Results
The shoot phenotypes of the SL synthesis mutants, rms1-2 andrms5-3, and the SL signalling mutants, rms3-1 andrms4-1 were monitored for 32 days after germination. The SL
mutant shoots were visibly similar to wild type after 7 days of growth
(Figure 1). The shoot height was similar in all lines but therms4-1 mutants were significantly more branched than the wild
type under both CO2 conditions (P = 0.0002; Figure 2).
After 14 days (Figure 3), the mutants were significantly shorter and
were significantly more branched than the wild type but there were no
significant differences arising from growth [CO2].
Since there were no significant genotype X environment interactions, all
genotypes responded similarly to eCO2.
After 28 days, elevated atmospheric [CO2]
significantly increased shoot fresh and dry weight (P = 0.0177 and P =
0.0111 respectively) by 21% and 23% respectively. However, there were
no significant genotypic differences in fresh or dry weight between the
lines (Figure 4 A, B). Elevated atmospheric [CO2]
significantly decreased the fresh to dry weight ratio in wild type
plants compared to those grown in air (P = 0.0212), but had no effect in
any of the SL mutants (Figure 4 C). However, the fresh to dry weight
ratio of the SL mutants was significantly higher than the WT plants.
At this time point, the SL synthesis mutants were visibly more branched
than the wild type (Figure 5). Growth under high CO2 for
32 days increased stem height (P < 0.0001; Figure 6 A) and
shoot branching (P = 0.0044; Figure 6 B) similarly in all of the lines.
However, the shoots of the wild type peas were 68% taller with a 65%
reduction in total branching compared to all of the SL mutants under
both ambient and high CO2 conditions. The shoot dry
weights were significantly higher in all the lines grown under high
[CO2] compared to those grown in air for 32 days (P
< 0.0001; Figure 7). However, all of the genotypes showed a
similar CO2 –dependent increase in shoot dry weight
(Figure 7). Thus elevated [CO2] effects on stem
extension and branching were independent of SLs.
Although growth under high CO2 had no effect on aphid
fecundity, there were significantly more aphids on the SL mutants
(rms3-1 and rms4-1 grouped) than the wild type pea plants
under both CO2 conditions (P = 0.0286; Figure 8).
Phytohormone levels were determined in aphid-infested shoots grown under
either ambient or elevated [CO2]. Relatively few
[CO2]-dependent changes in phytohormone levels were
observed. Growth under elevated [CO2] tended to
increase the JA levels, but this effect was only significant in therms3-1 plants (Figure 9). In contrast, trans -Zeatin was
decreased in the rms3-1 plants compared to the wild type andrms4-1 plants. Growth under elevated [CO2]
tended to decrease the levels of gibberellic acid in all the lines
(Figure 9). A linear regression analysis indicated that
18% of the variability in aphid
fecundity can be explained by the differences in gibberellic acid
concentrations (P = 0.0181; Figure 10).