Grain yield and associated photosynthesis characteristics during dryland winter wheat cultivar replacement since 1940 on the Loess Plateau as affected by seeding rate

R E G U L A R A R T I C L E *Corresponding author: Suiqi Zhang, State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling 712100, China. E-mail: sqzhang@ms.iswc.ac.cn Received: 01 August 2016; Revised: 10 December 2016; Accepted: 18 December 2016; Published Online: 02 January 2017 Sun, et al.: Photosynthesis improvement impacted the wheat yield increase 52 Emir. J. Food Agric ● Vol 29 ● Issue 1 ● 2017 growth (Gooding et al., 2002) and lodging (Easson et al., 1993). No marked differences are reported among varieties regarding the responses of grain yields to variations in plant density (Arduini et al., 2006; Brian et al., 2011). However, Black and Aase (1982) reported that USSR winter wheat cultivars were more capable of maintaining a high number of kernels per spike at a high plant density than USA cultivars. According to Marshall and Ohm (1987) and Anderson and Barclay (1991), optimal plant populations change according to their variety and local conditions. In Italy, narrow row spacings of between 12 and 18 cm and seeding densities of approximately 400 seeds m-2 are traditionally used for wheat (Arduini et al., 2006). In contrast, densities of approximately 250 seeds m-2 or less are considered optimal in the USA and China (Carr et al., 2003; Fang et al., 2010). The response of grain yields to seeding rate with cultivar replacement requires further research. At least six wheat cultivar replacements have been grown on the Loess Plateau since 1940s (Chen et al., 2003). Although increases in production slowed in the early 2000s (Zhang et al., 2009), significant genetic gains have been observed (Chen et al., 2003). According to a previous study in this region, greater wheat yields were strongly and positively correlated with increasing grain weights, which resulted from an increased filling rate (Zhang et al., 2008). The photosynthetic capacity per unit of leaf area after elongation is also an important source of this progress (Chen et al., 2012). Although different cultivars might vary differently when different seeding rates are used, it has not been considered. Thus, seven cultivars released since 1940 were studied using different seeding rates (1) to explore the evolution of wheat grain yields and agronomic traits with their responses to seeding rate and (2) to identify the correlations between photosynthetic characteristics and yield progress under different seeding rates. MATERIALS AND METHODS Plant materials, growth conditions and meteorological conditions Seven wheat cultivars released from 1940 to 2004 and once planted on the Loess Plateau (Table 1) were used. Field experiments were conducted on the Loess Plateau in Changwu (107°40′ to 107°42 E, 35°12′ to 35°16′N, 1200 m asl), Shaanxi Province, China, during the growing season from 2011 to 2012. The site was located in a semihumid area of the Loess Plateau, where rain-fed cropping with one harvest per year is standard practice. The soil is a dark loessial soil locally classified as a Heilu soil. The average annual precipitation (rain and snow) in the area is 584 mm, and mainly occurs from July to September. The precipitation during the experimental year (Fig. 1) was recorded at an automated meteorological station on the site. In the experimental year, the total precipitation was 388.4 mm during the fallow period (July-September) and 278.4 mm during the growing season (September-June of the next year). Experimental design The experiment was conducted using a randomized complete block with a split-plot design and three blocks, and the plot size was 12 m2 (14 rows, 4 m long and 20 cm row spacing). The main plots consisted of the different seeding rates [low: 100 seeds m-2, medium: 250 seeds m-2 (the value adopt by the local farmers), and high: 350 seeds m-2]. The subplots consisted of the seven cultivars used (Table 1). Fertilizer was applied [urea (N) 150 kg ha-1 and calcium superphosphate (P2O5) 120 kg ha -1] before planting, and no additional fertilizer was applied before harvest. Fungicides and pesticides were applied in each treatment at the shooting and grain filling stages to prevent attack by diseases and pests. Plant sampling and observations At maturity, four central rows (1 m long) were harvested, air-dried and weighed to determine the total aboveground biomass, grain yield and grain number per m2. Subsamples were used to record the height, grain number per spike and thousand-grain weight (TGW). The photosynthetic traits were recorded 3 days after flowering. The photosynthetic rate (Pn) of the flag leaf was measured between 9:30 and 11:30 AM using a LI-6400 portable photosynthesis system (LI-COR Inc., Lincoln, NE, USA) in every plot. These measurements were performed approximately halfway along the length of the flag leaf, which was exposed to full sunlight. The Pn values were calculated as the sum of the mean readings for five leaves per plot. The canopy characteristics were recorded using an LAI-2200 Plant Canopy Analyzer (LI-COR Inc., Lincoln, NE, USA) without direct sunlight. One above-canopy measurement and three below-canopy measurements at the soil surface were taken in each plot. The leaf area index (LAI), mean tilt angle (MTA) and diffuse non-interceptance (DIFN) were measured. Statistical analyses Analysis of variance tests were conducted using SAS V8.0 (SAS Institute, Inc., University of Texas, Arlington) and the mono factor analysis of variance method. Linear correlations between the phenotypic traits and the yield Sun, et al.: Photosynthesis improvement impacted the wheat yield increase Emir. J. Food Agric ● Vol 29 ● Issue 1 ● 2017 53 elements were determined using Pearson’s test in the SPSS 19.0 software (IBM, Armonk, New York). The absolute (1) (grain yield gains in mega-grams per hectare per year) or exponential (2) (the percentage grain yield gain per year) genetic gains of the grain yield and the related traits were modeled using the following equation: yi = a + bxi + u (1) or ln(yi) = a + bxi+ u (2) Where yi is the estimated mean grain yield in each trial of cultivar i, ln(yi) is the natural log of yi, and xi is the year in which cultivar i was released. The intercepts of both equations were estimated by a, and the slope (b) was used to measure the absolute or exponential grain yield gains (percent). The residual error was estimated by u (OrtizMonasterio et al., 1997). RESULTS Genetic improvements in yield with different seeding rates Consistent genetic gains were achieved for every seeding rate during the growing season (Table 2), with the medium rate producing the highest genetic gain (1.29%, R2=0.81, P<0.01). The medium seeding rate (local farmers’ selection) was always the best seeding rate, and only the cultivar from 2004 resulted in a greater yield as the seeding rate increased. The coefficient of variation (CV) significantly decreased with cultivar replacement. Contributions of agronomic traits to yield and genetic improvements The plant height at maturity decreased significantly with cultivar replacement (RLow=0.76, P<0.05; R 2 Medium=0.82, P<0.01; RHigh=0.84, P<0.01, respectively) in every Table 1: Representative cultivars of dryland winter wheat on the Loess Plateau from 1940 to 2004 Cultivars Planting decade on the Loess Plateau Year of release Pedigree Breeding sites Dwarf genes Mazha 1940s 1940 Landrace Shaanxi Province none Bima1 1950s 1951 Mazha/Biyu Shaanxi Province none Fengchan3 1960s 1964 Danmai 1/Xinong 6028×Bima1 Shaanxi Province none Hanxuan 1