Agricultural and Forest Meteorology 106 2001 289–301
Effects of elevated atmospheric CO
2
and drought stress on individual grain filling rates and durations of the
main stem in spring wheat
Aiguo Li
a,∗
, Yuesheng Hou
b
, Anthony Trent
c,1
a
NEINIH, Building 6 Rm 313, 6 Center Dr., MSC 2740, Behesda, MD 20892-2740, USA
b
Weed Science Laboratory, USDA-ARS, Washington State University, Pullman, WA 99164, USA
c
Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow, ID 83844-2339, USA Received 1 May 2000; received in revised form 21 August 2000; accepted 14 September 2000
Abstract
Rate and duration of individual grain growth determine final kernel weight and are influenced by environmental factors. The objectives of this research were to assess the effects of elevated CO
2
and drought stress on the grain filling rate and duration, and the weight of individual kernels. Spring wheat Triticum aestivum L. was grown in a free air CO
2
enrichment FACE system on the demonstration farm at the University of Arizona Maricopa Agricultural Center with a split-block design of four
replications. Mainplots were 550 or 370 mmol mol
− 1
of atmospheric CO
2
concentrations and subplots were two irrigation treatments. The weights of individual kernels from upper, middle, and lower spikelets of the main stem spike were fitted into
nonlinear cumulative logistic curves as a function of accumulated thermal units using SAS proc NLIN. Rate and duration of individual grain filling varied greatly depending on floret positions and environmental factors. The combination of these
changes determined the final weight of individual kernels. The rank order of kernel weights among kernel positions within a middle and lower spikelet was not affected by either elevated CO
2
or water stress treatments in this study. Elevated CO
2
often stimulated the rate of individual grain filling, whereas the well-watered condition extended duration of individual grain filling. Furthermore, kernels further from the rachis or nearest to the rachis were affected proportionately more than those
towards the center of a spikelet. The information from this research will be used to model wheat grain growth as a function of climate. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Elevated CO
2
; Drought stress; Wheat; Grain filling rate; Grain filling duration; Yield
Abbreviations: A, ambient CO
2
concentration; ATU, accumu- lated thermal unit; D, drought stress condition; E, elevated CO
2
concentration; FACE, free air CO
2
enrichment; MS, main stem; W, well-watered condition
∗
Corresponding author. Tel.: +1-301-402-0964; fax: +1-301-402-1883.
E-mail address: liaintra.nei.nih.gov A. Li.
1
Idaho Agricultural Experiment Station Research Paper No. 99722.
1. Introduction
Individual kernel weight, one of the three yield components in wheat, is determined by both rate and
duration of grain filling Wiegand and Cuellar, 1981. Individual kernel weight of mature grains varies
among various positions within a spike, and even
0168-192301 – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 2 2 1 - 5
290 A.G. Li et al. Agricultural and Forest Meteorology 106 2001 289–301
within a spikelet Bremner, 1972. At 9 days after anthesis, the largest kernels are located in the central
spikelet, whereas at maturity, the largest kernels are more towards the base of a spike. Within a spikelet,
when assimilate supply is limited, the first kernel proximal is the largest; when assimilate supply is
sufficient, the second and third kernels exceed the first kernel in weight Bremner and Rawson, 1978;
Simmons and Moss, 1978. Generally, the distal ker- nels fourth and fifth within a spikelet are smaller
than the proximal kernels first and second in size Simmons and Crookston, 1979.
Grain growth in wheat consists of three phases: a lag, a linear, and a plateau phase Wheeler et al., 1996.
During the initial lag phase, which follows anthesis and only lasts for a few days in wheat, the number
of cells per kernel is determined Brocklehurst, 1977 and there is little increase in grain dry weight. Grain
dry weight then increases linearly until a maximum dry weight is achieved. Differences in grain filling
rate or duration during this phase are important in ex- plaining variation in individual final kernel dry weight
Pinthus and Sar-Shalom, 1978; Simmons and Crook- ston, 1979; Gebeyehou et al., 1982. After the linear
phase, grain dry weight remains stable while the grain dries, or declines slightly, depending on the cultivar.
Proximal florets usually reach anthesis 2–4 days ear- lier than distal ones Evans et al., 1972; Simmons and
Crookston, 1979. This behavior can lead to a longer grain filling duration of proximal kernels because the
cessation of grain filling occurs at approximately the same time for all kernels in a spikelet Simmons and
Crookston, 1979. Further, some kernels grow faster than others due to a greater grain filling rate which
could be caused by ample assimilate supply and dif- ferences in kernel growth potential Bremner, 1972;
Bremner and Rawson, 1978. The contribution of grain filling rate and duration to final kernel weight
remains unclear. Darroch and Baker 1990 reported a positive association between the rate of linear grain
filling phase and final grain weight, while Gebeye- hou et al. 1982 found that both rate and duration of
grain filling were positively associated with the final grain weight.
Grain growth has been described by various math- ematical models Simmons and Crookston, 1979;
Wiegand and Cuellar, 1981; Bauer et al., 1985. Bauer et al. 1985 described individual kernel growth with
a cubic polynomial curve and determined that grain filling rates for lag phase, constant phase, and post
linear phase were, respectively, 0.022, 0.049, and 0.019 mg GDD
− 1
kernel
− 1
, whereas others predict grain filling rates in the linear phase of 0.04 and
1.98 mg spike
− 1
day
− 1
over various floret locations and in different varieties Simmons and Crookston,
1979; Wiegand and Cuellar, 1981. Simmons and Crookston 1979 reported that distal kernels exhib-
ited a lower growth rate than proximal kernels during the linear phase of grain filling.
The rate and duration of grain filling are affected by environmental factors. The relationship between
temperature and grain filling rate and duration has been well documented. A higher temperature accel-
erates rate and shortens duration of individual kernel grain filling Sofield et al., 1977, while a lower
temperature prolongs duration of individual kernel grain filling Wiegand and Cuellar, 1981. The op-
timum temperature for individual kernel growth is 12–18
◦
C Chowdhury and Wardlaw, 1978; Wardlaw and Moncur, 1995. On average, grain filling duration
is shortened about 3.3 days
◦
C
− 1
and reduction in ker- nel weight of 3–5 for each 1
◦
C increase above the optimum temperature Wiegand and Cuellar, 1981.
The rate of individual kernel growth did not respond to irradiance in cultivars where kernel number per
spike was affected by radiation, while with those in which kernel number was less affected by radiation,
the rate of individual kernel growth was highly re- sponsive to radiation, especially in the distal kernels
Sofield et al., 1977. The duration of individual ker- nel growth was not influenced by radiation Sofield
et al., 1977. Nitrogen has little effect on the rate of grain growth. If available nitrogen exceeded the
amount needed for the greatest kernel yield, however, the duration decreased Bauer et al., 1985. Informa-
tion on the effect of water on individual kernel growth is limited. No information on the effect of elevated
CO
2
on rate and duration of individual grain filling is available.
The objectives of this research were to: i estimate the weight of individual kernels on the lower, middle,
and upper spikelets; ii estimate the rate and dura- tion of individual grain filling in various positions on
the main stem spike; iii assess the effects of elevated CO
2
and water stress on the rate and duration of indi- vidual kernel growth.
A.G. Li et al. Agricultural and Forest Meteorology 106 2001 289–301 291
2. Materials and methods
