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).