Materials and methods
Materials
Spring wheat (Triticum aestivum ) is the main grain crop in semi-arid areas of Northwest China. Monkhead, the local variety, was widely grown in 1940s. Modern 92-46 was released in 2000 and was widely grown at the beginning of 21 century. The two varieties have similar phenology, but Monkhead is awnless and highly tillering, thus can be easily distinguished from 92-46 when harvested (Zhu and Zhang 2013).
Methods
We conducted a pot experiment in the greenhouse in Beijing Normal University from May until August in 2011. A randomized block design was used with three factors including root partition (two levels: plastic partition and mesh partition, Fig. 1), soil fertility (two levels: low fertility and high fertility) and varieties (two levels: Monkhead and 92-46). All experimental treatments were repeated 20 times with a total of 160 pots.
To set up partitions, we firstly put two nylon net bags (aperture size 20μ) filled with vermiculite into one pot (made by PVC pipes, 16cm diameter and 48cm long) which were equally separated by a plastic composite panel. Secondly, in mesh partition treatments, we removed the plastic composite panels, which permitted the movement of soil nutrients and root exudates from one side to the other while preventing direct contact between roots of the two individuals. In plastic partitions, the edges between the pots and the partitions were sealed with silicon (Fig. 1). In either plastic partition or mesh partition, all individuals have access to half volume of the pot, which enabled us to eliminate the effect of soil volume which might be confounded with effects of detection of neighbours (Semchenko et al. 2007). In low fertility treatments, pots were watered with nutrient solution (0.2 g L-1 Peter’s professional) until saturation every 14 days (1.6 L every 14 days), while in high fertility treatments, pots were watered every day (0.4 L every day).
All seeds were vernalized at 4 oC for one day, germinated at 25 oC. Two seedlings were transplanted into the pot with each in the centre of each half. Spike, stem&leaf and root were harvested at maturity for each individual plant. Roots were separated from the substrate and cleaned with tap water. All samples were dried at 65 oC to a constant weight and weighed. Seeds of each plant peeled out from ears were counted and weighed.
Statistical analyses
We used a three-way ANOVA to test for the effects of partition treatments, wheat varieties and soil fertility on tiller number and plant biomass. Differences between treatments were determined by running SNK multiple comparisons. ANOVA was conducted with SPSS statistical software (version 17.0; SPSS Inc., Chicago, IL, USA).
We compared static allometric relationships between treatments using standardized major axis regression (SMA), in which whole-individual biomass was taken as the independent variable, and biomass of different parts (i.e. root, stem&leaf and seed biomass) as the dependent variables (Müller et al. 2000; Weiner 2004). Given that biomass variables are interdependent, the Model Type II regression was used to reduce measurement errors associated with allometric data (Henry and Aarssen 1999; Niklas 2006). The slopes of regression lines were compared by running multiple post-hoc multiple comparisons. SMA analyses were conducted using the Standardized Major Axis Tests and Routines (SMATR) 2.0 (Warton et al. 2006). Biomass data were log-transformed before analysis when necessary to improve normality of residuals and homogeneity of variances.