Cotton (Gossypium spp.) is commonly grouped into eight diploid genomic groups, designated A-G and K, and an allotetraploid genomic group, AD. Gossypium raimondii (D ) and G. arboreum (A ) are the putative contributors to the progenitor of G. hirsutum (AD ), the economically important fibre-producing cotton species. Mitochondrial DNA from week-old etiolated seedlings was extracted from isolated organelles using discontinuous sucrose density gradient method. Mitochondrial genomes were sequenced, assembled, annotated and analysed in orderly. Gossypium raimondii (D ) and G. arboreum (A ) mitochondrial genomes were provided in this study. The mitochondrial genomes of two diploid species harboured circular genome of 643,914 bp (D ) and 687,482 bp (A ), respectively. They differ in size and number of repeat sequences, both contain illuminating triplicate sequences with 7317 and 10,246 bp, respectively, demonstrating dynamic difference and rearranged genome organisations. Comparing the D and A mitogenomes with mitogenomes of tetraploid Gossypium species (AD , G. hirsutum; AD , G. barbadense), a shared 11 kbp fragment loss was detected in allotetraploid species, three regions shared by G. arboreum (A ), G. hirsutum (AD ) and G. barbadense (AD ), while eight regions were specific to G. raimondii (D ). The presence/absence variations and gene-based phylogeny supported that A-genome is a cytoplasmic donor to the progenitor of allotetraploid species G. hirsutum and G. barbadense. The results present structure variations and phylogeny of Gossypium mitochondrial genome evolution.

SUMMARYUnderstanding the physiological mechanisms of biomass accumulation and partitioning in the grain, and the nitrogen (N) uptake associated with different plant densities and N management strategies, is essential for achieving both high yield and N use efficiency (NUE) in maize plants. A field experiment was conducted in 2013 and 2014, using five rates of N application and three plant densities (6·0, 7·5 and 9·0 plants/m2) in Quzhou County on the North China Plain (NCP). The objective was to evaluate whether higher plant density can produce more biomass allocated to the grain to achieve higher grain yield and to determine the optimal N management strategies for different plant densities. The highest grain yield and NUE were achieved in the 7·5 plants/m2 treatment; both the sub-optimal (6·0 plants/m2) and supra-optimal (9·0 plants/m2) plant densities resulted in diminished yield and NUE. Compared to 6·0 plants/m2, the 7·5 plants/m2 treatment displayed higher biomass accumulation during the grain-filling period and also exhibited more biomass allocated to kernels with similar total biomass accumulation compared with the 9·0 plants/m2 treatment, which contributed to its higher grain yield. The N uptake in the 7·5 plants/m2 treatment was similar to that in the 9·0 plants/m2 treatment up to pre-silking. However, the post-silking N uptake of the 7·5 plants/m2 treatment was 66·4 kg/ha, which was 29·1% higher than that of the 9·0 plants/m2 treatment. Furthermore, the highest maize grain yield was achieved in the 0·7 × optimal N rate (ONR × 0·7), ONR and ONR × 1·3 treatments for 6·0, 7·5 and 9·0 plants/m2, respectively, which suggests that different N management strategies are needed for different plant densities. In conclusion, selecting a planting density of 7·5 plants/m2 with an in-season root zone N management is a potentially effective strategy for achieving high grain yield and high NUE for maize production on the NCP.

SUMMARYCrop production in the Northeast Farming Region of China (NFR) is affected considerably by variation in climatic conditions. Data on crop yield and weather conditions from a number of agro-meteorological stations in NFR were used in a mixed linear model to evaluate the impacts of climatic variables on the yield of maize (Zea mays L.), rice (Oryza sativa L.), soybean (Glycine max L. Merr.) and spring wheat (Triticum aestivum L.) in different crop growth phases. The crop growing season was divided into three growth phases based on the average crop phenological dates from records covering 1981 and 2010 at each station, comprising pre-flowering (from sowing to just prior to flowering), flowering (20 days around flowering) and post-flowering (10 days after flowering to maturity). The climatic variables were mean minimum temperature, thermal time (which is used to indicate changes in the length of growth cycles), average daily solar radiation, accumulated precipitation, aridity index (which is used to assess drought stress) and heat degree-days index (HDD) (which is used to indicate heat stress) were calculated for each growth phase and year. Over the 1961–2010 period, the minimum temperature increased significantly in each crop growth phase, the thermal time increased significantly in the pre-flowering phase of each crop and in the post-flowering phases of maize, rice and soybean, and HDD increased significantly in the pre-flowering phase of soybean and wheat. Average solar radiation decreased significantly in the pre-flowering phase of all four crops and in the flowering phase of soybean and wheat. Precipitation increased during the pre-flowering phase leading to less aridity, whereas reduced precipitation in the flowering and post-flowering phases enhanced aridity. Statistical analyses indicated that higher minimum temperature was beneficial for maize, rice and soybean yields, whereas increased temperature reduced wheat yield. Higher solar radiation in the pre-flowering phase was beneficial for maize yield, in the post-flowering phase for wheat yield, whereas higher solar radiation in the flowering phase reduced rice yield. Increased aridity in the pre-flowering and flowering phases severely reduced maize yield, higher aridity in the flowering and post-flowering phases reduced rice yield, and aridity in all growth phases reduced soybean and wheat yields. Higher HDD in all growth phases reduced maize and soybean yield and HDD in the pre-flowering phase reduced rice yield. Such effects suggest that projected future climate change may have marked effects on crop yield through effects of several climatic variables, calling for adaptation measures such as breeding and changes in crop, soil and agricultural water management.

SUMMARYDrought risk is considered to be among the main limiting factors for maize (Zea mays L.) production in the Northeast Farming Region of China (NFR). Maize yield data from 44 stations over the period 1961–2010 were combined with data from weather stations to evaluate the effects of climatic factors, drought risk and irrigation requirement on rain-fed maize yield in specific maize growth phases. The maize growing season was divided into four growth phases comprising seeding, vegetative, flowering and maturity based on observations of phenological data from 1981 to 2010. The dual crop coefficient was used to calculate crop evapotranspiration and soil water balance during the maize growing season. The effects of mean temperature, solar radiation, effective rainfall, water deficit, drought stress days, actual crop evapotranspiration and irrigation requirement in different growth phases were included in the statistical model to predict maize yield. During the period 1961–2010, mean temperature increased significantly in all growth phases in NFR, while solar radiation decreased significantly in southern NFR in the seeding, vegetative and flowering phases. Effective rainfall increased in the seeding and vegetative phases, reducing water deficit over the period, whereas decreasing effective rainfall over time in the flowering and maturity phases enhanced water deficit. An increase in days with drought stress was concentrated in western NFR, with larger volumes of irrigation needed to compensate for increased dryness. The present results indicate that higher mean temperature in the seeding and maturity phases was beneficial for maize yield, whereas excessive rainfall would damage maize yield, in particular in the seeding and flowering phases. Drought stress in any growth stage was found to reduce maize yield and water deficit was slightly better than other indicators of drought stress for explaining yield variability. The effect of drought stress was particularly strong in the seeding and flowering phases, indicating that these periods should be given priority for irrigation. The yield-reducing effects of both drought and intense rainfall illustrate the importance of further development of irrigation and drainage systems for ensuring the stability of maize production in NFR.

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