4.2 Grain development in laboratory incubation
The grain weight in vitro cultured incubation was still decreased
largely by 18-38% in the stress treatments (Figure 4). This result
showed the impacts of stresses exposed around flowering persisted when
the sufficient source supply was provided. Through the dynamic of grain
weight, we found the early stress did not shorten grain filling period,
but the mean grain filling rate was reduced by 13-40% (Table 1).
Maize kernel development is mainly a process of carbohydrate metabolism
and starch synthesis (Singletary, Banisadr, & Keeling, 1997). Some
studies demonstrated that lack of assimilates within the embryo or
endosperm limited maize kernel development in drought during grain
filling stage (Westgate et al., 1994). In this study, the soluble sugar
used to starch synthesis in the stress was relatively high (Figure 7a),
suggested that the carbohydrate metabolism was not the main factor
affecting final kernel weight. This was agreement with previous research
in maize under water deficit (Oury, Tardieu, & Turc, 2016; Oury et al.,
2016). However, the starch content of grain was significantly decreased
in the DS, HS and DHS treatment (Figure 7b), indicating the
sugar-to-starch synthesis was interfered in stresses during maize grain
development (Jones et al., 1985; Singletary, Banisadr, & Keeling, 1994;
Zinselmeier, & C, 1999).
Grain weight was reduced because the
starch biosynthesis in endosperm
cell was interfered in stress (Jones, Roessler, & Ouattar, 1985).
Meanwhile, the rate of endosperm cell division was decreased with
reduced starch granules in stress during maize seed development
(Singletary, Banisadr, & Keeling, 1994).
The enzymes activities for starch
biosynthesis (surcose invertase, sucrose synthase, ADP Glc
pyrophosphorylase and starch synthase) played a key role in this process
(Figure 10). First, previous reports showed the sucrose invertase
produced adequate glucose and fructose to allow grain set to proceed
(Ruan, Jin, Yang, Li, & Boyer, 2010). In this study, sucrose invertase
activity in grain was decreased by 30-57% in stress treatments on the
early grain filling stage on 10th DAP for superior
grain (Figure 8a). Second, the sucrose synthase catalysed the cleavage
of sucrose to form UDPG and fructose, and its activity was linked to
sink strength in development grain (Kato, 1995; Yang, Zhang, Wang, Xu,
& Zhu, 2004). The activity of sucrose synthase was decreased by 30-45%
on the 10th DAP for superior grain in stress
treatments (Figure 8b) .Third, the ADP Glc pyrophosphorylase produced
ADPG, which was the primer of the starch chain regarded as the
rate-limiting enzyme in starch biosynthesis (Preiss, 1988; Smith and
Denyer, 1992; Yang, Zhang, Wang, Xu, & Zhu, 2004). The activity of ADP
Glc pyrophosphorylase was decreased by 21-67% on the
10th DAP and by 16-50 % on the 25thDAP for superior grain in the stress treatments compared with CK (Figure
8c). Finally, the previous found the starch synthase activity is
positively correlated with the rate of starch synthesis (Keeling, Bacon,
& Holt, 1993; Nakamura and Yuki, 1992). We also found starch synthase
activity was decreased by 22-34% on the 10th DAP in
stress treatments for superior grain (Figure 8d)
The starch synthesis is not only affected by enzymes activity but also
regulated by hormone content. It was reported that the
content of Z+ZR and IAA in rice
grains was related to the endosperm cell division and starch
biosynthesis (Huang et al., 2016; Zhang et al., 2009). The DS, HS and
DHS around flowering increased ZR content by 11-20% in superior and
inferior grains on 16th and 32th DAP
(Figure 9a). The correlation analysis showed the mean grain filling rate
was negatively correlated with the ZR content in both superior and
inferior grains (Table 2). For IAA content, it was reduced by 20-26% in
stress treatments on the 32th DAP for superior grain
(Figure 9b). The IAA content was positively correlated with grain
filling rate in superior grains at 32th DAP (Table 2).
The IAA content mainly increased grain development in mid-grain filling,
which was consistent with previous study (Yang, Zhang, Wang, Zhu, &
Wang, 2001). In this paper, the ABA and GA3 contents
were also changed by DS, HS and DHS treatment around flowering (Figure
9c, d), however, there was no significant correlation with the mean
grain filling rate (Table 2). The previous researches reported that the
grain filling rate was closely associated with an enhanced ABA level in
water-stressed grains (Yang, Zhang, Wang, Zhu, & Wang, 2001; Yang,
Zhang, Wang, Xu, & Zhu, 2004). In addition, the GAs content was
associated with the development of embryo at early grain filling stage
(Schussler, Brenner, & Brun, 1991).