Although there were no differences from the previous study
in the root dry weight (Kawasaki et al., 2015), plants grown at
0.4 or 2% O2 in the present study were affected by hypoxia treatment
(Table 1). This difference in the root dry weight depended
on the duration of hypoxia treatment; namely, hypoxia treatment
was conducted only in the light period in the previous study but
throughout the entire day in the present study. Thus, root respiration
might be inhibited by hypoxic conditions, and root dry
weight was decreased. However, in previous studies, decrease in
the leaf area and nitrogen content was observed under hypoxia
treatment only in the light period. Takeda et al. (1978) also showed
that leaf expansion was reduced by hypoxia treatment only in the
light period and concluded that some processes in the light reaction,
except photorespiration, were inhibited by hypoxia treatment.
Identifying the process inhibited by hypoxia treatment in the light
period awaits further study.
4.2. Reduction of A and electron transport
The A and chlorophyll content of plants grown at 0.4 or 2% O2
was significantly reduced or showed a tendency to decrease compared
with that of plants grown at 21% O2 (Fig. 4). This result was
different from that of short-term exposure to hypoxia. We propose
that the main cause of the A reaction under hypoxia was reduced
respiration and decreased nitrogen content, as mentioned above.
There was high positive correlation between nitrogen and chlorophyll
contents (Nakano et al., 1997); thus, it was considered that
the decrease of chlorophyll content at 0.4 and 2% O2 was caused by
the decrease of nitrogen content. Generally, respiration rates saturate
if the atmospheric O2 is above 5% for whole organs (Taiz and
Zeiger, 2010). Thus, the respiration of plants grown at 7 or 10% O2
should not be affected by hypoxia treatment. From these results,
we inferred that respiration was not inhibited in 7% O2 condition
and, conversely, that photorespiration was inhibited and resulted
in increased A.
We now consider the possibility of inhibition of electron transport
under hypoxic conditions. The PSII of plants grown at 0.4
or 2% O2 was reduced similarly to A (Fig. 4). Given that PSII
is calculated from qP and Fv/Fm, the decrease of PSII under
hypoxia was caused by the reduction of qP (Fig. 5). This reduction
corresponds to the inhibition of electron transport after PSII.
We did not identify the process after PSII that was inhibited by
hypoxia treatment, but we suspected that respiration and photorespiration
were inhibited under hypoxic conditions such as at
0.4 and 2% O2, and that this inhibition affected electron transport.
The role of photorespiration remains unclear, but Powles and
Osmond (1979) suggested that photorespiration is one means of
effectively dissipating excess photochemical energy. Kozaki and
Takeba (1996) showed that photorespiration drives electron transport
and protects plants from photooxidation. Therefore, it might
be that one of the causes of the A reduction at 0.4 and 2% O2
was the inhibition of electron transport followed by inhibition of
photorespiration.