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Published online 1 March 2007
Published in Crop Sci 47:711-714 (2007)
© 2007 Crop Science Society of America
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CROP PHYSIOLOGY & METABOLISM

Maize Kernel Moisture at Physiological Maturity as Affected by the Source–Sink Relationship during Grain Filling

Rodrigo G. Salaa,*, Fernando H. Andradea and Mark E. Westgateb

a CONICET, Unidad Integrada INTA Balcarce-Facultad de Ciencias Agrarias, Universidad Nacional de Mar del Plata, Ruta Nacional 226 km 73.5, CC 226 (7620) Balcarce, Buenos Aires, Argentina
b Dep. of Agronomy, 1301 Agronomy Hall, Iowa State Univ., Ames, IA 50010

* Corresponding author (salarode{at}yahoo.com.ar).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A wide range of values reported in the literature has precluded the use of grain moisture (GM) as an estimate of physiological maturity (PM) in maize (Zea mays L.). The reason for this variability in GM values remains unclear. Previous evidence suggests that the source–sink ratio could affect the dynamics of kernel water relations during grain filling and thus, GM at PM. To test this possibility, treatments were applied to manipulate the reproductive sink capacity or the assimilate availability during grain filling. Kernel dry weight, water content, and the dry weight to water content (D–W) ratio, were monitored throughout grain filling. A bilinear model relating dry weight and GM was used to estimate GM at PM for each treatment. Severely restricting source capacity during grain filling increased GM at PM. When the source capacity per kernel during grain filling was increased, however, GM at PM was not affected. A single model (r2 = 0.99, p < 0.001) described the relationship between relative dry weight and GM for all hybrids and treatments without source reduction during grain filling. The estimated value of GM at PM for this model was 34.9%. These results suggest that calculating GM late in grain filling can provide a reliable estimate of PM when the source capacity has not been severely restricted. A value of 35% moisture would be adequate in these situations. The D–W ratio of developing kernels was similar across all source–sink treatments until PM. Premature cessation of grain filling caused by defoliation increased the D–W ratio as the kernels continued to desiccate.

Abbreviations: D–W, dry weight to water content • GDD, growing degree days • GM, grain moisture • PM, physiological maturity • PWGK, plant weight gain per kernel.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
BIOMASS ACCUMULATION in maize (Zea mays L.) kernels begins shortly after fertilization and progresses in a sigmoid pattern in which three phases can be distinguished (Bewley and Black, 1985). The first phase, known as the lag phase, is a period of active cellular division and differentiation. It is characterized by a rapid increase in kernel fresh weight, mostly caused by the influx of water since little dry matter is accumulated. The second phase is marked by a reserve deposition in the kernel and is generally referred to as the effective grain-filling period. In the third phase of kernel development, kernels achieve their maximum dry weight and enter a quiescent state. At this moment, kernels are considered physiologically mature (Hillson and Penny, 1965; Carter and Poneleit, 1973; Brooking, 1990).

An accurate and simple method for estimating physiological maturity (PM) would be valuable for both seed producers and breeders. Various methodologies have been developed in this regard. The most accurate method to determine PM is to follow kernel dry-matter accumulation after anthesis. Using this method, however, PM can be determined no sooner than a week after it has occurred (Calderini et al., 2000). Black-layer formation (Daynard and Duncan, 1969) and milk-line progression (Afuakwa and Crookston, 1984b) are commonly used to estimate PM indirectly. There are problems associated with the use of these methodologies, however, including variability in appearance under different environmental conditions and an inability to determine its complete formation clearly (Carter and Poneleit, 1973; Afuakwa and Crookston, 1984b). Another methodology used to determine PM is based on grain moisture (GM). Traditionally, maize kernels have been considered mature when they reach GM values from 30 to 35% (Carter and Poneleit, 1973).

Several authors have suggested that water loss from kernels during grain filling is merely an exchange between dry matter and water (Millet and Pinthus, 1984; Brooking, 1990; Saini and Westgate, 2000). If so, the general pattern of dry-matter accumulation should be fairly similar irrespective of genotype or environment when expressed on a GM basis, and therefore, maize kernels should reach PM at about the same GM (Brooking, 1990; Saini and Westgate, 2000). Supporting this hypothesis, Westgate (1994) found that kernel development in water-deficient plants follows the same general pattern as in well-watered plants when expressed on a GM basis. Evidently, the drought imposed during grain filling did not affect the exchange between dry matter and water during kernel development. As a consequence, these plants reached PM at similar GM values. Others, however, have reported a range of GM values at PM (Rench and Shaw, 1971; Daynard, 1972; Carter and Poneleit, 1973; Afuakwa and Crookston, 1984a), which argues against this method of estimating PM under all circumstances. The reasons for the reported variability in GM values at PM remain unclear. Sala et al. (2007) studied the effect of source–sink manipulations on kernel–water relations. In their study, increasing reproductive sink capacity did not alter the dynamics of water content or dry-matter accumulation in the developing kernels. Restricting source capacity during the effective grain-filling period, however, affected kernel water content and the dynamics of dry-matter deposition differently. Their results indicated that source capacity during grain filling could influence GM at PM.

The objective of this study is to evaluate the effects of different source–sink combinations during grain filling on GM at PM. Specific objectives were (i) to evaluate the reliability of GM as a developmental indicator of PM across a range of growth conditions and (ii) to define a general GM reference value for considering the maize kernels physiologically mature.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Two field studies were conducted at the INTA Balcarce Research Station, Buenos Aires, Argentina (37°45‘ S, 58°18’ W), during the 2002–2003 and 2003–2004 growing seasons. Hybrid Dekalb 615 was sown on 15 October both years, following a randomized complete block design with three blocks. Each plot consisted of four rows 0.7 m apart and 12 m long. Final population density was 8 plants m–2. Treatments consisted of a control and two defoliation levels applied 20 d after silking to the whole plot: moderate, in which 50% of the leaves were removed, and severe, in which 85% of the leaves were removed. At the moment the defoliation treatments were performed, approximately 14 leaves were present. In the moderate treatment the uppermost seven leaves were kept on the plant, while in the severe treatment only the uppermost two leaves were retained. Plots were considered to be at silking when 50% of the plants in a row presented visible silks. Defoliations were performed by removing the leaf lamina starting from the bottom of the plant.

A third field trial was conducted during the 2004 growing season at the Bruner Research Farm of Iowa State University near Ames, IA (42°2' N, 93°37' W). Commercial hybrids Dekalb 5878, Fontanelle 4402, and Fontanelle 4741 were sown on 11 May 2004 at approximately 8 plants m–2 and thinned to one of three plant densities: 2, 4, or 8 plants m–2 by the fourth leaf stage. Each hybrid–by–population density combination was sown in a strip plot (one experimental unit for each hybrid by treatment combination) of eight rows 0.76 m apart and 36 m long. Within each plot individual plants were tagged and their silking date was recorded. The subapical ears of these plants were removed when present. Two pollination treatments were applied to each hybrid-density combination: not restricted or free, and restricted. For the latter treatment, randomly selected ears of at least 30 tagged plants of each plot were covered 1 d after silking to prevent further pollination. In the freely pollinated 8 plants m–2 density, a complete defoliation was applied in half of the plot to all three hybrids 20 d after silking (approximately 400 °C days after midsilking). All trials were conducted without any discernible nutrient or water limitations and free of biotic stresses.

In all three trials, grain samples were taken at 4- to 7-d intervals starting 10 to 20 d after silking until a final constant dry weight was achieved. At each sampling date, ears were harvested from three randomly selected plants in the central rows of each plot and transported to the laboratory in a humidified container, where 10 kernels were removed from the lower third of each ear. Kernels were weighed and dried to constant weight in a forced-air oven at 70°C to obtain kernel fresh and dry weights, respectively. GM was calculated as (1-dry weight/fresh weight) x 100. To estimate GM at PM for each treatment, a bilinear model was fitted to the dry weight and GM data (Brooking, 1990):

Formula

Formula
where DW is kernel dry weight (mg kernel–1), a is the Y-intercept (mg kernel–1), b is the increase in kernel weight per unit of decrease of GM (mg per kernel %–1), and c is GM at PM (%). Differences among treatments were based on the superimposition of the confidence intervals for the estimate of c (p < 0.05). To develop a model for all treatments with no source reduction independently of final kernel mass, values of dry weight were expressed relative to their respective maximum dry weight. Maximum dry weight was taken as the maximum value measured within each hybrid by treatment combination during the grain-filling period. Parameters described above were fitted using the iterative optimization technique in TableCurve V 3.0 (Jandel Scientific, Corte Madera, CA; Jandel Scientific, 1991).

For convenient comparison with previous works, growing degree days (GDD) were calculated starting at silking using mean daily air temperature and a base temperature of 0°C (Muchow, 1990). This method may have overestimated the rate of development on days when the mean temperature was low (Stewart et al., 1998) but was nonetheless considered adequate for the specific analyses in this work. Kernel water content was calculated as the difference between kernel fresh weight and dry weight. The ratio between dry matter and water content in the kernel (dry weight to water content [D–W] ratio) was calculated at each sampling date. Aboveground plant biomass samples for each genotype-treatment combination were taken in Balcarce (2003–2004) and Ames on five randomly selected plants 10 d after silking and at PM. Plants were separated into stalk, leaves, and ear. The plant weight gain during kernel growth was calculated as the difference between plant biomass at physiological maturity and at 10 d after silking. For the defoliated treatments (i.e., moderate and severe at Balcarce, and 8 plants m–2 completely defoliated at Ames) this variable was calculated as the difference in plant biomass without considering leaves. Kernel number per plant was estimated by counting the number of kernels per row and the number of rows on all the harvested ears. The source–sink ratio during grain filling for each treatment then was roughly estimated as the weight gain per plant divided by final kernel number (plant weight gain per kernel; PWGK).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
The developmental profiles of kernel dry matter, water content, and grain moisture for three contrasting treatments (2 plants m–2 freely pollinated, 8 plants m–2 freely pollinated, and 8 plants m–2 completely defoliated) on hybrid Dekalb 5878 are presented in Fig. 1 . Dry matter accumulation ceased soon after the plants were defoliated (Fig. 1a), while decline in water content was accelerated markedly in these treatments after reaching a maximum value (Fig. 1b). Percent GM, however, was similar for the three treatments until the defoliated treatment reached PM (Fig. 1c). In contrast, increasing the source capacity of the crop by reducing plant density (2 plants m–2 freely pollinated compared with 8 plants m–2 freely pollinated) did not alter the developmental pattern of the kernel significantly. These results show that in general terms, the pattern of kernel development is not affected by the relative source capacity of the crop, except at low source–sink ratios (Sala et al., 2007).


Figure 1
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  		Figure 1. Progress of (a) kernel weight, (b) kernel water content, and (c) kernel grain moisture as a function of growing degree days from silking for hybrid Dekalb 5878 grown without pollination restriction at 2 plants m–2 (2F, black) and 8 plants m–2 (8F, gray), and 8 plants m–2 completely defoliated (8D, white) in Ames, IA, during 2004. Data points are the mean of 10 kernels from an individual ear.

 
Values of GM at PM for growing seasons 2002–2003 and 2003–2004 at Balcarce and 2004 at Ames are presented in Tables 1 and 2. Control values (C in Table 1 and 8F in Table 2) were between 33.6 and 35%. Grain moisture at PM was not affected by treatments that increased resources available per kernel during grain filling, such as stand densities less than 4 plants m–2, restricted pollination treatments, or a combination of both (Table 2). Grain moisture values at PM averaged approximately 35% (range: 34.7–37.3%) across hybrids and treatments.


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  		Table 1. Grain moisture at physiological maturity for the defoliation treatments applied to hybrid Dekalb 615 sown at 8 plants m–2 at Balcarce, Buenos Aires, Argentina, during 2002–2003 (2003) and 2003–2004 (2004). Physiological maturity was determined as the time maximum dry weight was attained.

 

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  		Table 2. Grain moisture at physiological maturity for the different treatments applied to hybrids Dekalb 5878, Fontanelle 4402, and Fontanelle 4741 sown at Ames, IA, during 2004. Physiological maturity was determined as the time maximum dry weight was attained.

 
Defoliation treatments increased GM at PM. In the severe defoliation treatments (85% leaf removal) at Balcarce, GM at PM was 8.7 points higher on average than in the control treatments (Table 1). Removing 50% of the leaves also increased GM at PM compared with the control. This effect varied depending on the year, however, being greater in 2002–2003 than in 2003–2004. This variation could be related to the thermal environment during grain filling, which was warmer in 2003–2004 than in 2002–2003 (data not shown). Warmer average temperatures could have enhanced source capacity of the crop (Cirilo and Andrade, 1996). Given that sink capacity for the moderate treatment was similar between growing seasons (Sala et al., 2007), the source–sink ratio should have been greater in 2003–2004, all other things being equal. At Ames, complete defoliation of freely pollinated plants (8D in Table 2) also increased GM at PM for all three hybrids tested. Values of GM at PM were higher than those found at Balcarce, which is consistent with markedly lower source–sink ratios established during the effective grain-filling period for all three hybrids (Sala et al., 2007).

Afuakwa and Crookston (1984a) observed that GM at black-layer formation varied with the timing of defoliation during kernel development. Their study and many others (Rench and Shaw, 1971; Daynard, 1972; Carter and Poneleit, 1973) confirm that GM at PM can vary widely (15.4–37.3% in those cited) when physiological maturity is based on black-layer formation. This variability is likely associated with the unevenness in black-layer formation among kernels and between plants, as well as the longer time needed for black layer to form completely under cool conditions often encountered late in the season. Thus, caution is needed when using indirect estimators of PM such as black layer to estimate GM at PM (Brooking, 1990). Since Afuakwa and Crookston (1984a) measured GM at black layer and not at PM, their values are quite different from ours. Increased GM at PM was also observed when kernel development was interrupted by the occurrence of air frosts (Brooking, 1990).

As shown in Fig. 2a , a single model relating kernel dry weight and GM could be fitted for all treatments that did not impose a source restriction during grain filling (r2 = 0.99, p < 0.001). The estimated value of GM at PM for this model was 34.9%. This value is in general agreement with previous reports for maize (Brooking, 1990; Brown and Bootsma, 2002; Borrás and Westgate, 2006) and wheat (Calderini et al., 2000). As such, GM provides a fairly reliable method to estimate PM indirectly when the source capacity is not negatively affected (Fig. 2b). Under most situations, a value of 35% would be adequate to consider maize kernels physiologically mature. The availability of portable moisture meters, which have been previously reported to render reliable estimates of GM (Kang et al., 1978; Freppon et al., 1992), could facilitate the estimation of PM through this approach. This would be particularly beneficial for evaluating a large number of genotypes as in Sala et al. (2006).


Figure 2
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  		Figure 2. (a) Relationship between relative kernel dry weight (%) and kernel grain moisture (%) for developing maize kernels (nondefoliated treatments). Data are for hybrid Dekalb 615 sown at 8 plant m–2 in Balcarce, Buenos Aires, Argentina, during 2002–2003 and 2003–2004, and hybrids Dekalb 5878, Fontanelle 4402, and Fontanelle 4741 grown at 2, 4, and 8 plants m–2, with and without pollination restriction, in Ames, IA, during 2004. Triangles are the estimated values of grain moisture at physiological maturity for the defoliated treatments (see Tables 1 and 2): moderate (50% leaf removal; gray), severe (85% leaf removal; dark gray) and complete (100% leaf removal; black). (b) Relationship between plant weight gain per kernel (PWGK) during the effective grain-filling period (as a rough estimator of the source–sink ratio) and kernel grain moisture at physiological maturity (%) for hybrid Dekalb 615 sown at Balcarce, Buenos Aires, Argentina, during 2003–2004, and hybrids Dekalb 5878, Fontanelle 4402, and Fontanelle 4741 sown at Ames, IA, during 2004. The vertical dashed line indicates the threshold at which kernel dry weight begins to respond to a decrease in PWGK (Sala et al., 2007). Closed and open symbols are for nondefoliated and defoliated treatments, respectively.

 
Saini and Westgate (2000) suggested that maintenance of a favorable water status within the kernel allows metabolism to continue until a critical GM level is reached. This could explain why kernels in all the nondefoliated treatments reached PM around the same GM value (35%) and a single model was suitable to all hybrid-treatment combinations. This was not the case, however, in the defoliated treatments in which PM was reached at higher GM values. In these treatments, a severe and sudden lack of assimilate supply likely limited the duration of kernel growth. These results indicate that the duration of grain filling in maize can be limited by the availability of assimilates or by the capacity for continued metabolism as moisture content decreases within the developing kernel.

Kernel water content dynamics during grain filling seem to be closely coordinated with dry matter deposition during the effective grain-filling period. One way to visualize these coordinated processes is to follow the D–W ratio of the kernels throughout grain filling. Figure 3a shows that this D–W ratio was nearly identical during the grain-filling period for all nondefoliated treatments. In kernels of defoliated plants (50, 85, and 100% leaf removal), D–W values were similar to those of control kernels until they reached PM. Different genotypes with contrasting potential kernel weights, grown under different environmental conditions and exposed to an array of source–sink treatments (Sala et al., 2007) maintained the same D–W relationship during grain filling until they reached PM (Fig. 3a). After PM of the defoliated treatments, the D–W ratio of these treatments began to increase earlier and more rapidly than in the controls. The dramatic increase in the D–W ratio reflected the strong decline in kernel water content after cessation of kernel growth (8F and 8D in Fig. 1a and b). Thus, this ratio could be used as a sensitive indicator of situations where the source–sink ratio is reduced. Previous works have suggested that GM could be used to document the progress of kernel development (Brooking, 1990; Borrás and Westgate, 2006). Because D–W ratio and GM are mathematically related (Fig. 3b), both variables could be used as developmental scales for kernel growth during grain filling. The similarity of the D–W ratio until PM for the majority of the treatments in this study supports this possibility.


Figure 3
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  		Figure 3. (a) Change in the ratio of dry matter to water content (D–W ratio) in developing maize kernels as a function of thermal time (°C day) from silking for the control nondefoliated plants (black circles), 50% defoliated plants (gray squares), and 85 or 100% defoliated plants (white circles). See Tables 1 and 2 for details on the treatments. TI = time of treatment imposition (defoliation), PMD = average physiological maturity for 85 or 100% defoliated plants, PMMD = average physiological maturity for the 50% defoliated plants, and PMU = average physiological maturity for the control nondefoliated plants. Thermal time to physiological maturity was calculated as an average for the different group of treatments (data extracted from Sala et al. [2007]). (b) Relationship between the D–W ratio and grain moisture (GM) in developing maize kernel. Data are for hybrid Dekalb 5878 grown at 2 plant m–2 without pollination restriction, in Ames, IA, during 2004.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Grain moisture provides a simple method to estimate the PM of developing maize kernels when the source capacity during grain filling is not negatively affected. Under these situations, a value of 35% GM would be adequate to consider maize kernels physiologically mature. Restricting source capacity during grain filling, however, generally increases GM at PM due to premature cessation of kernel growth. Thus, when the assimilate supply to the kernels is severely restricted GM is no longer a useful estimator of PM. The ratio of dry matter to water content in the kernel was remarkably stable across a wide range of source–sink manipulations until kernels reached PM. After PM, treatments that caused premature cessation of grain filling (e.g., defoliated treatments) showed a rapid increase in the D–W ratio.


   ACKNOWLEDGMENTS
 
This work was partially supported by the Instituto Nacional de Tecnología Agropecuaria (INTA). R.G. Sala holds a scholarship from CONICET, the research council of Argentina. The authors thank Agustín Fonseca, Sebastian Schneider and Maria Hartt-Eckerman for help with the experiments and Laura Echarte for critically reading the manuscript. This work is a part of a thesis submitted by R.G. Sala for the doctoral degree, Universidad Nacional de Mar del Plata, Argentina.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication June 14, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





This Article
Right arrow Abstract Freely available
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Right arrow Full Text (PDF) Free
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Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Sala, R. G.
Right arrow Articles by Westgate, M. E.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Sala, R. G.
Right arrow Articles by Westgate, M. E.
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