3.3 Photosynthetic traits
The Pn of K. humilis and P. annua decreased significantly
with the drought stress (84.16% and 87.36%, respectively, under the D2
treatment) and combined drought and heat stress increase (51.71% and
60.95%, respectively, under the D2H2 treatment)
(P <0.05). After the drought and high temperature
stress, the Pn was maintained at a low level of 0.52–0.81
μmol·m-2·s-1 and a relatively high
level of 4.70–4.84 μmol·m-2·s-1,
respectively. The Pn of O.
ochrocephala and S. pulchra decreased significantly under both
the drought and the combined drought and heat stress
(P <0.05). The Pn of S. pulchra first increased
significantly (12.16% under the H1 treatment) and then decreased
significantly (71.19% under the H2 treatment) (P <0.05)
with the increase of the high temperature stress; the Pn of O.
ochrocephala changed in a similar manner(Figure 3A).
The Tr of K. humilis and P. annua decreased significantly
under drought stress (63.10% and 69.20%, respectively, under the D2
treatment). The Tr of the same species decreased significantly under the
H1 treatment (46.53% and 24.37%, respectively) and increased under the
H2 treatment (20.80% and 25.55%, respectively). The Tr characteristics
of O. ochrocephala under different stress conditions were
consistent with those of S. pulchra; the drought and high
temperature stress increase resulted in a significant Tr decrease
(69.87% and 69.10%, respectively, under the D2 treatment; 63.95% and
64.40%, respectively, under the H2 treatment)
(P <0.05), as this remained at a relatively low level of
4.70–4.84 μmol·m-2·s-1. An increase
in the compound drought and heat stress, initially resulted in a
decrease and then in an increase in the Tr, which reached a significant
level in S. pulchra (P <0.05) (Figure 3B).
The Gs variation of K. humilis and P. annua was the same
under the three treatments, as it initially increased and then decreased
with the stress increase. The Gs decrease of K. humiliswas higher than its Gs increase (P <0.05), while the
opposite was true for the Gs of P. annua . The Gs variation ofO. ochrocephala was the same under the three treatments, as it
initially increased and then decreased with the stress increase; Gs was
more sensitive under the D1, H1, and D1H1 treatments (it increased by
95.88%, 288.00%, and 112.93%, respectively)
(P <0.05). The Gs of S. pulchra showed a
significantly decreasing trend under different treatments (68.79%,
78.44%, and 78.63% under the D2, H2, and D2H2 treatments,
respectively) (P <0.05) (Figure 3C).
The Ci of K. humilis showed a decreasing trend under the
stress treatments and it decreased significantly (by 49.30%) under the
D2H2 treatment (P <0.05). The Ci of P. annuafirst decreased and then increased with the stress increase. The Ci in
this species decreased more under the D1 and H1 treatments (26.36% and
24.57%, respectively) than under the D1H1 treatment (8.58%)
(P <0.05). The Ci of O. ochrocephala decreased
significantly under high temperature stress (20.11% and 46.29% under
the H1 and H22 treatment, respectively) (P <0.05), and
maintained a higher level of 330.72 μmol/mol under drought stress. The
Ci of S. pulchra did not change in a consistent manner as a
response to the stress treatments. Under compound stress, the Ci ofS. pulchra decreased significantly (21.45% under the D1H1
treatment) (P <0.05) (Figure 3D).