| dc.description.abstract | 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. | |
