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).