4.1 The reduced grain weight in field
Drought and heat stress are two main factors adversely affecting crop
productivity. In the present study, the grain weight was decreased by
6-7% in DS treatment, 5-6% in HS treatment and 8-11% in DHS treatment
under field condition (Figure 4), which was consistent with the previous
reports (Jones, Roessler, & Ouattar, 1985; Westgate et al., 1994).
Multiple underlying mechanisms may contribute to the reduced grain
weight in stresses in field. From a source (leaf) perspective, the
drought and heat stress accelerated leaf senescence and decreased
photosynthetic capacity (Hlaváová et al., 2018; Zhou, Zhou, He, Zhou, &
Zhou, 2019). In this study, we found the leaf area and SPAD were
decreased by the early stress at the middle-late growth stage (from 99
DAS to 123 DAS) (Figure 6a, b). The net photosynthesis rate was
decreased by 16% in DS treatment, 13% in HT treatment and 40% in DHS
treatment on DAS 123 (Figure 6c). The inhibited leaf photosynthesis
mainly resulted from the decrease of maximum photosynthetic electron
transport rate and carboxylation efficiency (Wang et al., 2015).
Furthermore, grain weight was reduced because the grain filling period
was short in stresses (Chen et al., 2013; Mayer, Rattalino, & Maddonni,
2014; Plaut, Butow, Blumenthal, & Wrigley, 2004). Grain filling
duration of superior grains was decreased by 4.7 days in DS treatment,
4.6 days in HS treatment and 2.2 days in DHS treatment at fast-increase
period (Table 1). The results above showed that DS, HS and DHS treatment
around flowering could shorten grain filling duration in superior grain,
without sufficient increases in grain filling rate to compensate,
resulted in grain weight reduction, which was inconsistent with previous
researches (Jones, Roessler, & Ouattar, 1985; Yang et al., 2006). Some
studies also showed that the drought and heat stress shortened the grain
filling period, but the grain weight could be compensated by faster
grain filling rate, enhanced pre-stored carbohydrate remobilization, and
accelerated endosperm cell division (Jones, Roessler, & Ouattar, 1985;
Yang & Zhang, 2006).