4. Discussion
Diurnal variation of rice Fv/Fm in this
study was consistent with what reported by Wu et al . (2007) and
Panda et al . (2011) (Fig. 2). The decrease of
Fv/Fmat midday was thought to be photoprotection under high-light intensity
and high-temperature rather than photodamage to PSII (Roden & Ball.
1996; Huang et al ., 2006), resulted
from reversible inactivation of PSII
reaction centers, which was responsible for the midday depression in
photosynthesis (Roden & Ball, 1996;
Panda et al ., 2011). The negative correlation between
Fv/Fm and PPFD and temperature supported
photo-protection theory (Fig. 3).
Elevated [CO2] enhanced PSII thermotolerance showed
by the greater Fv/Fm under high
temperature (Taub et al .,
2000; Pan et al ., 2018). Stepwise increase of
[CO2] (SI) further enhanced high-light and heat
resistance than short-term abrupt increase of [CO2]
(AI160), demonstrated by alleviated midday depression in
Fv/Fm across three growth stages (Table
2; Fig. 2). However, constant increase of [CO2] (CI)
did not show this advantage than short-term abrupt increase of
[CO2] (AI200). Different effects of
the two treatments of elevated [CO2] suggested that
simulations closer to the actual pattern of [CO2]
increment may be more effective in reflecting crop responses to elevated
[CO2]. Consistent with conjecture of Taub et
al . (2000), increased PSII thermotolerance under elevated
[CO2] may be associated with lower initial
(Fo) at midday, while Pan et al . (2018) reported
that elevated [CO2] protected PSII by reducing the
heat stress-induced reactive oxygen species accumulation. The different
results remind us that more mechanism researches should be carried out.
Regardless of different treatment of [CO2] increase,
the midday decrease in Fv/Fm of rice
were 8%, 11% and 7% at jointing,
heading and grain-filling stages, respectively (Table 2), lower than
37% at booting stage reported by Wu et al . (2007). In addition
to the differences in growth periods, it may be related to the different
cultivar of rice used in our study (japonica) and the experiment of Wuet al . (2007) (indica). When it came to the tolerance to high
light, as reported by Li et al . (2002) that the decrease of
Fv/Fm in high-light resistant japonica
rice
cv.
9516 was the least, while the depression in strong-light-sensitive
indica rice cv. shanyou 63 was the most. Even for two photosensitive
indica rice, the Fv/Fm of rice cv. IR42
reduced more than cv. FR13A at midday (Panda et al ., 2001).
Planting high-light and high-temperature tolerated varieties of rice may
help to maintain higher PSII efficiency at noon, thus reducing the
midday downregulation of photosynthesis.
Short-term elevation of [CO2] (one growing season)
enhanced Fv/Fm in japonica rice cv.
Fujiyama-5 (Ziska & Teramura, 1992). Multi-year
[CO2] elevation (four generations) caused higher
Fv/Fm than short-term increase of
[CO2], whether it was a constant increase or a
stepwise increase of [CO2] (Fig. 2), indicating that
long-term increase of [CO2] enhanced beneficial
changes to improve photosynthesis in rice. As reported by X. Li et
al . (2019), multi-generational exposure to elevated
[CO2] could reinforce the response occurred in
short-term exposure, so that long-term response of crop to increasing
[CO2] would not be completely predicted by
short-term response to elevated
[CO2].
The SI treatment, consistent with actual increases pattern of
atmospheric [CO2], was more conducive to strengthen
the advantage mentioned above. It can be indicated by the result that
the higher Fv/Fm occurred under SI than
AI160 across all three growth stages, while
Fv/Fm in CI was only higher than
AI200 at jointing stage (Table 2; Fig. 2). Roden & Ball
(1996) showed that the depression in
Fv/Fm was associated with the reduced
reaction center as reflected by an increase in the minimum fluorescence
(Fo) and the increased levels of nonstructural
carbohydrates. Since the corresponding nonstructural carbohydrates were
not determined, we could not derive the relationship between them, but
the reduced Fo corresponded to the increased
Fv/Fm under SI and CI (Fig. S1),
suggesting that long-term exposure to higher [CO2]
increased rice reaction center, which helps PSII capture more light
quantum for electron transfer.
Significant interactions between [CO2] treatments
and observation time points on Fv/Fm (p = 0.003 at heading and p = 0.087 at jointing),
PIABS ( p = 0.046) and φEo (p = 0.066) suggested that attention should be paid to the
observation time points in studying the effects of increased
[CO2] on PSII efficiency (Table 2). Measurements
that span a long time in a day may amplify or mask treatment effects due
to diurnal variation. Predawn observations of plants that have been
dark-adapted overnight may be more effectively to reflect the effects of
treatments (Kalaji et al ., 2014). Values observed at sunset were
almost completely restored to those in the state of predawn, which can
be used for analysis. In addition to the treatment effects of elevated
[CO2], the observations at noon and afternoon may
also involve the response of plants to high-light and high-temperature,
and the ability to recover.
Contrary to the results of Zhu et al . (2018) that short-term
elevation of [CO2] decreased ψo of
wheat, in this study, the marked increase of ψo under SI
than AI160 at grain filling stage reflected that SI
increased plastoquinone pool, thus reduced the accumulation of QA, along
with the unhindered donor side of PSII and electron transfer chain (Fig.
4; Fig. S2). This attribution was supported by previous studies
revealing that the accumulated
QA- and reduced PQ
pool caused higher fluorescence of OJ-phase (Kalaji et al ., 2011;
Tsimilli-michael, 2019). While, CI did not affect ψo,
speculating that long-term increment of [CO2]
mitigated adverse effects of short-term elevation of
[CO2], and the stepwise increase would be more
beneficial to PSII. On the other hand, this might the difference between
wheat and rice in response to increased [CO2].
The
φEo,
which was the product of ψo and
Fv/Fm, showed stronger correlation with
ψo than Fv/Fm(Table 4), indicating that elevated
[CO2]-induced changes in electron transport had more
to do with the changes in ψo than in
Fv/Fm. The process of transferring
electrons by captured photons in the reaction center may be the
rate-limiting step of the initial photochemistry. The dominant role of
ψo was also reported by Jiang et al . (2008). The
correlation coefficient between PIABS and
Fv/Fmwas the highest, suggesting that Fv/Fmcontributed the biggest fraction to the increase in
PIABS due to elevated [CO2] (Table
4).
The significant decrease in ψo, φEo and
PIABS along with the growth periods indicated that rice
regulate the PSII efficiency according to growth needs, which may lead
to a reduction in photosynthesis (Table 5). The lower attenuation under
CI and SI indicated that long-term increase of [CO2]
slowed down the reduction of PSII efficiency from jointing to grain
filling stage, which may be a strategy to maintain higher
photosynthesis.