Responses of dry matter distribution and water use of summer maize (Zea mays L.) to intercropped cultivars competition on the Loess Plateau of China

On the Loess Plateau, maize morphological structure and yield performance are restricted by low rainfall and limited soil nutrient. Resource competition in intercroped cultivation have a postive effect on individual establishment and biomass allocation of maize cultivars. It was necessary to research root morphology and biomass allocation of maize under the combined effect of rainfall, planting density and intercropping for exploring effects of intercropping models on grain yield and water use efficiency (WUE) of maize in the Loess Plateau. To this end, a field experiment was conducted at Changwu Agri-ecological Station in the Loess Plateau, Chinese Academy of Science which is located in the classic dry farmming region in the semi-arid region of the Loess Plateau in Northwest China. Two maize cultivars ('Z958' and 'S16a') were intercroped under two planting densities (45 000 plants·hm^-2 and 75 000 plants·hm^-2) with 1:1 lines ratio. The aboveground and belowground growth and biomass accumulation were investigated at different growth stages of maize in tow rainfall year types, 2011 with rainfall of 647 mm and 2012 with rainfall 497 mm. The study also measured and analyzed the correlation among biomass allocation, root distribution, grain yield and WUE. The results showed that:1) soil water deficiency had a negative effect on root surface area (SA) of 'S16a' at flowering stage under low intercropped planting density. Also while SA of 'Z958' decreased by 30.5% under high intercropping density, WUE increased with increasing intercropping density. After two years of experimentation, root length density (RLD) in the 0-20 cm soil layer significantly increased under 'Z958' and 'S16a' intercropping. Also with increasing planting denstiy, rainwater deficiency led to deeper root growth to enhance water uptake. This in turn enhanced root competition for water and eventually obviously increased RLD in the 30-40 cm soil layer. Root distribution of 'Z958' was higher than that of 'S16a' for the two planting densities. 2) Biomass accumulation under two cultivars intercropping was genotypically different, with enhanced 'Z958' growth during vegetative period and enhanced 'S16a' growth during reproductive period. The individual biomass of two maize cultivars decreased with increasing intercropping density. The increase in dry weight at reproductive growth period was higher for 'S16a' than for 'Z958' in 2011 under low intercropped planting density. With high density and drought condition, individual biomass accumulation decreased under maize cultivars intercropping after flowering. 3) There was on average 6.0% increase in harvest index (HI) under high intercropping density and HI increased with increasing rainfall. Root and shoot growth was normal due to light competition under sufficient precipitation and low planting density. Soil drought and high intercropping density resulted in significant decrease in root to shoot rate (RSR) of 'Z958' as root competition for water increased. 4) In drought year (2011), competitive advantage was fully apparent in the two cultivars intercropping system, with yield and WUE increases of respectively 10.3% and 21.4%, 28.2% and 42.0% under two intercropping densities. Furthermore, yield and WUE of intercropped 'Z958' were 17.6% and 50.0% higher than that of 'S16a' for the two-year experimental period. Finally, 'Z958' showed rational biomass distribution and response to soil drought when intercropped with 'S16a' via reducing redundant root growth and decreasing excessive resouces use. Effective root morphological adjustment and biomass distribution of 'Z958' were responsible for the yield and WUE increase.