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a ExSeed Genetics L.L.C., 1573 Food Sciences Building, Iowa State University, Ames, IA 50011-2062 USA
b Pioneer Hi-Bred International, Inc., P.O. Box 1004, Johnston, IA 50131-1004 USA
c Dep. of Agronomy, Iowa State Univ., Ames, IA 50011 USA
edwilhelm{at}hotmail.com
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Abbreviations: AGPase, adenosine diphosphate-glucose pyrophosphorylase • DAP, days after pollination • HU, heat units • PAR, photosynthetically active radiation • SSS, soluble starch synthase
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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10% in the U.S. Corn Belt (Thompson, 1966). In a field study of maize, Muchow (1990) did not see a yield loss associated with high temperature during grain filling. Muchow (1990) found that during five growing seasons in Northern Australia, yield was unaffected by temperatures, which ranged from 25.4 to 31.6°C during the period from pollination to 80% maximum grain size. A growth chamber study by Badu-Apraku et al. (1983) shows a more dramatic yield loss associated with high temperatures during the period of grain filling . They observed a 42% loss in grain weight per plant when day/night temperature from 18 d post-silking to maturity was increased from 25/15 to 35/15°C, a 6°C rise in average daily temperature. The interaction of heat stress with other environmental factors in the field, such as drought stress, makes it difficult to study the effect of high temperature on maize yield in isolation. Furthermore, it may be difficult to separate the effects of heat stress occurring during grain filling from a previously occurring heat stress. The use of controlled environments makes it possible to study more precisely how high temperature treatment affects maize grain filling. However, controlled-environment studies should strive to mirror conditions in the field as closely as possible. As described below, conditions of root temperature and photyosynthetically active radiation (PAR) intensity can differ between the field and controlled environment and have been shown to be important factors that help determine how plants respond to high temperature.
Roots growing under a canopy of maize plants in the field are well buffered from extremes in air temperature during grain filling (J.K. Radke, 1995, personal communication). The buffering capacity of the soil to extremes in air temperature is well illustrated by Radke's average daily maximum/minimum temperature recordings during the month of August in 1992 and 1994 at the canopy level (29.3/13.9°C) and at soil depths of 5 cm (24.7/17.8°C), 15 cm (21.8/19.3°C), and 50 cm (19.6/19.2°C). In contrast, the root temperature of plants grown in pots will reflect the ambient temperature unless efforts are made to control soil temperature independently of temperature experienced by aerial portions of the plant. Furthermore, roots have been shown to be highly sensitive to high temperature (e.g., >25°C), especially compared with plant shoots. Kuroyanagi and Paulsen (1988) studied the effects of four root/shoot temperatures (25/25, 35/25, 25/35, and 35/35°C) during grain filling on dry weight accumulation of developing wheat (Triticum aestivum L.) grains. Grain dry weight of plants with roots held at 35°C was dramatically reduced when compared with plants with roots at 25°C. However, in plants with roots at the lower temperature, a 10°C shift in shoot temperature had a smaller effect on grain growth. Roots of tepary bean (Phaseolus acutifolius Gray) and common bean (Phaseolus vulgaris L.) were also found to be more sensitive to high temperature than plant shoots (Udomprasert et al., 1995).
Plants exposed to the low PAR intensities occurring under growth chamber conditions are less tolerant of high temperature stress. Wardlaw (1970) demonstrated that low PAR intensity during grain filling of wheat significantly reduced grain dry weight, with the effect most pronounced at high temperature. Spiertz (1977) exposed wheat plants grown in a growth chamber to 12 combinations of temperature and PAR following anthesis. The study showed that low PAR intensities decreased grain growth by a greater amount under high temperature than under normal temperature. For the aforementioned reasons, root temperatures and PAR intensity comparable to the field should be incorporated into an experimental design to portray realistic effects of high temperature stress on maize plants.
The mechanisms in cereals that are affected by heat stress and limit kernel growth are not well understood. Mature kernel dry weight is determined by the product of the rate and duration of grain growth, both of which are influenced by temperature (Tashiro and Wardlaw, 1989). Generally, in lower temperature ranges (10–25°C), cereals respond to increasing temperature with an increase in the rate of grain filling per unit of time. At temperatures greater than a critical maximum (25–35°C), the gain in the rate of grain filling begins to diminish, and at supra-optimal temperatures (40–45°C) the grain-filling rate drops precipitously. Rising temperatures also produce a progressive decline in grain-filling duration. Hence, at high temperatures, yield losses are the result of a loss in both grain-filling rate and duration.
Grain-filling duration may be determined by a number of factors including sucrose availability to the kernel (Afuakwa et al., 1984) and activity levels of enzymes involved in sugar and starch metabolism in the kernel (Singletary et al., 1994). Similarly, the rate of grain filling may be affected by sucrose concentration in the kernel (Jenner, 1970) and activity levels of enzymes in the pathway of starch biosynthesis (Jenner et al., 1993; Keeling et al., 1993, 1994).
The effects of temperature on starch biosynthesis in the kernel have received much attention because starch accounts for most of the dry weight in cereal grains. In wheat grains, losses in starch accumulation caused by heat stress are believed to be linked to a reduction in the activity of soluble starch synthase (SSS) (Hawker and Jenner, 1993; Jenner et al., 1993; Keeling et al., 1993). The mechanisms limiting starch synthesis in chronically heat-stressed maize kernels are not so well understood. Singletary et al. (1994) studied 13 enzymes of sugar and starch metabolism in maize kernels grown in vitro and exposed to a range of chronic heat stresses. The activities of ADPglucose pyrophosphorylase (AGPase) and SSS were reduced the most, and their activities were prematurely terminated compared with other enzymes. The authors concluded that reductions in starch synthesis under heat stress are closely tied to the duration of activity for these enzymes. Keeling et al. (1994) assayed 11 enzymes of starch synthesis extracted from kernels exposed to a short-term (3 h) high temperature stress in vitro. The activity of SSS was reduced most by high temperature and reached a maximal rate at 25°C. Other enzymes (with the exception of branching enzyme) increased in activity up to a temperature of 45°C. Reductions in the rate of SSS were similar to losses in the rate of starch synthesis caused by heat stress.
The objective of this study was to extend our understanding of the effects of chronic high temperature during grain filling on kernel growth, composition, and starch metabolism in maize plants. Unlike previous controlled-environment studies, we independently regulated root zone temperatures and PAR to simulate field conditions. To do this, plants were greenhouse-grown during summer and early fall for a high PAR level, and root zone temperature was maintained at day/night temperatures of 25/20°C for both temperature treatments. Seven inbred lines, representing different heterotic groups, were selected for the study.
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Materials and methods |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Analyses of variance were conducted for each variable of response in a split-plot experimental design. The main plot was a greenhouse divided by a glass wall into a control temperature room and a high temperate room. Temperature treatments (two) were assigned to the main plots, and inbreds (seven) were assigned to subplots. Subplots consisted of three plants per inbred, and analyses of variance were determined using average values of the three plants. Plants within each temperature treatment were randomly placed and rerandomized weekly. The main plots were replicated across three planting dates in 1994: 6 May, 22 June, and 27 July. A total of 123 plants were used in this study.
Kernel Sampling and Analyses
Kernels were collected from maize ears at three dates during grain filling determined by the accumulation of HU following pollination (Table 1)
. The daily accumulation of HU was calculated with the following formula:
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8 h into the light cycle, a strip of husk was peeled from the proximal end of the ear to expose two rows of kernels, and 10 to 15 kernels were removed from a single row in the central portion of the ear. Kernels were immediately frozen in liquid N and stored at -80°C. Ears were sprayed with 0.440 g L-1 Benlate 50W (methyl 1-[(butylamino)carbonyl)]-1H-benzimidazol-2-ylcarbamate(CA)) fungicide to prevent fungal growth, and the husk was returned to its original place and secured to the ear with rubber bands. The rate and duration of grain filling was calculated according to Johnson and Tanner (1972). Rate was calculated on the basis of HU (mg kernel-1 HU-1) and on the basis of time (mg kernel-1 d-1). Duration was expressed as total HU accumulated and as total days.
Density of mature grains was measured with a multi-pycnometer, model MVP-1 (Quantachrome Corp., Syosset, NY). Protein, oil, and starch concentrations of mature kernels were determined with an Instalab 600 NIR product analyzer, model 15299-1810V5 (Dickey-John Corp., Auburn, IL).
Enzyme Extraction and Assay
All chemicals and enzymes were obtained from Sigma Chemical Co., St. Louis, MO. The radiochemical adenosine diphospho-D-[U-14C]Glc (1.06 x 1013 Bq mol-1) was obtained from Amersham Corporation, Arlington Heights, IL. All tissue used in enzyme assays was from kernels collected at the second harvest (250 postpollination accumulated HU). After removing the pericarp and embryo from frozen kernels with forceps, the endosperm was lyophilized, finely ground in liquid N with a mortar and pestle, and returned to -80°C.
The enzyme extract was prepared by adding 100 mg of endosperm tissue to 2 mL of extraction buffer [50 mM Hepes-NaOH (pH 7.5), 5 mM MgCl2, 1 mg mL-1 bovine serum albumin, 1 mM dithiothreitol], and homogenizing at 25000 rpm for 20 s (PowerGen 700, Omni International, Warrenton, VA). The homogenate was centrifuged (30000 x g, 20 min., 4°C) and the supernatant saved for assay of soluble enzymes. The activities of ATP-dependent fructokinase, UTP-dependent fructokinase, glucokinase, and sucrose synthase were measured in dialyzed extracts, with dialysis conducted overnight in extraction buffer at 4°C.
Assays used to measure ATP-linked fructokinase, UTP-linked fructokinase, glucokinase, sucrose synthase, PPi-linked phosphofructokinase, ATP-linked phosphofructokinase, and AGPase were described by Singletary and Below (1990), with the modification of 40 mM UDP for the sucrose synthase reaction mix. Phosphoglucomutase, phosphoglucoisomerase, and UDPglucose pyrophosphorylase were assayed by the procedures of Doehlert et al. (1988) using 100 mM Hepes (pH 7.7), 1 mM UDP-Glc, 50 µM Glc-1,6-bisphosphate, and 2.5 U mL-1 of the coupling enzymes. With the exception of sucrose synthase, these enzyme activities were measured by mixing reactions in a microtiter plate and monitoring the change in absorbance at 340 nm with a BIO-TEK Instruments (Winooski, VT) Model EL 340 automated microplate reader. Substrate-independent background rates were measured and subtracted from the complete assay to obtain the enzyme activity. To minimize variability between triplicate assays run in microtiter wells, Tween-20 (2 x 10-5 mL mL-1 final concentration) was included in reaction mixtures.
The activity of SSS was determined using 50 µL of undiluted extract as part of a 200-µL reaction mix [100 mM Bicine (pH 8.3), 500 mM sodium citrate, 5 mM EDTA (ethylenediaminetetraacetic acid), 10 mM glutathione, 10 mg mL-1 rabbit liver glycogen, and 1 mM ADP-Glc (14C), 220 disintegrations min-1 nmol-1]. Radiolabeled ADP-Glc was added to start the reaction. The reaction was stopped at 10 min by boiling for 2 min. Unreacted ADP-Glc [14C] was separated from radiolabeled glucan by passing the reaction mix through OH- ion exchange resin columns as described by Jenner et al. (1995). Radioactivity was measured using a Beckman (Fullerton, CA) model LS 5000TD scintillation counter. Background counts were determined by running the assay with boiled extract.
Sucrose synthase and SSS assays were performed in duplicate. Enzyme activities of both temperature treatments were measured at 25°C and reported. For the high temperature treatment, values were also reported after adjustment with the temperature coefficient (Q10) of respective enzymes to estimate the in vivo catalytic activity in heat-stressed kernels. The Q10 adjustment was made according to the following formula (adapted from Hochachka and Somero, 1973):
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Results and discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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0.09) and inbred (P 0.04) for all grain quality and growth parameters measured (Table 3)
. This verifies that the contrasting temperatures and germplasm chosen for this study produced a notable impact on maize seed growth and development. In contrast to temperature and inbred effects, tests for significance of the inbred x temperature interaction revealed P values >0.10, except for grain-fill rate (Table 3). Hellewell et al. (1996) also noted that kernel growth of oat (Avena sativa L.) cultivars responded the same to contrasting temperature regimes. Nevertheless, the absence of a genotype x temperature interaction in our study and in that by Hellewell et al. (1996) is more the exception than the rule. Cultivars of wheat (Wardlaw et al., 1989a, 1989b; Wardlaw and Moncur, 1995), barley (Hordeum vulgare L.; Savin et al., 1996), and rice (Oryza sativa L.; Yoshida and Hara, 1977) commonly display differences in seed growth under conditions of supra-optimal grain fill temperatures as opposed to normal temperatures. The same has also been reported for maize. Lu et al. (1996) exposed developing maize kernels to high temperature stress independently of the rest of the plant and found a striking difference in growth response of different inbreds. We believe that in making genetic comparisons related to kernel heat-stress tolerance it is important to recognize that temperature stress applied in different fashions can produce disparate degrees of genotype sensitivity. For instance, maize kernels of eight inbreds displayed similar levels of growth susceptibility to high temperature when grown in vitro (Singletary and Banisadr, 1992), but when plants of the same inbreds were raised in growth chambers and heat stressed during grain filling (25 vs. 35°C; root temperature not controlled) genotypic differences in kernel heat-stress tolerance were much more pronounced. Hence, in view of mixed results that are reported for genotypic heat-stress tolerance, it is important to emphasize that the results of our study do not rule out the possibility that developing kernels of some maize genotypes may tolerate high temperatures better than others. Nevertheless, because the inbreds we chose represented several heterotic groups and maturities, and because of the difference in the way root temperature was controlled in our work vs. other indoor studies with maize (see discussion below), we are led to conclude that the effect of heat stress on developing kernels of maize probably does not differ widely among elite inbreds.
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The parallel decreases in starch, protein, and oil contents caused by chronic heat treatments suggest that losses in kernel dry weight may be produced by one or more mechanisms that broadly affect whole kernel growth. Assimilate transport could be one such mechanism (Wardlaw et al., 1995). The photosynthetic system is another mechanism that may be affected by high temperature during grain filling. For example, Harding et al. (1990) demonstrated that high temperature during reproductive development in wheat seems to affect kernel growth rate and duration by initially accelerating thylakoid component breakdown. Mechanisms of transport and photoassimilation must be considered in studies of heat stress, but perhaps equally important is the effect of high temperature on starch biosynthesis. Numerous studies involving maize (Jones et al., 1984; Keeling et al., 1994; Singletary et al., 1994), wheat (Bhullar and Jenner, 1985), and barley (MacLeod and Duffus, 1988) have shown a negative effect of heat stress on starch deposition in the kernel. A similar series of data showing a negative relationship of protein and oil syntheses to high grain-fill temperature does not exist. We speculate that the negative influence of heat stress on kernel growth in cereals relates to perturbation of starch biosynthesis, but heat stress can also affect the syntheses of other storage products through an indirect effect via interconnected metabolism. It has been reported that coordinated transcriptional regulation between starch and protein syntheses occurs in maize seed (Giroux et al., 1994) and potato (Solanum tuberosum L.) tubers (Muller-Rober et al., 1992), which could account for the like response of starch and protein contents to heat stress. Similarly, the ratio of maize endosperm to embryo weight within a specific genotype remains relatively constant regardless of kernel size (Paddick and Sprague, 1939). Because starch is the primary constituent of the endosperm and oil is found mainly in the embryo, it is possible that a drag on starch biosynthesis could concomitantly restrict development of scutellar cells or diminish the deposition of oil in the kernel.
Enzyme Analysis
A survey of enzymes in the pathway of sugar and starch metabolism was performed to determine if the effect of heat stress on kernel growth was associated with altered rates of enzymatic activity. Endosperms were collected (250 postpollination accumulated HU) during the period when kernel growth rate was also being measured and, hence, entailed a stage of temperature-induced growth impairment. Enzyme activity in crude extracts was measured at 25°C for kernels of both temperature treatments. On this basis, high temperature had little effect on most of the enzymes studied, but significantly reduced the endosperm activities of AGPase, glucokinase, sucrose synthase, and SSS (Table 6)
. Although enzyme activities were measured at 25°C, enzymes in kernels growing at the elevated temperature presumably would have functioned at increased catalytic rates, based upon the temperature coefficient (i.e., Q10). To obtain a better comparison between enzymatic activities in control and heat-stressed kernels, estimates of in vivo activities in the heat-stressed kernels were calculated using Q10 values previously determined for the same maize endosperm enzymes according to the same reaction conditions described herein (Table 2; Wilhelm, 1995). Under this premise, activities in developing heat-stressed kernels were equal to or greater than corresponding activities in control kernels in nearly all cases. The only exception was AGPase, where the Q10-adjusted activity was 15% less than control kernels (Table 6).
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Studies linking specific enzymes with the rate of starch deposition in cereals have suggested SSS as a key control point in the pathway. Catalytically, SSS activity measured in vitro becomes less efficient with increasing temperature (Hawker and Jenner, 1993; Jenner et al., 1995), and the enzyme is known to be labile when it, or isolated kernels it is extracted from, are held in vitro at high temperature for short periods (Keeling et al., 1993). Also, the flux control coefficients of other enzymes in the pathway of sugar and starch metabolism in cereal grains, such as branching enzyme, AGPase, and sucrose synthase (Singletary et al., 1997) are much lower than that reported for SSS (Hawker and Jenner, 1993; Jenner et al., 1993; Keeling et al., 1993). Hence, SSS seems to be responsible for the limitation in starch synthesis that occurs under high temperatures. However, in spite of these factors, there are several points that call into question the role of SSS in the temperature sensitivity of starch biosynthesis. Most importantly, simple genetic comparisons of wheat cultivars that show varying degrees of tolerance to heat stress have failed to relate whole-plant differences to catalytic differences in SSS activity measured at high temperatures (Jenner et al., 1993). We have confirmed this lack of correlation using the same cultivars tested by Jenner et al. (1993) and also after comparing SSS and growth data from heat-stressed kernels of several maize inbreds (unpublished results). We also found that starch synthesis in isolated (i.e., in vitro) wheat kernels shows large sensitivities of SSS to temperature (Keeling et al., 1993), but the same was not true for kernels heat treated on the plant (Kuroyanagi and Paulsen, 1988; Hawker and Jenner, 1993). This raises a concern over the agronomic relevance of the in vitro heat treatments, as we have already noted. It is noteworthy that the activity profile of SSS is compressed in time and amount very similarly to AGPase, and the in vitro measured activity of SSS is low relative to most enzymes of sugar and starch biosynthesis (Caley et al., 1990; Singletary et al., 1994). In our study, Q10-corrected SSS activity was slightly increased, which may directly contribute to the small increase in grain-fill rate. Similarly, heat induces a gradual decline in SSS activity, which can lead to a premature end to starch biosynthesis (Singletary et al., 1994).
The response of AGPase is, in some ways, the opposite of SSS in response to high temperatures. In vitro, AGPase activity is stable at high temperature (Kennedy and Isherwood, 1975; Keeling et al., 1993) and its activity profile during development of maize under heat stress is more reduced in magnitude and timing than that of SSS (Ou-Lee and Setter, 1985; Singletary et al., 1994). Work by Duke and Doehlert (1996) involving heat-stressed kernels of maize also demonstrates that AGPase activity is constricted more than several other enzymes of sugar metabolism. In addition, only 5 d of growth at 30°C, compared with 25°C, reduced the expression of AGPase genes Brittle-2 and Shrunken-2 by 50 and 70%, respectively. On the other hand, simple genetic comparisons of wheat cultivars known to vary in heat-stress tolerance failed to relate AGPase activity to whole-plant differences, similar to SSS (Jenner et al., 1993). In our study of 11 enzymes, Q10-adjusted AGPase activity was the most reduced by high temperature. This suggests that high temperature affects AGPase more than the other enzymes, which may contribute to a reduction in the efficiency of starch and dry matter accumulation or cause a premature termination of grain-filling (Singletary et al., 1994). However, previous studies indicate that reductions in starch accumulation generally do not occur without a decrease in AGPase activity of 40% or more (Singletary et al., 1994, 1997). This calls into question the importance of a 15% reduction in AGPase activity in reducing final seed size, as in our study.
The roles of glucokinase and sucrose synthase in determining starch accumulation in heat-treated tissues are less clear. In our study, high temperature had an adverse effect on the activity of both of these enzymes before Q10-adjustment. Previous studies have shown high-temperature sensitivity of glucokinase in maize endosperm tissue (Singletary et al., 1994) and sucrose synthase in both maize (Singletary et al., 1994) and barley (MacLeod and Duffus, 1988), and reductions in activity of these enzymes may be linked to a reduction in starch biosynthesis.
In conclusion, the seed growth response to heat stress was not significantly different across seven maize genotypes. A heat stress of moderate magnitude, but of much greater duration than normally occurs in the U.S. Corn Belt, reduced kernel dry weight by a small but significant amount (7%). Reductions in kernel dry weight were attributable to proportional losses in starch, protein, and oil contents, indicating that a mechanism affecting whole grain metabolism is responsible for the loss. Grain-filling duration in HU increased under heat stress, but could not compensate for the decrease in grain-filling rate, producing kernel dry weight losses. The chronic high temperature significantly reduced the activities of four enzymes of the starch biosynthetic pathway: AGPase, SSS, glucokinase, and sucrose synthase, but only AGPase activity was reduced when activities were Q10-corrected to the growth temperature. These results indicate that these enzymes are closely linked to reductions in starch content and may explain the reductions in mature seed dry weight.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Received for publication October 2, 1998.
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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