also severely affected when treated either with pretilachlor and/or UV-B while glutamate dehydrogenase exhibited a stimulatory response. The study suggests that both the species of Azolla showed considerable damage under pretilachlor and UV-B treatments alone, however, in combination the effect was more intense. Further, in comparison to A. pinnata, A. microphylla exhibited greater resistance against tested doses of both the stresses, either alone or in combination.

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