Limitation of photosynthetic carbon metabolism in South African soybean genotypes in response to low night temperatures
Strauss, Abram Johannes
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Soybean (Glycine max (L.) Merr.) is a key source of protein for humans and animals, but is unfortunately chilling sensitive. A single episode of low night temperature (dark chilling) is sufficient to inhibit pod formation in soybean. Genotypes respond differently to dark chilling, with some exhibiting a degree of tolerance, while others are extremely sensitive. Since many soybean-producing regions in South Africa are located at high altitudes with very low daily minimum temperatures, production potential is often limited. Identification of genotypes best suited for cultivation in areas that experience dark chilling episodes could provide parental material for inclusion in breeding programs aimed at developing more dark chilling tolerant genotypes, ensuring higher yields per unit area. The main objectives of this study were to evaluate a large number of South African soybean genotypes for chilling tolerance and to benchmark their response against those of two foreign genotypes of well-known chilling tolerance. Moreover, a detailed physiological and biochemical characterisation of the chilling response of two of these local genotypes of contrasting chilling tolerance was undertaken with emphasis on the contributing role of low soil temperatures towards the inhibition of photosynthesis. Thirty South African soybean genotypes of unknown chilling tolerance and two foreign genotypes of well known, but contrasting chilling tolerance, were grown under controlled growth conditions in growth chambers. Chlorophyll a fluorescence measurements were employed as a rapid screening tool for dark chilling tolerance. Plants were dark chilled (6°C) for seven consecutive nights, but kept at normal day temperatures (26°C). Chlorophyll a fluorescence (0-J-I-P) transients were recorded before the end of each night of dark chilling and analysed by the so-called JIP-test to translate stress-induced alterations in the transients to changes in biophysical parameters quantifying the stepwise energy flow through photosystem ll (PSII). The performance index (PIABs), a multi-parametric expression that combines the three main functional steps taking place in PSII, was used as a measure of dark chilling tolerance. It was successfully demonstrated how the 0 -J-I-P chlorophyll a fluorescence transients could be utilised in screening for dark chilling tolerance in large numbers of soybean genotypes. Elaboration of the PIABs resulted in the formulation of a novel parameter, the so-called chill factor index (CFI), which revealed large differences in dark chilling response among the genotypes. The CFI of the two foreign reference genotypes correlated with their known difference in chilling tolerance. The CFI was used to rank the thirty genotypes according to dark chilling tolerance, and two local genotypes were selected for further detailed studies, namely Highveld Top, representing a chilling tolerant genotype, and P AN809, representing a highly chilling sensitive genotype. Further investigations focused on the effects of low soil temperatures, in combination with low air temperatures (whole plant chilling, WPC), compared to the situation where only low air temperatures, but normal soil temperatures, were experienced (shoot chilling, SC). This was an important consideration because besides the inhibition of photosynthesis by low air temperatures, low soil temperatures could also inhibit symbiotic nitrogen fixation, which may indirectly aggravate effects on photosynthesis. Besides chlorophyll a fluorescence, these detailed studies also involved monitoring of vegetative development with the plastochron index, C02 gas exchange analysis, determination of chlorophyll and ureide content in leaves, and assaying the activities and contents of key photosynthetic enzymes. Although a clear distinction could be made between the overall effects induced by the WPC and SC treatments, there were also similarities, especially regarding initial effects. This included the reduction of stomatal conductance, inhibition of PSII function and inhibition of C02 saturated rates of photosynthesis (Jmax), which were similar in both WPC and SC treatments of PAN809. These initial effects could therefore be ascribed mainly to chilling stress experience by shoots and leaves. However, reduced soil temperatures aggravated the effects of dark chilling, especially in PAN809. When above and below-ground metabolism was taken into consideration, it was clear that continued exposure to dark chilling set into motion a sequence of events in P AN809 leading to severe inhibition of photosynthesis. The inhibition of PSII function in WPC-treated plants of P AN809 was further aggravated when low soil temperatures inhibited symbiotic nitrogen fixation, causing reduced ureide production and export from the nodules to the shoot, leading to a chlorotic phenotype. Similar symptoms were absent in Highveld Top. When leaf ureide content dropped below a certain threshold level ill the WPC-treated plants ofPAN809, additional constraints on photosynthesis developed. As N-limiting conditions developed in these leaves, chloroplastic fructose-1,6-bisphosphatase (cFBPase) was targeted specifically, leading to reduced cFBPase protem content and specific activity. As a result, RuBP regeneration capacity was severely compromised as indicated by decreased RuBP content and further suppression of photosynthetic rates that was observed exclusively ill WPCtreated plants ofPAN809. In conclusion, the study demonstrated that the genetic pool of South African soybean genotypes exhibits a large degree of variation in dark chil1mg tolerance. The genotypes Highveld Top and P AN809 were investigated further, focusing on the effect of sub-optimal soil temperatures on above and below-ground metabolism. These treatments facilitated unravelling of the sequence of events leading to the development of a chlorotic shoot phenotype in PAN809 and additional constraints being imposed on photosynthesis of which selective targeting of cFBPase was a key factor. It was clearly demonstrated that, in a chilling sensitive genotype such as P AN809, the response of photosynthesis to dark chilling depended on both shoot and root-derived factors. The large difference ill the response of the two genotypes to the WPC-treatment made it possible to identify the keysites of dark chilling inhibition.
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