4.2 Flowering dynamics and yield formation in response to heat
stress
This study clearly showed that high temperature around flowering
disturbed normal flowering pattern and reduced seed set in maize,
agreeing well with previous studies (Cicchino, Rattalino Edreira, &
Otegui, 2010; Rattalino Edreira et al., 2011; Wang et al., 2019). On
average across inbred lines, tasseling, pollen shedding, and silking
time were all advanced by high temperature, but the negative effects on
flowering dynamics were larger for pollen shedding than for silking
(Figure 6 and 7). In this case, anthesis-silking interval (ASI) were
extended, thus disturbing synchrony between pollen shedding and silking
as well as reducing seed set (Rattalino Edreira et al., 2011; Wang et
al., 2019). ASI was proved a key determinant of kernel formation in many
abiotic stresses (Fuad-Hassan, Tardieu, & Turc, 2008). The close
correlations between ASI and kernel number also confirm that ASI
strongly determined kernel formation in heat stress (Figure 8). However,
high temperature between 11-leaf stage and tasseling caused a delay in
flowering events, and the delay was larger in silking than in pollen
shedding (Cicchino, Rattalino Edreira, & Otegui, 2010; Cicchino,
Rattalino Edreira, Uribelarrea, et al., 2010). The inconsistencies in
flowering responses to high temperature probably derived from high
temperature degree, crop growth stage, and genotype used in different
previous studies. High temperature >35 ℃ that is imposed
shortly before silking is seemingly able to advance tasseling and pollen
shedding but delay silking in maize (Lizaso et al., 2018; Wang et al.,
2019). A long-term warmer temperature can accelerate crop development,
shorten vegetative phase in field conditions, and advance flowering
events (Lizaso et al., 2018). There are high temperature thresholds for
different flowering events over which flowering pattern will be greatly
changed (Sanchez et al., 2014). Tasseling, pollen shedding, and silking
responded differently to high temperature in the present study,
indicating they have different high temperature thresholds, but the
relevant information is lacking. Date in each of flowering events varied
largely among maize lines even within the same genetic group, revealing
some lines are sensitive to high temperature in male or female
reproductive organs. Based on this, maize hybrids that are tolerant to
high temperature in both male and female flowers probably can be
selected and bred.
This study also revealed that high temperature that occurs in different
short periods of time around flowering resulted in different effects on
maize yield (Figure 9). Pre-silking high temperature reduces kernel
number as a result of the disturbed flowering pattern (Cicchino,
Rattalino Edreira, & Otegui, 2010; Lizaso et al., 2018), reduced pollen
shedding number, and reduced pollen viability (Wang et al., 2019). By
contrast, post-silking high temperature resulted in more negative
effects on kernel formation, consistent with findings in a
temperature-controlled study (Wang et al., 2020), which indicated that
the first five days post-silking were most sensitive to high temperature
for maize kernel formation. In this short period, silk growth,
male-female crosstalk, pollen germination, pollen tube growth, and
double fertilization successively proceed (Zhou, Juranic, &
Dresselhaus, 2017). The present results indicate that one or all of
these processes are susceptible to high temperature (Dupuis & Dumas,
1990). We found that a short period of high temperature post-silking can
reduce silk emergence rate and inhibit pollen tube growth and hence
reduce maize kernel number (unpublished). Since these reproductive
processes are very complex, evidences concerning high temperature
impacts on them are very rare in maize (Zinn, Tunc-Ozdemir, & Harper,
2010; Thomas & Franklin-Tong, 2013). Besides, high temperature at two
weeks after silking also have significantly negative correlations with
kernel number and yield (Figure 9), indicating high temperature during
early grain filling period can also result in kernel abortion, which was
also found in wheat (Talukder et al., 2014). High temperature in early
grain filling can disturb sugar and starch metabolism (Commuri & Jones,
2001; BarnabÁS, JÄGer, & FehÉR, 2008) and limit assimilate partitioning
to kernels (Carcova, Uribelarrea, Borras, Otegui, & Westgate, 2000) and
hence result in kernel abortion.
Compared to kernel number, kernel weight was less affected by high
temperature during flowering and early grain filling period (Figure 1
and 9, Rattalino Edreira et al., 2011; Wang et al., 2019), but the
effects were also significant for three maize line groups. The reduction
in kernel weight is dependent on both source and sink activities (Zhang
et al., 2017; Janni et al., 2020). High temperature has limited effects
on leaf photosynthesis capacity because maize as a typical
C4 crop has a higher CO2 concentration
in leaf tissue (Crafts-Brandner & Law, 2000; Crafts-Brandner &
Salvucci, 2002). Sinsawat, Leipner, Stamp, & Fracheboud (2004)
indicated that the threshold temperature that can lead to irreversible
effects on leaf photosynthesis was 45 ℃ in maize, and there were no such
high temperature events in our field-grown maize. The accelerated leaf
senescence is supposed to be a larger contributor to kernel weight
reduction under high temperature condition as compared with the reduced
leaf photosynthesis in maize (Tian et al., 2019). In addition, high
temperature during early grain filling can directly reduce sink
activities by reducing endosperm cell number (Nicolas, Gleadow, &
Dalling, 1985) and starch accumulation (Yang, Gu, Ding, Lu, & Lue,
2018). The similar results were also found in wheat (Hurkman et al.,
2003), rice (Yamakawa & Hakata, 2010).