A.J. Strauss

The role of low soil temperature in the inhibition of growth and PSII function during dark chilling in soybean genotypes of contrasting tolerance

Dark chilling affects growth and yield of warm-climate crops such as soybean [Glycine max (L.) Merr.]. Several studies have investigated chilling-stress effects on photosynthesis and other aspects of metabolism, but none have compared effects of whole-plant chilling (WPC; shoots and roots) with that of aboveground chilling in legumes. This is important because low root temperatures might induce additional constraints, such as inhibition of N(2) fixation, thereby aggravating chilling-stress symptoms. Effects of dark chilling on PSII, shoot growth, leaf ureide content and photosynthetic capacity were studied in two soybean genotypes, Highveld Top (chilling tolerant) and PAN809 (chilling sensitive), in experiments comparing effects of WPC with that of shoot chilling (SC). Both treatments inhibited shoot growth in PAN809 but not Highveld Top. Also, WPC in PAN809 caused a decrease in leaf ureide content followed by severe chlorosis and alterations in O-J-I-P fluorescence-rise kinetics, distinct from SC. A noteworthy difference was the appearance of a Delta K peak in the O-J-I-P fluorescence rise in response to WPC. These genotypic and treatment differences also reflected in the degree of inhibition of CO(2) assimilation rates. The appearance of a Delta K peak, coupled with growth inhibition, reduced ureide content, chlorosis and lower CO(2) assimilation rates, provides mechanistic information about how WPC might have aggravated chilling-stress symptoms in PAN809. We introduce a model explaining how chilling soil temperatures might trigger N-limitation in sensitive genotypes and how characteristic changes in O-J-I-P fluorescence-rise kinetics are linked to changes in carbon and nitrogen metabolism.

Regulation of Respiration and the Oxygen Diffusion Barrier in Soybean Protect Symbiotic Nitrogen Fixation from Chilling-Induced Inhibition and Shoots from Premature Senescence

Symbiotic nitrogen fixation is sensitive to dark chilling (7°C–15°C)-induced inhibition in soybean (Glycine max). To characterize the mechanisms that cause the stress-induced loss of nodule function, we examined nodule structure, carbon-nitrogen interactions, and respiration in two soybean genotypes that differ in chilling sensitivity: PAN809 (PAN), which is chilling sensitive, and Highveld Top (HT), which is more chilling resistant. Nodule numbers were unaffected by dark chilling, as was the abundance of the nitrogenase and leghemoglobin proteins. However, dark chilling decreased nodule respiration rates, nitrogenase activities, and NifH and NifK mRNAs and increased nodule starch, sucrose, and glucose in both genotypes. Ureide and fructose contents decreased only in PAN nodules. While the chilling-induced decreases in nodule respiration persisted in PAN even after return to optimal temperatures, respiration started to recover in HT by the end of the chilling period. The area of the intercellular spaces in the nodule cortex and infected zone was greatly decreased in HT after three nights of chilling, an acclimatory response that was absent from PAN. These data show that HT nodules are able to regulate both respiration and the area of the intercellular spaces during chilling and in this way control the oxygen diffusion barrier, which is a key component of the nodule stress response. We conclude that chilling-induced loss of symbiotic nitrogen fixation in PAN is caused by the inhibition of respiration coupled to the failure to regulate the oxygen diffusion barrier effectively. The resultant limitations on nitrogen availability contribute to the greater chilling-induced inhibition of photosynthesis in PAN than in HT.

Open-top chamber facility to study air pollution impacts in South Africa. Part II: SO2–drought interactions on yield, photosynthesis and symbiotic nitrogen fixation in soybean

South Africa has an extremely energy-intensive economy, resulting in substantial air pollution through its coal-fired power stations. Modelled sulphur dioxide (SO2) concentrations on the central Highveld mostly range between 10 and 50 ppb, exceeding 50 ppb in source areas. Well-watered and drought-stressed soybean (Glycine max) plants were exposed to different SO2 concentrations in open-top chambers to study the physiology of SO2 injury by measuring in parallel growth, biomass accumulation, photosynthetic gas exchange (as a function of internal CO2 concentration and photon flux density), chlorophyll a fluorescence, in vitro Rubisco activity and symbiotic nitrogen fixation. A strong concentration-dependent SO2-induced inhibition was displayed in all variables. After fumigation for only 7 d, photosynthesis was reduced without any accompanying visual injury symptoms, even at the 50 ppb treatment level. Exposure to SO2 also resulted in large decreases in biomass accumulation of both well-watered and drought stressed plants. Seed yield reduction of up to 57% occurred in plants exposed to the highest SO2 concentration and simultaneously subjected to drought stress. Root nodule ureide content was lowered at all treatment levels, but was lowered more in the SO2-treated plants subjected to drought stress. The photosynthetic gas exchange data showed a severe decrease in carboxylation and quantum efficiency pointing at increasing mesophyll limitation. The chlorophyll a fluorescence data, pointing at impaired electron transport and formation of end electron acceptors as well as the in vitro activity of Rubisco, 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.

Photosynthetic differences between Microcystis aeruginosa and Oscillatoria simplicissima in relation to species succession in the Vaal River, South Africa

In 1991 Oscillatoria simplicissima replaced Microcystis aeruginosa in the Vaal River, South Africa. This study explored the reason for this succession by looking at the photosynthetic performance of these two cyanoprokaryotes at different temperatures, light intensities and N:P ratios. Microcystis aeruginosa showed higher growth rates and chlorophyll a concentrations than O. simplicissima for most treatments, except the N-deficient treatments. It seems that M. aeruginosa can maintain growth under a range of different conditions and can keep forming mass occurrences, but has high light requirements for photosynthesis and maintenance and can tolerate a much higher light intensity without experiencing photo-inhibition. Oscillatoria simplicissima, on the other hand, is able to harvest available light more efficiently at 25 °C and 15 μmol m2 s-1, experiences less damage or loss at antenna level and retains more of its electron transport capabilities than Microcystis, giving it a competitive edge over M. aeruginosa under these environmental conditions.

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.

Changes in O-J-I-P Fluorescence Rise Kinetics During Dark Chilling Provide Insight into Genotype-Specific Effects on Photosynthesis and N2 Fixation in Soybean

Dark chilling affects growth and yield of warm-climate crops such as soybean [Glycine max (L.) Merr.]. Several studies have investigated chilling stress effects on photosynthesis and otheraspects of metabolism, but none have compared effects of whole plant chilling (shoots and roots) with that of aboveground chilling in legumes. Thisis important since low root temperatures might induce additional constraints, such as inhibition of N2 fixation, thereby aggravating chilling stress symptoms. Effects of dark chilling on photosystem II, shoot growth, leaf ureide content and photosyntheticcapaCity were studied in two soybean genotypes, Highveld Top (chilling tolerant) and PAN809 (chilling sensitive), in experiments comparing effects of whole plant chilling (WPC) with that of shoot chilling (SC). Both treatments inhibited shoot growth in PAN809, but not Highveld Top. Also, WPC in PAN809 caused a decrease in leaf ureide content followed by severe chlorosis and alterations in O-J-I-P fluorescence rise kinetics, distinct from SC. A noteworthy difference was the appearance of a ?K-peak in the O-J-I-P fluorescence rise in response to WPC. These genotypic and treatment differences also reflected in thedegree of inhibition of CO2 assimilation rates. The appearance of a ΔK-peak, coupled with growth inhibition, reduced ureide content, chlorosis and lower CO2 assimilation rates, provide novel mechanistic information about how WPC might have aggravated chilling stress symptoms in PAN809.