Constraints on photosynthesis of C3 and C4 crop plants by trichloroacetic acid, an atmogenically generated pollutant

Abstract

This study examined the physiological and biochemical bases of the inhibition of photosynthesis by trichloroacetic acid (TCA) in Phaseolus vulgaris L. and Zea mays L., representing dicotyledonous and mono-cotyledonous plants with C3 and C4 photosynthetic pathways respectively. TCA is a phytotoxic compound transformed in the atmosphere from chloro-organic substances (C2-CHCs) present in solvents and cleaning agents used in the metal and textile industry, as well as in combustion products released from coal-fired power stations and biomass burning. Photosynthetic gas exchange and fast chlorophyll fluorescence kinetics were measured in 3-week-old plants exposed for 3 days to TCA concentrations ranging from 0.05–3.20g TCA-Na m− 2. Photosynthetic inhibition by TCA was observed in both species, this being greater at the highest level of TCA supplied in Z. mays (85% decrease) than in P. vulgaris (37% decrease), though at low levels of TCA supplied some photosynthetic stimulation was also observed in P. vulgaris. Analysis of Rubisco activity revealed that TCA decreased the activation state of the enzyme in both species, especially in Z. mays, a possible consequence also of a measured decrease in stomatal conductance. However, the decline in net CO2 assimilation rate at saturating CO2 levels (Jmax) was also indicative of a decrease in RuBP regeneration capacity. This ostensible mesophyll limitation was supported by chlorophyll fluorescence measurements, which indicated a disruption of electron transport in photosystem II (PS II) reaction centres requisite for effective RuBP regeneration. It is concluded that increased emission of C2-CHCs by industry and veld-fire catastrophes pose a threat particular to grain production by monocotyledonous crops especially in southern Africa which is experiencing greater food demands from a rapidly expanding population.