Effects of irrigation and nitrogen management on hybrid maize seed production in north-west China

Scienti ﬁ c irrigation and nitrogen management is important for agricultural production in arid areas. To quantify the effect of water and nitrogen management on yield components, biomass partitioning and harvest index ( HI ) of maize for seed production with plastic ﬁ lm-mulching, ﬁ eld experiments including different irrigation and N treatments were conducted in arid north-west China in 2013 and 2014. The results indicated that kernel number per plant ( KN ) was signi ﬁ cantly affected by irrigation and N treatments. However, 100-kernel weight was relatively stable. Reducing irrigation quantity signi ﬁ cantly increased stem partitioning index ( PI stem ) and leaf partitioning index ( PI leaf ), and decreased ear partitioning index ( PI ear ) at harvest, but lowering N rate (from 500 to 100 kg N $ hm – 2 ) did not signi ﬁ cantly reduce PI stem , PI leaf , and PI ear at harvest. HI was signi ﬁ cantly reduced by reducing irrigation quantity, but not by reducing N rate. Linear relationships were found between KN , PI stem , PI leaf , PI ear at harvest and HI and evapotranspiration ( ET ).


Introduction
Irrigation and nitrogen management is the most critical element of agricultural production in arid areas [1][2][3][4][5] . Scientific and rational management of irrigation and N is important for the high yield and high efficiency of agriculture in arid areas. The economic yield of crops is closely related to the changes in yield components, biomass partitioning and harvest index (HI). Quantifying the response of yield components, biomass partitioning and HI to different water and N treatments cannot only provide available parameters for crop yield modeling, but also provide the scientific basis for crop irrigation and N management.
Kernel number per plant (KN) and 100-kernel weight (KW) are two major components of yield. There are many reports about the effects of water and N on yield components of maize [6][7][8][9][10] . However, the effects of water and N on yield components vary with its growing environment, thus studying crop response to water and N for specific climate, soil conditions, and agronomic practice is still needed. In addition, previous studies have focused on maize yield, but there have been relatively few reports on effects of irrigation and N management on yield components of maize for seed production.
Biomass partitioning is closely related to crop cultivar and environmental factors [11][12][13][14] . However, there are many studies on biomass partitioning, which mostly concentrated on the root-shoot ratio. It was found that soil water deficit significantly reduces shoot dry mass in maize but only reduces root dry mass slightly, thus it increases rootshoot ratio [15] . Other studies indicated that under drought conditions, crop growth rate and biomass production are reduced to decrease water consumption, and more biomass is transferred to the roots to maintain a higher root-shoot ratio [16][17][18][19][20][21][22][23][24][25] . It was also showed that the proportion of root dry mass does not increase under drought conditions [26] . Plant biomass partitioning has mostly been analyzed using aboveground and belowground parts, but aboveground measurements of biomass partitioning into different organs (stem, leaf and ear) are also needed [13] . In addition, there are relatively few examples of continuous measurements of changes of biomass partitioning over the whole growth period.
HI, the ratio of economic yield and biomass, reflects the capacity of crop photosynthate to be converted to economic product, and is an important index in evaluating the yield of crop cultivars and cultivation practices. It was found that water stress significantly decreases HI in maize, even reducing it to zero [27] . Other studies likewise indicated that water stress reduces HI [28][29][30] . However, proper reduction of irrigation quantity can increase HI and N rate had no effect on HI [9,15,[31][32][33][34] , although N deficit decreases HI [35] . Despite the many studies on HI, the response of HI to different irrigation and N treatments is still poorly understood.
Thus a two-year field experiment on yield components, biomass partitioning and HI in maize for seed production with plastic film-mulching under different irrigation and N treatments was conducted in arid north-west China. The objectives of this study were to (1) quantify the response of yield components, biomass partitioning and HI in maize for seed production under different irrigation and N treatments, and (2) develop relationships between yield components, biomass partitioning and HI and evapotranspiration (ET) under different N rates. The latter was to provide parameters for crop yield modeling and a scientific basis for irrigation and N management of maize under these growing conditions.

Experimental site and description
Field experiments were conducted at Shiyanghe Experimental Station of China Agricultural University, located near Wuwei, in Gansu Province, China (37°52′ N, 102°50′ E, 1581 m) during April to September, 2013 and 2014. The experimental site is in an inland arid desert climate zone where light and heat resources are abundant, with mean annual duration of sunshine of over 3000 h, mean frost free days of over 150 days, mean annual temperature of 8°C and annual accumulated temperature ( > 0°C) of 3550°C. The site had limited water resources, with annual precipitation of 164 mm, mean annual pan evaporation approximate of 2000 mm, and the groundwater table was below 25 m [36] . The soil texture was a light sandy loam, with mean soil dry bulk density of 1.40 g$cm -3 , mean field capacity (FC) 0.30 cm 3 $cm -3 and mean permanent wilting point 0.10 cm 3 $cm -3 for the 0-100 cm layers.

Experimental methods
In 2013, three irrigation treatments, namely 65-70 (W1), 55-60 (W2) and 45%-50% FC (W3), and three nitrogen (N) treatments, namely 500 (local N rate, N500), 400 (N400) and 300 kg N$hm -2 (N300), totaling 9 treatments were applied. Maize (Zea mays cv. Funong340) was sown on 20 April and harvested on 11 September. In 2014, the experimental design was adjusted in response to the results in 2013. Three irrigation treatments, namely 65-70 (W1), 55-60 (W2) and 45%-50% FC (W3), and three N treatments, namely 500 (N500), 300 (N300) and 100 kg N$hm -2 (N100), totaling 9 treatments were applied. Maize was sown on 15 April and harvested on 20 September. The irrigation method was border irrigation. Irrigation treatment was controlled according to the lower limit of FC. In each irrigation treatment, water was applied to FC when soil water content reached the controlled lower limit. Two experiments were conducted in a randomized complete block design with three replicates. N fertilizer was applied as urea (46% N). Phosphorus (P 2 O 5 ) and potassium (K 2 O) fertilizers were applied at 240 and 50 kg$hm -2 , respectively, in both years. Fertilizer application time and method were similar for all treatments. Maize was planted with one line of male plants and five lines of female plants with plastic film-mulching. Plant spacing was 0.25 m, row spacing was 0.4 m, and planting density was 100000 plants per hectare. Female plants were manually emasculated before flowering. Except for irrigation and N fertilizer, other farming measures were similar for each treatment.
Plot area was 86.8 (12.4 Â 7.0) m 2 and the plots were separated by ridges (0.3 m wide and 0.5 m high) with 1 m wide strips around the inside of each plot as the protected area. Each plot was divided into three sub-plots, and one sub-plot was used for biomass destructive sampling and the other two sub-plots for the measurements of soil water content, and yield components.

Meteorological data
The meteorological data including precipitation (P), solar radiation (Rs), air temperature (T), wind speed at 2 m aboveground (u 2 ) and relative humidity (RH) during the whole growth period, were continuously measured by a standard automatic weather station (Hobo, Onset Computer Corporation, Cape Cod, Massachusetts, USA) about 100 m away from the experimental field (Table 1). The data were taken at 5 s interval, and 15 min averages were calculated and recorded using a data logger. The reference Note: Rs, solar radiation; T, air temperature; RH, relative humidity; P, precipitation; ET 0 , reference evapotranspiration.

Soil moisture content
In each plot, two TRIME tubes were installed to allow measurement of moisture content using a time domain reflectometer (TRIME-PICO-IPH, IMKO, Ettlingen, Germany). Measurements were made at the depths of 20, 40, 60, 80 and 100 cm every 7 days, and the mean soil water content over the 0-100 cm depths was used to determine irrigation time. The reflectometer measurements were calibrated gravimetrically. Irrigation quantity was determined by the difference of actual soil water content and field capacity ( Table 2).

Actual crop evapotranspiration
ET for each treatment was calculated as: where ET is crop evapotranspiration, ΔW the change in soil water storage between two soil moisture content measurements, I irrigation water applied during the growth period, P precipitation, S g the capillary rise from the lower soil layer to the crop root zone, D the amount of drainage water, and R f the amount of runoff. S g was ignored due to the deeper water table in this area. R f was zero due to the basin irrigation system. D was ignored because the upper limit of irrigation was field capacity.

Yield components
At the end of each season, 20 female plants were randomly chosen from each plot and harvested for KN and KW. Grains were first dried at 105°C for 30 min, and then dried at 60-70°C to constant mass. One hundred grains were randomly chosen from each plot and weighed to give KW. KN was obtained from grain yield per plant divided by the individual seed weight calculated from KW.

Biomass and its partitioning index
Three plants from each plot were cut at ground level every 10 to 20 days to determine aboveground biomass production, and biomass partitioning into stem (including stems and sheaths), leaf (including green leaves and dead leaves) and ear (including peel, core axis and grain). Each part was separately dried at 105°C for 30 min, and then dried at 60-70°C to constant mass. Stem partitioning index (PI stem ), leaf partitioning index (PI leaf ) and ear partitioning index (PI ear ) were calculated as follows: where S mass is stem dry mass (g), L mass leaf dry mass (g), and E mass ear dry mass (g).

Harvest index
At the end of each season, 20 female plants were randomly chosen from each plot and harvested for grain yield. Grains were first dried at 105°C for 30 min, and then dried at 60-70°C to constant mass. The final grain yield was expressed on the basis of water content of 13%. HI (%) was calculated as: where Y is yield (t$hm -2 ) and B final aboveground biomass (t$hm -2 ).

Statistical analysis
Analysis of variance (ANOVA) was performed using the general linear model (univariate procedure) from SPSS 21.0 software (IBM SPSS Statistics, USA). ANOVAs were done with irrigation and N fertilizer as the main effects and including their interactions. All the treatment means were compared for any significant differences using the Duncan's multiple range tests at significance level of P£0.05. Regression analyses were performed using Microsoft Excel. There was more rainfall in 2014 than 2013, and experimental design was slightly different for 2 years, resulting in a differential seasonal response. Therefore, the effects of different irrigation and N treatments on maize for seed for 2013 and 2014 were analyzed separately.

Results and discussion
3.  (Table 3). Compared with N500 in 2014, N300 and N100 reduced KN by 4.4% and 15.5%, respectively. Therefore, lowering irrigation quantity significantly reduced KN, while reducing N rate had less effect on KN, which was similar to the effect on yield. In addition, the interaction of irrigation and N had no significant effect on KN in both years ( Table 3).
The effect of irrigation quantity on KW was significant (P < 0.05) in 2013, but not significant in 2014 (Table 3). Compared with W1 in 2013, W2 increased the KW by 3.5%, but W3 reduced it by 9%, respectively. Compared with W1 in 2014, W2 and W3 reduced the KW by 2.1% and 3.2%, respectively. The effect of N rate on KW was not significant in either year ( Table 3), indicating that increasing N rate did not improve the KW. In addition, the interaction of irrigation and N had no significant effect on KN in either year (Table 3). Jia et al. indicated that a low irrigation quantity (263 mm) significantly decreases KW and number in maize. A low N rate (100 kg$hm -2 ) decreased kernel weight significantly, but affected KN depending on irrigation quantity and cultivar [10] . Nesmith and Ritchie showed that water deficit decreased the yield because water deficit reduced the number of well-developed kernels before the blossom stage [7] . Claassen and Shaw found that water stress reduced KN before silking and pollination stages, and it reduced KW at or after silking and pollination stage [6] . In this study, high irrigation quantity gave the highest KN, but lowering irrigation quantity decreased KN markedly and KW slightly. A possible reason is that severe drought affected the quantity and activity of pollen in male plants before anthesis, leading to a reduction in the number of well-developed kernels infemale plants, thus reducing KN. As well-developed kernels were relatively few in the reproductive growth stage, the female plants were basically able to provide enough carbohydrates to meet the needs of kernel development, even under water stress, so lowering irrigation quantity affected KW only slightly. In this study, low N rate had satisfactory KN and KW of maize for seed and high N rate only affected theses lightly, possibly because of high N fertilization in the arid Hexi Corridor region of north-west China. Pandey et al. and Moser et al. showed that reducing the quantity of irrigation and N decreased KN and KW of maize significantly, and it reduces KN more significantly than KW [8,9] . Others studies have also shown that water stress reduces KN of maize significantly, which is similar to our study [6,38] .
The KN of W1N500 in 2013 and 2014 were 156 and 213 kernels per plant, respectively. Generally, lowering irrigation quantity strongly reduced KN. However, compared with W1N500, W1N300 in 2013 had a lower KN but it was not a significant decrease (Table 3). Likewise, compared with W1N500, W1N100 in 2014 had a lower the KN, but it was not a significant decrease (Table 3). Thus low N rates (100-300 kg$hm -2 ) could largely maintain KN of maize for seed when soil moisture content is above 65%-70% FC in this region.

Biomass partitioning
PI stem showed a single peak, reaching the maximum (about 0.7) about 80 days after sowing. PI stem was not significantly different between different treatments at the early growth stage, but it was significantly different during later growth stage (Fig. 1a, Fig. 1d). PI stem in different treatments at harvest ranged from 0.321 to 0.470 and 0.325 to 0.441 in 2013 and 2014, respectively ( Table 4). The effect of irrigation quantity on PI stem at harvest was significant in both years (Table 4). In 2013, compared to W1, W2 and W3 increased PI stem at harvest by -9.2% and -29.3%, respectively. In 2014, compared to W1, W2 and W3 increased PI stem at harvest by 4.5% and 18.4%, respectively. However, the effects and interaction of N rate were not significant in either year (Table 4).
PI leaf decreased with the advance of growth stage (Fig. 1b, Fig. 1e). PI leaf under different irrigation and N treatments at harvest ranged from 0.160 to 0.214 and 0.137 to 0.165 in 2013 and 2014, respectively ( Table 4). The effect of irrigation quantity on PI leaf at harvest was significant in both years (Table 4). In 2013, compared to W1, W2 and W3 increased PI leaf at harvest by 10.4% and 19.8%, respectively. In 2014, compared to W1, W2 and W3 increased PI leaf at harvest by 7.5% and 15.6%, respectively. However, the effects and interaction of N rate were not significant in either year (Table 4).
PI ear increased after the flowering stage (Fig. 1c,  Fig. 1f). PI ear under different irrigation and N treatments at harvest ranged from 0.326 to 0.519 and 0.394 to 0.530 in 2013 and 2014, respectively ( Table 4). The effect of irrigation quantity on PI ear at harvest was significant in both years (Table 4). In 2013, compared to W1, W2 and W3 decreased PI ear at harvest by 10.1% and 27.9%, respectively. In 2014, compared to W1, W2 and W3 decreased PI ear at harvest by 14.7% and 26.5%, respectively. However, the effects and interaction of N rate were not significant in either year (Table 4).
Thus PI stem and PI leaf increased significantly but PI ear decreased significantly with the decrease in irrigation quantity, indicating that maize for seed generally allocated more assimilates to stems and leaves and fewer assimilates to ears under drought conditions. However, PI stem , PI leaf and PI ear did not significantly change with the decrease of N input, indicating that for a certain range of N rates (from 500 to 100 kg$hm -2 ), lowering N input had no effect on assimilate allocation among aboveground organs of maize for seed. According to source-sink theory, lower assimilate allocated to the ear can result from lower source strength, transport capacity or sink strength. Marcelis et al. indicated that sink strength is the important factor determining biomass partitioning in the whole plant [39] . For cereals, grain is the most important sink organ after flowering [40] . Thus lower sink strength can lead to lower assimilate allocation to ear, and reducing irrigation quantity decreased sink strength, and increasing N input did not increase sink strength in this study.

Harvest index
The effect of irrigation quantity on HI was extremely significant (P < 0.01) in both years (Table 3). In 2013, compared to W1, W2 and W3 reduced HI by 22.4% and 54.4%, respectively. In 2014, compared to W1, W2 and W3 reduced HI by 11.6% and 38.2%, respectively. The effect of N rate on HI was not significant in either year (Table 3). Although the effect of N rate on HI was not significant, HI was reduced more significantly when N rate dropped from 300 to 100 kg$hm -2 . In addition, the interaction of irrigation quantity and N rate had no significant effect on HI in either year (Table 3).
Farré and Faci found that HI of maize decreased significantly with increased of water stress, ranging from 0.51 to 0.03, indicating that HI is very sensitive to the irrigation quantity [28] . Other studies have also shown that water stress reduces HI [29,30] , which was similar to our findings on maize for seed production. However, Kang et al. showed that water deficit at the seedling and elongation stages increased HI but reduced aboveground biomass significantly with lower yield loss in the semi-arid Loess Plateau of north-west China [15] . Zhang et al. conducted a field experiment with winter wheat, involving six irrigation treatments (from 0 to 5 irrigation applications) in the North China Plain for 6 years, and found that HI decreased with increased water supply, and only in very dry seasons (seasonal rainfall was less than 80 mm) was the HI of the rain-fed treatment reduced [32] . It also concluded that reducing irrigation quantity increases HI of winter wheat in northern China, because grains are filled more quickly than those of well-watered controls and less assimilate remained in the temporary storage organs stem and sheath [31] . These results are different from maize for seed production in this study. There are two possible reasons for this. Firstly, the previous studies were conducted in the semi arid or humid climate with mean annual rainfall of 584, 400-600 and 600 mm, respectively [15,31,32] , so HI is less dependent on irrigation. However, our study was conducted in extremely dry conditions with mean annual rainfall of 164 mm, so HI Fig. 1 Variations of stem partitioning index (PI stem ), leaf partitioning index (PI leaf ) and ear partitioning index (PI ear ) of maize for seed production against days after planting (DAP) in 2013 (a-c) and 2014 (d-f) might be more sensitive to irrigation. Secondly, HI of maize for seed production is controlled by both male and female plants, so water deficit may affect the quantity of pollen in the male plants and the activity of filaments in female plants, which can lead to lower HI. But the effects of different water and N treatments on the number of pollen grains in the male plants and the activity of filaments in female plants are unclear and need further study.
In contrast, N application had no effect on HI of tropical maize [9,34] and temperate maize [33] . In this study, N rate also had no significant effect on HI. This could have been related to high initial soil N content and high N rate. However, Pandey et al. found that N stress decreased HI of maize in the Sahelian climate, and Hammad et al. found similar result [35,41] . In this study, HI decreased more significantly when N rate was reduced from 300 to 100 kg$hm -2 , indicating that further reducing N input may significantly reduce HI.

Correlation of growth measures and evapotranspiration
KN had a clear linear relationship with ET (Fig. 2a), which was similar to the result of Otegui et al. [42] . However, no significant correlation was found between KW and ET (data not shown), since KW did not change under the different irrigation and N treatments.
HI had a weak linear relationship with ET under different water and N treatments (Fig. 2b), indicating that HI could be related to not only total ET over the whole growth period, but also ET at different growth stages. Kang et al. found a quadratic relationship between HI and ET in winter wheat (HI = -5Â10 -6 ET 2 + 0.0034 ET-0.2352, R 2 = 0.4763), which contrasts with our result [43] . This may be related to the different climatic conditions and crop species in their study.
PI stem , PI leaf and PI ear at harvest showed a linear relationship with ET (Fig. 2c, Fig. 2d, Fig.2e), which can provide basis for modeling the partitioning of biomass among different aboveground organs.

Conclusions
Lowering irrigation quantity significantly reduced KN, and reducing nitrogen fertilizer rate also decreased KN. However, KW was relatively stable under different Table 4 Effect of irrigation quantity and N rate management on stem partitioning index (PI stem ), leaf partitioning index (PI leaf ) and ear partitioning index (PI ear ) of maize for seed production with film-mulching at harvest in 2013 and 2014 Note: Means within each column followed by different letters are statistically different at P < 0.05. NS, not significant; ***, significance at P < 0.001; **, significance at P < 0.01;*, significance at P < 0.05.
irrigation and N treatments. Lowering irrigation quantity significantly increased PI stem and PI leaf , and decreased PI ear at harvest, but lowering N input (from 500 to 100 kg N$hm -2 ) did not significantly reduce PI stem , PI leaf , and PI ear at harvest. HI was significantly reduced by lowering irrigation quantity, while it was not significantly reduced by lowering N input. Linear relationships were found between KN, HI, PI stem , PI leaf , and PI ear at harvest and ET.