Influence of Nitrogen Fertilizer on Growth and Yield of

Published July 18, 2014 Crop Ecology & Physiology

Influence of Nitrogen Fertilizer on Growth and Yield of Grain Sorghum Hybrids and Inbred Lines George Y. Mahama, P. V. Vara Prasad,* David B. Mengel, and Tesfaye T. Tesso ABSTRACT

Grain sorghum [Sorghum bicolor (L.) Moench] is an important crop in semiarid regions of the world because of its drought tolerance. Nitrogen is one of the most limiting nutrients in crop production due to low availability and loss. We hypothesize that there are differences in physiological and yield traits among grain sorghum genotypes in response to N. The objectives of this study were to determine the responses of sorghum genotypes (hybrids and inbred lines) to N fertilizer and the relationship between their physiological and yield traits. Field experiments were conducted at two locations in Kansas for two seasons (2010 and 2011). Genotype × N regimes and year × genotype interactions were significant for leaf chlorophyll, aboveground biomass, grain yield, and seed number. Overall, the hybrids were superior to inbred lines for grain yield and total aboveground biomass, but grain yields of inbred lines TX2783 and TX7000 were comparable to hybrids. Maximum total aboveground biomass, leaf chlorophyll index, and grain yield were obtained at 90 kg N ha–1. Across years, application of 45 and 90 kg N ha–1 resulted in an increase in yield of 13 and 48% over 0 kg N ha–1, respectively. No strong relationship was detected between genotypes and leaf chlorophyll index or chlorophyll a fluorescence and grain yield, but there was a strong relationship between seed number and total aboveground biomass and grain yield. Leaf chlorophyll index and chlorophyll a fluorescence did not provide physiological basis for differences in N response among the genotypes for grain yield.

Grain sorghum is widely grown in the semiarid

regions of the world as animal feed, human food, or bioenergy feedstock. Its tolerance to drought and superior adaptation to marginal environments make it an attractive crop in rainfed production systems. Because it originated in the semiarid region of Africa, sorghum has adaptive traits for stressful environments and wide genetic variability for traits including tolerance to low nutrient supplies and efficiency in utilizing water and nutrients. Nitrogen is a crucial component of plant nutrition, and its deficiency limits productivity of crops more than any other element. Previous research has demonstrated that application of N increased biomass and grain yield of sorghum (Wortmann et al., 2007; Kaizzi et al., 2012). Increases in sorghum grain yield were mainly associated with improving panicle number, grain number per panicle, and grain weight (Buah and Mwinkara, 2009; Buah et al., 2012). Mousavi et al. (2012) observed that application of N up to 150 kg ha–1 increased grain number, grain yield, and harvest index in sorghum. Greater yields and components of yields with increases in N application were also observed in maize (Zea mays L.) and wheat (Triticum aestivum

Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506. Received 18 Feb. 2014. *Corresponding author ([email protected]). Published in Agron. J. 106:1623–1630 (2014) doi:10.2134/agronj14.0092 Available freely online through the author-supported open access option. Copyright © 2014 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

L.; Mullen et al., 2003; Demotes-Mainard and Jeuffroy, 2004; Ma et al., 2006; Arnall et al., 2013). Higher N levels led to higher leaf chlorophyll index for staygreen sorghum during anthesis (Addy et al., 2010), an important factor in determining not only onset of leaf senescence but also the rate of postanthesis leaf senescence. Increases in leaf photosynthesis rates were observed under higher N levels in grain sorghum hybrids (Cechin, 1998). Although leaf senescence in sorghum during grain filling is affected greatly by the water supply/demand balance, it is also affected by the N supply/demand balance. The potential N translocation rate from vegetative plant parts to the grain in cereals has been related to the amount of translocatable N available in these plant parts at anthesis (Jamieson and Semenov, 2000). Sorghum genotypes with improved post-flowering drought tolerance and stay-green genotypes have been reported to be efficient users of N (Borrell et al., 2000a). Thomas and Smart (1993) characterized the staygreen trait (i.e., the phenotypes that exhibit delayed senescence) as having higher chlorophyll content during the grain-filling period and maturity. Genotypic differences in postanthesis N uptake can affect leaf senescence patterns and grain yield (Borrell and Hammer, 2000; Borrell et al., 2000a, 2000b). If grain N demand exceeds total soil N uptake, an accelerated translocation of N from stems and leaves can occur (Tribio and Tribio-Blondel, 2002), leading to complete grain filling and increased grain yield.

Abbreviations: DAS, days after sowing; DTF, days to flowering; HI, harvest index; HT, high temperature; NUE, nitrogen use efficiency.

A g ro n o my J o u r n a l • Vo l u m e 10 6 , I s s u e 5 • 2 014

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Despite large genetic variability in grain sorghum for grain yield, little research has been done on the responses of various genotypes or hybrids to N, particularly on the influence of N on various physiological and yield traits. Enhanced understanding of genetic and physiological characteristics of sorghum genotypes, their responses to N, and associations among various traits is needed. We hypothesize that there are differences in physiological and yield traits among grain sorghum genotypes in response to N. The objectives of this study were to determine the responses of sorghum genotypes (hybrids and inbred lines) to N fertilizer and the relationship between their physiological and yield traits. MATERIALS AND METHODS Field experiments were conducted at two locations in 2010 and 2011 to evaluate the responses of grain sorghum hybrids and inbred lines to varying N fertilizer levels. Locations for the study were the East Central Experiment Field near Ottawa, KS (38°32¢12.7² N; 95°14¢38.7² W), on a Woodson silt loam soil (fine, smectitic, thermic Abruptic Argiaquoll), and Ashland Bottoms Research Farm near Manhattan, KS (39°08¢35.3² N; 96°37¢39.2² W), on a Reading silt loam soil (fine-silty, mixed, superactive, mesic Pachic Argiudoll). The experiment was implemented on conventional tillage at Ashland Bottoms and on no-tillage at Ottawa. The previous crops in Manhattan and Ottawa were soybean [Glycine max (L.) Merr.] and maize, respectively, for 2010, and the previous crop was soybean at both locations in 2011. Experimental Design The experiment was conducted in a split-plot arrangement in a randomized complete block design with four replications. Each plot dimension was 6.0 by 3.0 m (four rows). The middle two rows were used for data collection to eliminate any border effects, and each replication was separated by two alleys to eliminate N fertilizer border effects. The test genotypes consisted of six hybrids and six inbred lines with varying genetic backgrounds and known drought tolerance characteristics (pre-flowering and post-flowering drought tolerance) (Table 1). Three different N regimes were used to evaluate the response of genotypes in all the environments: control (no inorganic

N supplied), half the recommended rate (45 kg N ha–1), and optimum rate (90 kg N ha–1). The N regimes were assigned as main plots and the genotypes as subplots. Soil Sampling and Analyses At each location, composite soil samples were taken from each replication from a depth of 15 cm. Sampling was done using a hand probe, and samples consisted of 12 to 15 individual cores mixed to form individual composite samples. The soil was analyzed for pH, available P, exchangeable K, soil organic matter (SOM), S, and chloride. Profile ammonium and nitrate were also measured in soil collected at 60-cm depth. Soil physical properties such as sand, silt, and clay were determined for each replication at a depth of 15 cm at both locations. Analyses were conducted by the Kansas State University Soil Testing Lab. Hydrometer method was used for determining soil texture. Soil samples were treated with sodium hexametaphosphate to complex Ca2+, Al3+, Fe3+, and other cations that bind clay and silt particles into aggregates. Organic matter was suspended in the solution and density of the soil suspension was determined with a hydrometer. Soil pH was estimated using a 1:1 slurry method with a 10 g scoop of soil and 10 mL of deionized water, and pH was measured by dual-probe automated pH analyzer (Labfit Pty. Ltd., Burswood, Western Australia). Also Mehlich 3 P was analyzed by HCl-ammonium fluoride extraction method. Extractable (plant-available) K and Na were determined by the ammonium acetate (1 M, pD 7.0) extraction method. This analysis was performed by an inductively coupled plasma (ICP) spectrometer (Model 3110 Flame Atomic Absorption Spectrometer, PerkinElmer Corp., Norwalk, CT). Walkley–Black method was used to determine organic matter and colorimetric analysis of the solution was performed with PC910 Fiber Optic Spectrophotometer (Brinkmann Instruments, Inc., Westbury, NY). Chloride was analyzed by calcium nitrate extraction method and colorimetric analysis through mercury thiocyanate method. Turbidimetric method was used for the determination of sulfate-S. A suspension of BaSO4 precipitate in a slightly acid medium when excess BaCl2·2H2O was added to a solution containing sulfate-S was developed. The density of the suspension was then read in a colorimeter. Also, for soil-extractable nitrate, 1 M KCl extraction

Table 1. Key characteristics and sources of genotypes used in the experiment during 2010 and 2011 seasons. Genotypes 23012 26056 95207 99480 CSR1114/R45 TX3042xTX2737 B35 SC35 SC599 TX2783 TX430 TX7000

Type Hybrid Hybrid Hybrid Hybrid Hybrid Hybrid Lines Lines Lines Lines Lines Lines

Characteristics† PreFDT, PostFDT PreFDS, PostFDT PreFDT, PostFDS PreFDS, PostFDT PostFDT PostFDS Stay-green (charcoal rot–resistant) Stay-green (charcoal rot–resistant) Stay-green (Stalk rot–resistant) Non stay-green Non stay-green Non stay-green (charcoal rot–susceptible)

Source Crosbyton Crosbyton Crosbyton Crosbyton Experimental hybrid Experimental hybrid Public inbred Breeding material Breeding material Public inbred Public inbred Public inbred

† PreFDS: Pre-flowering drought-susceptible. PreFDT: Pre-flowering drought-tolerant. PostFDT: Post-flowering drought-tolerant. PostFDS: Post-flowering drought-susceptible.

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(2 g in 20 mL, 15 min.) and cadmium reduction/colorimetry was used. Ammonia was extracted from soil samples with 1 M KCl (2 g in 20 mL, 30 min) and measured by an indophenol colorimetric reaction. The results of soil analyses are presented in Table 2.

Table 2. Physical and chemical characteristics of soils from study area in 2010 and 2011. Characteristics Location Sand, % Silt, % Clay, % pH Mehlich-3 P, mg kg–1 K, mg kg–1 Organic matter, % Chlorine, mg kg–1 SO4–S, mg kg–1 NH4–N, mg kg–1 NO3–N, mg kg–1

Crop Management The N fertilizer source was urea (46% N). The fertilizer was hand-broadcast 10 to 14 d after emergence along the rows of each plot to ensure that N was evenly distributed. Standard spacing of 75 cm between rows, was used at both locations. Planting was done in May and June at both locations and in both years. Weeds were controlled with pre-emergence herbicides applied at labeled rates using a tractor-mounted boom sprayer. At Manhattan, Lumax 2.84 at the rate of 2.9 L ha–1 and Bicep II Magnum at the rate of 3.3 L ha–1 were sprayed. At Ottawa, atrazine (1-chloro-3-ethylamino-5-isopropylamino-2, 4, 6-triazine) and 2, 4-dichlorophenoxyacetic acid (2, 4-D) at the rate of 1.1 L ha–1 were applied. Hand-weeding was also done as necessary throughout the growing season to reduce weed pressure. Plots were mechanically harvested after physiological maturity using a two-row plot combine with which grain samples were collected to determine moisture content and test weight. Yields at both locations were then corrected to 135 g kg–1 moisture content. Observations and Growth Measurements At both locations, five plants were tagged in the two middle rows for phenological measurements (days to 50% flowering and days to physiological maturity; Vanderlip, 1993), growth traits (number of green leaves, total number of leaves, and percentage of leaf senescence at physiological maturity), physiological measurements (leaf chlorophyll index [SPAD value] and chlorophyll a fluorescence). Leaf senescence was calculated as the percentage of the difference between total number of leaves produced per plant and number of green leaves at any particular time, expressed over the total number of leaves produced per plant. Physiological measurement such as leaf chlorophyll index of the leaves was measured with the aid of a soil plant analysis development (SPAD) 502 chlorophyll meter (Minolta Corp., Tokyo, Japan). This meter records optical density measurements at two wavelengths, converts them into digital signals, and then into a SPAD value (Minolta, 1989). Measurements were taken at 38 DAS from the uppermost fully expanded leaves from five different plants in each plot and averaged to one value per plot. Flag leaves were used for measuring SPAD value at flowering and physiological maturity. In addition, chlorophyll a fluorescence (Fv/Fm) was recorded during vegetative stage, flowering, and physiological maturity with the aid of a handheld pulse-modulated chlorophyll fluorometer, OS 30 (Opti-Science, Hudson, NH). Chlorophyll a fluorescence measurements were taken between 1100 and 1500 h from the regions on the leaves that were dark-adapted using clips left in place for at least 30 min. Following a dark adaptation, measurements were taken by inserting the fluorometer tip, opening the clip shutter, then giving a flash of light from the fluorometer that activated the PS II reaction centers of the photosynthetic apparatus (Ristic et al., 2007).

2010 Manhattan Ottawa 4.5 8.7 69 64 27 27.3 6.4 6.2 52.3 11.2 387 122 2.6 2.8 6.2 14.4 5.8 5.0 3.0 3.7 13.8 2.2

2011 Manhattan Ottawa 6.0 2.0 75 72 19 26 7.5 6.0 56.1 14.9 293 117 1.5 1.8 4.3 5.7 3.4 7.2 4.1 7.1 7.7 16.6

At physiological maturity, the aboveground portion of 10 plants from each plot (middle rows) were harvested for grain yield and yield components (harvest index, 200-seed weight, and seed number m–2). All samples were dried at 60°C in a forced-air oven for 72 h and weighed. Based on plant population per plot, total biomass and grain yield were expressed as kg ha–1. Harvest index was calculated as a ratio of grain yield to total biomass (stover + grain). Statistical Analyses Statistical analyses were performed using PROC MIXED procedure of SAS 9.1 (SAS Institute, 2003). Genotype and year were treated as fixed effects, and replication was treated as a random effect. Mean separation for significant effects was performed using Tukey’s honestly significant difference test. Correlation (PROC CORR) analyses were used to determine the relationships between grain yield, leaf chlorophyll index, chlorophyll a fluorescence, aboveground total biomass, harvest index, 200-seed weight, and seed number. Tests for homogeneity of variances (Hartley, 1950) across sites showed that variances were homogenous. Data from each site-year were therefore pooled. RESULTS Climatic Conditions Precipitation and temperature varied between locations and years of the study. In Manhattan (Ashland Bottoms), the mean minimum and maximum temperatures from May through October were 13.7 and 28.8°C, respectively, for 2010 and 15.6 and 29.5°C, respectively, for 2011. In Ottawa, mean minimum and maximum temperatures from the same period were 14.3 and 28.9°C for 2010 and 15.8 and 29.6°C for 2011, respectively (Fig. 1). At both locations and years, temperature peaked in August, which coincided with flowering or early grain filling for most genotypes. Rainfall amounts were higher in 2010 than in 2011 in Manhattan and Ottawa, thus making 2011 a dry year compared with 2010. Total precipitation in 2010 and 2011 growing seasons were 551 mm and 403 mm, respectively, in Manhattan, and 667 and 344 mm, respectively, in Ottawa. Crop Development Effects of both genotype and N regime were significant (P < 0.05) on days to flowering (DTF). Only genotype had a significant effect on days to maturity (Table 3). Application of fertilizer significantly affected DTF. Overall, fertilized hybrids


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Fig. 1. Daily minimum and maximum temperatures and precipitation in Manhattan and Ottawa in 2010 and 2011 growing seasons.

flowered earlier than the inbred lines. Within the hybrids, flowering ranged from 56 to 67 DAS, whereas within the inbred lines, flowering ranged from 68 to 75 DAS. In addition, fertilized plants flowered 3 to 5 d earlier than those in treatments with no inorganic fertilizer. Days to maturity varied from 107 to 130 d with a mean of 115 d. Averaged across N regimes, Genotypes 23012, 95207, and CSR1114/R45 were the early maturing genotypes (107 d), whereas SC35 (130 d) was the late-maturing genotype (Table 4). At physiological maturity, genotype had a significant effect on green leaf retention and percentage of senesced leaves. Among the genotypes, the stay-green types (B35, SC35, and SC599) had the highest green leaf retention, but it was not statistically different from the post-flowering, droughttolerant (23012, 26506, 99480, and CRS114/R45) genotypes. The effect of N regime on green leaves was similar at 0 and

45 kg N ha–1 but significantly higher at 90 kg N ha–1. When averaged across N regimes, the percentage of leaves senesced was lower among the stay-green genotypes (35.2, 35.8, and 37.5% for B35, SC35, and SC599, respectively) (Table 4). The hybrids and the non–stay-green genotypes were not statistically different for percentage of senesced leaves. In addition, averaged across genotypes and years, percentage of senesced leaves was significantly higher at 0 kg N ha–1 (57.3%) compared with 45 kg N ha–1 (51.7%). The lowest percentage of senesced leaves was observed at 90 kg N ha–1 (48.9%) (Table 4). Genotypes and N regimes had significant (P < 0.05) effect on chlorophyll a fluorescence (Fv/Fm). When averaged across N regimes, genotypes SC35 and SC599 had the highest Fv/Fm (0.750 and 0.753), although it was not statistically different from genotypes CSR114/R45, B35, and TX430 (0.711, 0.744, and 0.737). Chlorophyll a fluorescence was similar at 0 and 45 kg N ha–1 (0.724 and 0.732) but significantly higher at 90 kg N ha–1 (0.753). There was a significant interaction (P < 0.05) of genotype and N regime on leaf chlorophyll index (Table 3 and Fig. 2a). Averaged across the genotypes, leaf chlorophyll index was significantly higher at 90 kg N ha–1 (59.6) compared with 45 kg N ha–1 (54.8) and 0 kg N ha–1 (44.4); however, leaf chlorophyll index was not statistically different at 45 and 90 kg N ha–1 among Genotypes 26056, 95207, SC599, and TX7000 (Fig. 2a). The interaction effect of genotype and N regime on aboveground biomass at physiological maturity was significant (P < 0.05) (Table 3, Fig. 2b). When averaged across years, it was evident that hybrids 26056 and 99480 had the highest total biomass at 90 kg N ha–1 (11640 kg ha–1) compared with 0 kg N ha–1 (8804 kg ha–1) and 45 kg N ha–1 (10065 kg ha–1). In addition, similar total aboveground biomass was obtained at all the N regimes among Genotypes 95207, TX430, and TX7000. Overall, maximum biomass was obtained at 90 kg N ha–1, followed by 45 kg N ha–1, and the least was at 0 kg N ha–1 (Fig. 2b). Yield and Yield Components A significant (P < 0.05) genotype × N interaction was occurred on grain yield (Table 3, Fig. 2c). When averaged across the N regimes, Genotype 99480 had the highest grain yield but did not differ significantly from Genotypes 23012, 26056, and CSR1114/R45. In addition, genotypes SC35 and B35 had the lowest grain yield compared with

Table 3. Significance (P values) for main effects and interactions from analysis of variance for genotypes and N rate effects on growth, yield, and yield components of grain sorghum genotypes grown in 2010 and 2011. Effect df DTF† DTM Green leaves Senesced leaves SPAD value Fv/Fm Aboveground biomass Grain yield Seed numbers, m–2 200-seed weight Harvest index

Genotype (G) 11 0.0001 0.0001 0.0850 0.0010 0.0032 0.0276 0.0001 0.0001 0.0001 0.0001 0.0001

N Level (N) 2 0.0363 0.3510 0.0264 0.0101 0.0061 0.0251 0.0082 0.0021 0.0101 0.0502 0.0461

Y×G 11 0.1311 0.0511 0.1977 0.3411 0.0134 0.0974 0.0001 0.0001 0.0001 0.0588 0.1122

Y×N 2 0.4326 0.7327 0.3481 0.0650 0.3684 0.1243 0.2306 0.0587 0.065 0.2436 0.0763

G×N 22 0.0886 0.1125 0.1572 0.0549 0.0069 0.2753 0.0124 0.0020 0.0049 0.0636 0.0590

Y×G×N 22 0.1218 0.9189 0.4280 0.1493 0.2734 0.2729 0.1080 0.0526 0.0493 0.1927 0.0638

† DTF: Days to 50% flowering; DTM: Days to maturity; SPAD: Leaf chlorophyll index; Fv/Fm: Chlorophyll a fluorescence.

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Table 4. Genotype and N regimes means averaged across years for phenology, physiology, growth, and yield traits of 12 grain sorghum genotypes grown in 2010 and 2011.

Treatment

DTF† DTM ––––– d –––––

Green leaves

Senesced leaves %

SPAD value

Fv/Fm

Grain Biomass yield ––––– kg ha–1 –––––

Seeds m–2

200 seed weight g

HI ratio

Genotypes Hybrids 23012 26056 95207 99480 CSR1114/R45 TX3042xTX2737 Lines B35 SC35 SC599 TX2783 TX430 TX7000

58fg‡ 61f 56g 67de 61f 59fg

107g 111fg 107g 120cd 107g 110fg

74ab 75a 70cd 71bc 69cd 68cd

127ab 130a 113ef 119cd 123bc 117de

11a 11a 10a 7c 7c 7c

0 45 90

68a 66ab 63c

116 116 116

7b 7b 10a

8bc 9ab 7c 9ab 8b 7c

50.0c 47.0d 56.2b 50.0c 55.5b 63.1a

50.2d 52.2cd 50.9d 50.4dcd 52.1cd 51.6cd

35.2e 56.7a 38.8e 54.7ab 37.5e 51.9cd 58.8ab 51.6cd 61.1a 53.2bc 56.2b 50.2d Nitrogen levels 57.3a 44.4c 51.7b 54.8b 48.9c 57.2a

0.707c 0.704c 0.721b 0.703c 0.711bc 0.736ab

10,178bc 10,570ab 9,627bcd 11,626a 11,679a 9,714bcd

4254ab 4278ab 3574c 4540a 4156sb 3713bc

17,344ab 15,921bc 12,985de 18,620a 14,896cd 13,507cd

5.02cd 5.45b 5.52b 4.89de 5.56b 5.58b

0.42a 0.40ab 0.41ab 0.43a 0.39ab 0.39ab

0.744ab 0.750a 0.753a 0.729ab 0.737ab 0.710bc

9,215cd 8,520d 9,072cd 9,981bc 8,650d 10,011bc

2031de 1682e 2499d 3393c 2559d 3538e

7,789g 6,275g 10,924ef 14,773cd 8,669fg 13,890cd

5.26bc 5.38b 4.72de 4.59e 5.92a 5.37b

0.31d 0.28d 0.33cd 0.41ab 0.37bc 0.39ab

0.724b 0.732b 0.753a

8,121c 10,028b 12,471a

3148c 3541b 4646a

12,427b 12,555b 13,917a

5.21b 5.26ab 5.56a

0.37b 0.37b 0.41a

† DTF: Days to 50% flowering; DTM: Days to maturity; SPAD value: Leaf chlorophyll index; Fv/Fm: Chlorophyll a fluorescence; HI: Harvest index. ‡ Values within a column followed by the same letter are not significantly different at the 0.05 level by Tukey honest test.

other genotypes. Grain yield increased linearly with increasing N fertilizer regimes. Maximum grain yield was obtained at 90 kg N ha–1, followed by 45 kg N ha–1, and the lowest yield was obtained with 0 kg N ha–1. However, no statistical difference was observed at 0 and 45 kg N ha–1 among genotypes 23012, CSR114/R45, TX3042xTX2737, TX2783, and TX7000. For all N treatments, grain yield was similar among inbred lines B35, SC35, and SC599, and all stay-green genotypes. Among all genotypes, hybrids 26056 and 99480 had the highest positive response to N fertilizer application (Fig. 2c). The effect of genotype × N regime on seed numbers (m–2) was also significant (P < 0.05) (Table 3, Fig. 2d). Maximum seed numbers were obtained at 90 kg N ha–1 compared with 0

and 45 kg N ha–1. In addition, seed numbers were similar at all N rates for Genotypes 95207, CSR114/R45, B35, and SC35, which showed no increase in seed numbers with increasing N regimes (Fig. 3d). There were strong genotypic differences (P < 0.001) for 200seed weight (g) and harvest index (HI) (P < 0.0001) (Table 3) when averaged across N regimes. Two hundred-seed weight ranged from 4.72 to 5.92 g averaged across years and genotypes. Genotype TX430 had the highest 200-seed weight, whereas Genotypes 99480, SC599, and TX2783 had the lowest. The low seed weights among these genotypes were somewhat compensated for, however, by their higher seed numbers m–2 . Harvest index, on the other hand, ranged from 0.28 to 0.43

Fig. 2. Genotype × N interaction effect on (a) leaf chlorophyll index (b) aboveground biomass at maturity (c) grain yield, and (d) grain numbers (m –2) of 12 grain sorghum genotypes grown in 2010 and 2011.

Fig. 3. Year × genotype interaction effects on (a) leaf chlorophyll index, (b) aboveground biomass at maturity, (c) grain yield, and (d) seed numbers (m –2) of 12 grain sorghum genotypes grown in 2010 and 2011.


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among the genotypes. Overall, HI was not statistically different among hybrids or inbred lines. The effect of N fertilizer on HI was significant (P < 0.05). The response of HI to N application was similar in the 0 kg N ha–1 (0.37) and 45 kg N ha–1 (0.37) but significantly higher at 90 kg N ha–1 (0.41). Effect of Year on Genotypes There was significant (P < 0.05) effect of year × genotype interaction for leaf chlorophyll index, total biomass at maturity, seed number (m–2), and grain yield (Fig. 3). Leaf chlorophyll index was significantly higher in 2010 than in 2011 (Fig. 3a); in both years, genotypes B35 and SC35 had the highest leaf chlorophyll index. Similarly, total aboveground biomass at maturity was significantly higher in 2010 than in 2011. When averaged across years, genotypes 26056, 99480, and CSR1114/R45 produced the highest total biomass in 2010; in 2011, genotypes 99480, CSR1114/R45, and B35 had the highest total aboveground biomass. The response of genotypes SC35, SC599, TX430, and TX7000 were similar in both years for total aboveground biomass (Fig. 3b). Grain yield was significantly higher among the genotypes in 2010 than in 2011; however, similar response was observed among genotypes B35, SC35, and TX430 in both years. The hybrids were superior to the inbred lines for grain yield, but inbred lines TX2783 and TX7000 yields were comparable to those of the hybrids (Fig. 3c). A similar response was observed for seed numbers (m–2) in both years (Fig. 3c). Correlations Grain yield poorly correlated with leaf chlorophyll index and chlorophyll a fluorescence (r = 0.20 and r = 0.23, respectively). Grain yield had strong relationships with total aboveground biomass at physiological maturity (r = 0.74) and grain numbers (m–2) (r = 0.93). In addition, grain yield was positively correlated with HI (r = 0.36) but not with 200-seed weight (Fig. 4). Percentage Change There were significant differences in response of genotypes to N levels for total aboveground biomass and grain yield. These responses were quantified as percentage changes in total aboveground biomass or grain yield as N changed from 0 to 45 kg N ha–1 or 0 to 90 kg N ha–1 (Fig. 4). Overall, based on N response, genotypes were divided into three categories: genotypes with percentage change 40%. Based on the criterion indicated above for total aboveground biomass, all genotypes (except TX2783) had percentage change 32°C). The most sensitive stages in sorghum were identified as 10 d before flowering and at flowering (Prasad et al., 2008). Drought during anthesis causes a

drastic reduction in yield and yield components (Hammer and Broad, 2003), which could explain the yield advantage of 2010 over 2011. Seed numbers (m–2), 200-seed weight, and HI were all affected by HT stress. High temperature stress can directly affect seed yield by influencing seed-filling duration and rate, both of which are highly sensitive to HT stress (Prasad et al., 2006a, 2006b). The differences in the seed size observed in this current study might be due to decreased seed-filling duration as a result of the high temperatures (>32°C) during the growing season. Genetic variation for HI has been reported in different crops (Royo and Blanco, 1999; Kumudini et al., 2002). In the present study, HI was not close to the maximum HI of 0.55 that has been reported as a reflection of genetic potential of most current sorghum hybrids (Hammer and Muchow, 1994), perhaps because the genotypes in our study have not yet been fully developed and released for commercial production. Number of seeds (m–2) was the yield component that was most associated with grain yield changes in both years. These results are consistent with those of Bidinger and Raju (2000) in pearl millet [Pennisetum glaucum (L.) R. Br.]; and Craufurd and Peacock (1993) in grain sorghum. This result underlines the role of N use in determination of seed numbers. The significant effect of genotype × N regime interaction on seed numbers concurs with the findings of Bertin and Gallais (2000). Harvest index was also found to be significant and positively correlated with grain yield. CONCLUSIONS Sorghum hybrids and inbred lines varied in their response to N fertilizer. There were significant differences in physiological and yield traits among the genotypes. Overall, the hybrids were superior to the inbred lines for grain yield and total aboveground biomass. Maximum total aboveground biomass, leaf chlorophyll index, and grain yield were obtained


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at 90 kg N ha–1, followed by 45 kg N ha–1, and the lowest yield was obtained at 0 kg N ha–1. No strong relationship was found between chlorophyll index or chlorophyll a fluorescence and grain yield in this set of genotypes, but a strong relationship was found between seed number and grain yield, and total aboveground biomass and grain yield. Across years, application of 45 and 90 kg N ha–1 resulted in an increase in yield of 13 and 48% over 0 kg N ha–1, respectively. Similarly, 45 and 90 kg N ha–1 increased leaf chlorophyll index by 23 and 29% over 0 kg N ha–1, respectively. Leaf chlorophyll index and chlorophyll a fluorescence did not provide the physiological basis for differences in N response among genotypes for yield traits. ACKNOWLEDGMENTS Financial assistance from the former USAID Collaborative Research Support Programs of International Sorghum and Millet (INTSORMIL) and Sustainable Agricultural and Natural Resource Management (SANREM), Kansas Grain Sorghum Commission, and K-State Center for Sorghum Improvement is appreciated. This is contribution no. 14-274-J from the Kansas Agricultural Experiment Station. REFERENCES Addy, S., C.E. Niedziela, Jr., and M.R. Reddy. 2010. Effect of nitrogen fertilizer on stay-green and senescent sorghum hybrids in sand culture. J. Plant Nutr. 33:185–199. doi:10.1080/01904160903434253 Arnall, D.B., A.P. Mallarino, M.D. Ruark, G.E. Varvel, J.B. Solie, M.L. Stone et al. 2013. Relationship between grain crop yield potential and nitrogen response. Agron. J. 105:1335–1344. doi:10.2134/agronj2013.0034 Baumhardt, R.L., J.A. Tolk, and S.R. Winter. 2005. Seeding practices and cultivar maturity effects on simulated dryland grain sorghum yield. Agron. J. 97:935– 942. doi:10.2134/agronj2004.0087 Bertin, P., and A. Gallais. 2000. Genetic variation for nitrogen use efficiency in a set of recombinant maize inbred lines 1. Agrophysiological results. Maydica 45:53–66. Bidinger, F.R., and D.S. Raju. 2000. Mechanisms of adjustment by different pearl millet plant types to varying plant population densities. Crop Sci. 40:68–71. doi:10.2135/cropsci2000.40168x Borrell, A.K., and G.L. Hammer. 2000. Nitrogen dynamics and physiological basis of stay– green in sorghum. Crop Sci. 40:1295–1307. doi:10.2135/ cropsci2000.4051295x Borrell, A.K., G.L. Hammer, and A.C.L. Douglas. 2000a. Does maintaining leaf area in sorghum improve yield under drought? I. Leaf growth and senescence. Crop Sci. 40:1037–1048. doi:10.2135/cropsci2000.4041037x Borrell, A.K., G.L. Hammer, and R.G. Henzell. 2000b. Does maintaining green leaf area in sorghum improve yield under drought? II. Dry matter production and yield. Crop Sci. 40:1037–1048. doi:10.2135/cropsci2000.4041037x Buah, S.S.J., J.M. Kombiok, and L.N. Abatania. 2012. Grain sorghum response to NPK fertilizer in the Guinea savanna of Ghana. J. Crop Improv. 26:101–115. doi:10.1080/15427528.2011.616625 Buah, S.S.J., and S. Mwinkara. 2009. Response of sorghum to nitrogen fertilizer and plant density in Guinea Savanna Zone. Agron. J. 8:124–130. doi:10.3923/ ja.2009.124.130 Cechin, L. 1998. Photosynthesis and chlorophyll fluorescence in two hybrids of sorghum under different nitrogen and water regimes. Photosynthetica 35:233– 240. doi:10.1023/A:1006910823378 Craufurd, P.Q., and J.M. Peacock. 1993. Effect of heat and drought stress on sorghum. II. Grain yield. Exp. Agric. 29:77–86. doi:10.1017/S0014479700020421 Demotes-Mainard, S., and M.H. Jeuffroy. 2004. Effects of nitrogen and radiation on dry matter and nitrogen accumulation in the spike of winter wheat. Field Crops Res. 87:221–233. doi:10.1016/j.fcr.2003.11.014 Dwyer, L.M., A.M. Anderson, B.L. Ma, D.W. Steward, M. Tollenaar, and E. Gregorich. 1995. Quantifying the non-linearity and chlorophyll meter response to corn leaf nitrogen concentration. Can. J. Plant Sci. 75:179–182. doi:10.4141/ cjps95-030 Hammer, G.L., and I.J. Broad. 2003. Genotype and environment effects on dynamics of harvest index during grain filling in sorghum. Agron. J. 95:199–206. doi:10.2134/agronj2003.0199

1630

Hammer, G.L., and R.C. Muchow. 1994. Assessing climatic risk to sorghum production in water-limiting sub-tropical environment: Development and testing of a simulation model. Field Crops Res. 36:221–234. doi:10.1016/0378-4290(94)90114-7 Hartley, H.O. 1950. The use of range in analysis of variance. Biometrika 37:271–280. doi:10.1093/biomet/37.3-4.271 Jamieson, P.D., and M.A. Semenov. 2000. Modeling nitrogen uptake and redistribution in wheat. Field Crops Res. 68:21–29. doi:10.1016/ S0378-4290(00)00103-9 Kaizzi, K.C., J. Baylebeka, O. Semalulu, I. Alou, W. Zimwanguyizza, A. Nasamba et al. 2012. Sorghum response to fertilizer and nitrogen use efficiency in Uganda. Agron. J. 104:83–90. doi:10.2134/agronj2011.0182 Kumudini, S., D.J. Hume, and G. Chu. 2002. Genetic improvement in short-season soybeans (Glycine max L.): II. Nitrogen accumulation, remobilization, and partitioning. Crop Sci. 42:141–145. doi:10.2135/cropsci2002.0141 Ma, B.L., K.D. Subedi, D.W. Stewart, and L.M. Dwyer. 2006. Dry matter accumulation and silage moisture changes after silking in leafy and dual purpose corn hybrids. Agron. J. 98:922–929. doi:10.2134/agronj2005.0299 Minolta. 1989. SPAD-502 owner’s manual. Industrial Meter Div. Minolta Corp., Ramsey, NJ. Mousavi, S.G.R., M.J. Seghatoleslami, and R. Arefi. 2012. Effect of N fertilization and plant density on yield and yield components of grain sorghum under climatic conditions of Sistan, Iran. J. Plant Ecophysiol. 2:141–149. Muchow, R.C., and T.R. Sinclair. 1994. Nitrogen response of leaf photosynthesis and canopy radiation use efficiency in field-grown maize and sorghum. Crop Sci. 34:721–727. doi:10.2135/cropsci1994.0011183X003400030022x Mullen, R.W., K.W. Freeman, W.R. Raun, G.V. Johnson, M.L. Stone, and J.B. Solie. 2003. Identifying an in-season response index and the potential to increase wheat yield with nitrogen. Agron. J. 95:347–351. doi:10.2134/ agronj2003.0347 Mutava, R.N., P.V.V. Prasad, M.R. Tuinstra, K.D. Kofoid, and J. Yu. 2011. Characterization of sorghum genotypes for traits related to drought tolerance. Field Crops Res. 123:10–18. doi:10.1016/j.fcr.2011.04.006 Pheloung, P.C., and K.H.M. Siddique. 1991. Contribution of stem dry matter to grain yield in wheat cultivars. Aust. J. Plant Physiol. 18:53–64. doi:10.1071/ PP9910053 Piekkielek, W.P., and R.H. Fox. 1992. Use of a chlorophyll meter to predict side dress nitrogen requirements for maize. Agron. J. 84:59–65. doi:10.2134/agronj199 2.00021962008400010013x Prasad, P.V.V., K.J. Boote, and L.H. Allen, Jr. 2006a. Adverse high temperature effects on pollen viability, seed-set, seed yield, and harvest index of grain sorghum are more severe at elevated carbon dioxide due to higher tissue temperatures. Agric. For. Meteorol. 139:237–251. doi:10.1016/j.agrformet.2006.07.003 Prasad, P.V.V., K.J. Boote, L.H. Jr. Allen, J.E. Sheehy, and J.M.G. Thomas. 2006b. Species, ecotype and cultivar differences in spikelet fertility and harvest index of rice in response to high temperature stress. Field Crops Res. 95:398–411. doi:10.1016/j.fcr.2005.04.008 Prasad, P.V.V., S.R. Pisipati, R.N. Mutava, and M.R. Tuinstra. 2008. Sensitivity of grain sorghum to high temperature stress during reproductive development. Crop Sci. 48:1911–1917. doi:10.2135/cropsci2008.01.0036 Ristic, Z., U. Bukovick, and P.V.V. Prasad. 2007. Correlation between heat stability of thylakoid membranes and loss of chlorophyll in winter wheat under heat stress. Crop Sci. 47:2067–2073. doi:10.2135/cropsci2006.10.0674 Royo, C., and R. Blanco. 1999. Growth analysis of five spring and five winter triticale genotypes. Agron. J. 91:305–311. doi:10.2134/agronj1999.00021962009100 020020x SAS Institute. 2003. SAS propriety software release 9.1 for Windows. SAS Inst., Cary, NC. Schepers, J.S., D.D. Francis, M. Vigil, and F.E. Below. 1992. Comparison of corn leaf nitrogen concentration and chlorophyll meter readings. Commun. Soil Sci. Plant Anal. 23:2173–2187. doi:10.1080/00103629209368733 Thomas, H., and C.M. Smart. 1993. Crops that stay green. Ann. Appl. Biol. 123:193–219. doi:10.1111/j.1744-7348.1993.tb04086.x Tribio, E., and A.M. Tribio-Blondel. 2002. Productivity and grain or seed composition: A new approach to an old problem-Invited paper. Eur. J. Agron. 16:136–186. Vanderlip, R.L. 1993. How a sorghum plant develops. Kansas State Univ., Manhattan. van Oosterom, E.J., S.C. Chapman, A.K. Borrell, I.J. Broad, and G.L. Hammer. 2010. Functional dynamics of the nitrogen balance of sorghum. II. Grain filling period. Field Crops Res. 115:29–38. doi:10.1016/j.fcr.2009.09.019 Wortmann, C.S., M. Mamo, and A. Dobermann. 2007. Nitrogen response of grain sorghum in rotation with soybean. Agron. J. 99:808–813. doi:10.2134/ agronj2006.0164
