1 Introduction
NASA GISS showed that the average global temperature has increased by
about 0.8 ℃ since 1880 (Carlowicz, 2010), and two-thirds of the warming
has occurred since 1975 at a rate of about 0.20 ℃ per decade
(Masson-Delmotte et al., 2018). The increased temperature and extreme
heat stress are continuously restricting agricultural production and
threating global food and feed security (Shiferaw, Prasanna, Hellin, &
Bänziger, 2011; Rojas-Downing, Nejadhashemi, Harrigan, & Woznicki,
2017; Khaliq, Iqbal, & Zafar, 2019;). As one of most important stable
crops, maize (Zea mays L.) is planted over a wider range of
altitudes and latitudes than other food crops and under different
temperatures ranging from cool to hot. In each region, heat stress may
occur at different stages during maize growth, and the occurrences of
extreme heat frequently overlap with reproductive growth stage (Hedhly,
Hormaza, & Herrero, 2009), hence reducing yield tremendously (Ferris,
Ellis, Wheeler, & Hadley, 1998; Wang et al., 2019)
Heat stress can impact maize yield formation at different growth stages,
out of which flowering is considered the most susceptible stage
(Cicchino, Rattalino Edreira, & Otegui, 2010; Siebers et al., 2017).
Temperature beyond 30.5 ℃ advances tasseling and pollen shedding
(Schoper, Lambert, Vasilas, & Westgate, 1987; Sanchez, Rasmussen, &
Porter, 2014) but has smaller effects on silking dynamics, hence
extending anthesis – silking interval (ASI) (Rattalino Edreira, Budakli
Carpici, Sammarro, & Otegui, 2011; Lizaso et al., 2018; Wang et al.,
2019). Pollen shedding number dramatically reduces in > 36
℃ condition due to the failed anther dehiscence (Wang et al., 2019), and
pollen viability is greatly reduced by temperature > 38 ℃
because of the disturbed pollen structure and components in maize
(Herrero & Johnson, 1980; Muchow & Carberry, 1989; Sanchez et al.,
2014). In the study of Cicchino, Rattalino Edreira, Uribelarrea, &
Otegui (2010), nonheated fresh pollens were supplied to heated silked
ears and remained at heat condition for 15 days, and kernel number
reduced by more than 60%. A cross pollination test (i.e., normal fresh
pollens and normal silks were crossed pollinated and were transferred to
heat stress) showed that pollination and fertilization processes were
also sensitive to high temperature in maize (Wang et al., 2019).
Post-silking heat stress even in a short episode likely resulted in
kernel abortion and reduced kernel number which implied that early seed
development was sensitive to heat stress in maize (Sehgal et al., 2018).
Pollen tube growth, fertilization, and the subsequent growth and
development of the embryo and endosperm can be disrupted by
post-pollination high temperature in rice (Oryza sativa L., Shi
et al., 2017), sorghum (Sorghum bicolor L., Chiluwal et al.,
2020), and wheat (Triticum aestivum L., Jagadish, 2020). The relevant
evidences, however, are still limited in maize, and the mechanisms
remain to be tested (Smith, 2019).
Compared to kernel number, the extent to which high temperature reduces
kernel weight was much smaller during flowering in maize (Wilhelm,
Mullen, Keeling, & Singletary, 1999; Suwa et al., 2010; Wang et al.,
2019). Kernel weight in 14-day high temperature 40/30 ℃ (day/night) that
bracket the silking stage was at the same level with in 32/22 ℃ (Wang et
al., 2019). However, a 6-day continuous 35 ℃ temperature that begun 5 d
after pollination reduced kernel weight by 95% for maize inbred lines
B73 (Commuri & Jones, 2001), suggesting early grain filling period is
sensitive to high temperature. Many studies indicated that heat stress
at early grain filling increases the protein content and reduces the
starch content in grains (Thitisaksakul, Jiménez, Arias, & Beckles,
2012; Mayer, Savin, & Maddonni, 2016; Yang, Gu, Ding, Lu, & Lu, 2018).
In the study of Ben-Asher, Garcia, & Hoogenboom (2008), the net
photosynthetic rate (Pn) in maize was reduced by 50 –
60% in high temperature of 35/30 – 40/35 ℃ compared to 25/20 – 30/25
℃. The insufficient photosynthates accumulation for grain filling
resulted from the decreased enzyme activities and relative transcript
levels of genes (Duke & Doehlert, 1996). Additionally, heat stress at
early grain filling in wheat accelerated grain filling rate but
shortened the duration; the former did not compensate for loss of the
latter, thereby resulting in a small grain size and low kernel weight
(Farooq, Bramley, Palta, & Siddique, 2011). The relevant studies of
heat stress occurring after pollination in maize, however, are limited,
and are still unable to fully reveal the mechanism of its influence on
kernel weight.
It has been confirmed that maize hybrids with different genetic
backgrounds responded differently to heat stress around flowering (Suwa
et al., 2010; Rattalino Edreira et al., 2011; Rattalino Edreira &
Otegui, 2012, 2013; Rattalino Edreira, Mayer, & Otegui, 2014). Tropical
maize hybrids conferred a higher capacity for enduring heat effects than
temperate maize when forming kernels by synchronizing anthesis and
silking time (Rattalino Edreira et al., 2011). Evidences also indicated
that tropical maize lines showed a broader adaption to
multi-environmental conditions especially for the lines that derived
from lowland sites (Jiang et al., 1999; Abadassi & Herve, 2000).
Although tropical maize contains many ecotypes with interesting
adaptation traits, this germplasm has some undesirable traits such as
late maturity, excessive plant or ear height, and low harvest index
(Abadassi & Herve, 2000; White, Vincent, Moose, & Below, 2012;
Edmeades, Trevisan, Prasanna, & Campos, 2017). Hence, tropical
germplasm was incorporated into temperate lines to achieve high yield
potential, adaptability and stability of maize (Lafitte & Edmeades,
1997; Lewis & Goodman, 2003; Wolde, Keno, Tadesse, Bogale, & Abebe,
2018). Mushayi, Shimelis, Derera, Shayanowako, & Mathew (2020) compared
117 maize hybrids at five location in South Africa and found that
hybrids derived from temperate × tropical inbred lines exhibited a
higher yield and broader adaption compared to the hybrids developed from
either temperate or tropical germplasms. These results clearly reveal
the important implication of germplasm when selecting of and breeding
for superior hybrids for broad and specific adaptation to the target
environments. However, responses of different maize germplasms (i.e.,
temperate, tropical, and temperate × tropical) to high temperature or
extreme heat events have not widely evaluated and compared so far in
terms of flowering dynamics and kernel formation.
Therefore, 162 maize inbred lines including 40 temperate lines, 45
tropical lines, and 77 temperate × tropical inbred lines were sown at
different sowing dates in four experimental years (i) to evaluate their
responses to heat stress around flowering and during the early grain
filling period; and (ii) to explain the underlying mechanisms from the
perspective of flowering characteristic.