P.D.R. Van Heerden

Effect of Trifluoroacetate, a Persistent Degradation Product of Fluorinated Hydrocarbons, on C3 and C4 Crop Plants

TFA is a pollutant generated in the atmosphere by the degradation of hydrofluorocarbons (HFCs), as well as other anthropogenic sources such as teflon-cookware. The aim of this study was to quantify the physiological and biochemical basis of the inhibition of photosynthesis imposed by trifluoroacetate (TFA) in Phaseolus vulgaris and Zea mays, crop plants representing C3 and C4 photosynthesis respectively. Photosynthetic gas exchange and fast phase fluorescence kinetics were measured in parallel over a 12-day period in plants cultivated in a hydroponic system under strictly controlled conditions. Although initial stimulation of some photosynthetic parameters was observed at low TFA concentrations, marked inhibition occurred at higher concentrations. The inhibitory effect was reflected by chlorophyll a fluorescence and photosynthetic gas exchange derived parameters. Analysis of photosynthetic gas exchange revealed that besides constraints on mesophyll processes such as Rubisco activity and RuBP regeneration capaCity, stomatal limitation also increased with increasing TFA concentration, especially in P. vulgaris. Although the mechanism of inhibition was similar in both species, the C4 plants were affected more severely. In depth analysis of the fast phase fluorescence transients pointed at TFA-induced uncoupling of the oxygen evolving complex (OEC) and inhibition of electron transport beyond QA including constraints on ferredoxin- NADP+ oxidoreductase (FNR)-function.

SO2-Drought Interaction on Crop Yield, Photosynthesis and Symbiotic Nitrogen Fixation in Soybean (Glycine Max)

South Africa has an energy-intensive economy, resulting in substantial SO2 pollution. Well watered and drought stressed soybean (Glycine max) were exposed to SO2 in open-top chambers to study the physiology of SO2 injury by measuring in parallel: growth, biomass accumulation, photosynthetic gas exchange, chlorophyll a fluorescence, in vitro Rubisco activity and symbiotic nitrogen fixation. A strong concentration dependent SO2-induced inhibition was displayed in all parameters. A reduction in photosynthesis occurred without any accompanying visual injury symptoms. SO2 exposure also resulted in large reductions in biomass and seed yield and root nodule ureide content. The chlorophyll a fluorescence data, pointing at impaired electron transport and formation of end electron acceptors as well as the in vitro Rubisco activity, supported the gas exchange data. Inhibition of photosynthesis, proved to be the main constraint imposed by SO2. SO2-stress was aggravated by simultaneous drought stress.

Inhibition of Symbiotic Nitrogen Fixation by Dark Chilling in Soybean

Legume crops are an important source of protein, oil and secondary metabolites and they are also used as a natural nitrogen source in agriculture, particularly in Africa, because of presence of nitrogen-fixing bacteria in specialized organs called “nodules”. The symbiotic association between the plant and nitrogen-fixing bacteria in nodule formation has been the subject of intensive study but much less information is available on the mechanisms that cause the breakdown of symbiosis particularly during stress. Although the symptoms and progression of nodule senescence has been described, much remains to be discovered regarding the mechanisms that trigger the end of symbiosis and the genes and proteins that underpin nodule senescence (Puppo et al., 2005).