net photosynthesis in stresses
To identify the source limit to grain weight by the early stresses, the leaf area, relative chlorophyll content (SPAD) and net photosynthesis rate of ear leaves was investigated in field in Exp. 1 (Figure 6). Because of the stresses since 63 DAS, leaf area decreased on 66 DAS in both DS and DHS treatments (Figure 6a). When the stresses were relieved, leaf area recovered for DS treatment on 75 DAS. However, leaf area was consistently lower in stress treatments compared with CK since 89 DAS. The large decrease for leaf area in stress treatments was observed in the middle-late grain filling stage. On 123 DAS (near physiology maturity), the leaf area was decreased by 33% in DS, 33% in HS and 36% in DHS.
The SPAD value was also decreased by the early stress at the middle-late growth stage (from 99 DAS to 123 DAS) (Figure 6b). On 123 DAS, the SPAD value was decreased by 30% in DS, 38 % in HS and 40% in DHS. The net photosynthesis rate was significantly reduced when stresses occurred on 69 DAS, but it recovered on 75 DAS when stress was eliminated in both DS and HS treatments (Figure 6c). The net photosynthesis rate continued to be lower in DHS treatment than CK after stress episodes. On 123 DAS, net photosynthesis rate was decreased by 16% in DS, 13% in HS and 40% in DHS. Furthermore, we found the reduction in leaf area, SPAD value, and net photosynthesis rate on 123 DAS compared with CK was in line with the decrease in 1000-kernel weight at physiological maturity (Figure 6d, e, f).