The concept of critical nitrogen (Nc) concentration can be implemented to diagnose in-season plant nitrogen (N) status for optimizing N fertilizer management. The Nc dilution curves have been established for rice (Oryza sativa L.) grown in different climatic regions, yet no attempt has been made to develop the Nc dilution curve for double cropping rice regions. This study was undertaken to develop the Nc dilution curves for double cropping rice in south China for assessment of in-season N status and to establish the relationships N nutrition index (NNI) and relative yield (RY) for in-season prediction of rice grain yield. Three different N application rate field experiments using six Indica rice varieties, including two early rice hybrids and four late rice hybrids were carried out in east China. The Nc dilution curves based on whole plant N concentration were determined and described as, Nc=3.37W-0.44 for early rice and Nc=3.69W-0.34 for late rice. The constant N concentration at early growth stage was 3.31 and 3.15% DM for early and late rice, respectively. Late rice showed a higher capacity of N accumulation and a lower rate of N decline per unit shoot biomass as compared to early rice. The curves for present study were different from the existing reference curves for Indica and Japonica rice grown in different rice growing regions. Integrated N nutrition index (NNIint) based on Nc was used to estimate RY at different growth periods using linear regression functions. The results showed that the critical curves and relationship between NNIint and RY could be used as a reliable indicator of N status diagnosis, grain yield prediction as well as to provide technical support in N management for double cropping rice in south China.

Quantitative study of root morphological characteristics of plants is helpful for understanding the relationships between their morphology and function. However, few studies and little detailed and accurate information of root characteristics were reported in fine-rooted plants like rice (Oryza sativa L.). The aims of this study were to quantitatively classify fine lateral roots (FLRs), thick lateral roots (TLRs), and nodal roots (NRs) and analyze their dynamics of mean diameter (MD), lengths and surface area percentage with growth stages in rice plant. Pot experiments were carried out during three years with three rice cultivars, three nitrogen (N) rates and three water regimes. In cultivar experiment, among the three cultivars, root length of ‘Yangdao 6’ was longest, while the MD of its FLR was the smallest, and the mean diameters for TLR and NR were the largest, the surface area percentage (SAP) of TLRs (SAP_T) was the highest, indicating that Yangdao 6 has better nitrogen and water uptake ability. High N rate increased the length of different types of roots and increased the MD of lateral roots, decreased the SAP of FLRs (SAP_F) and TLRs, but increased the SAP of NRs (SAP_N). Moderate decrease of water supply increased root length and diameter, water stress increased the SAP_F and SAP_T, but decreased SAP_N. The quantitative results indicate that rice plant tends to increase lateral roots to get more surface area for nitrogen and water uptake when available assimilates are limiting under nitrogen and water stress environments.

Precise quantification of plant nitrogen (N) nutrition status is essential for crop N management. The concept of critical N concentration (Nc) has been widely used for assessment of plant N status. This study aimed to develop a new winter wheat Nc dilution curve based on leaf area duration (LAD). Four field experiments were performed on different cultivars with different N fertilization modes in the Yangtze River basin and Yellow River basin in China. Results showed that the increase in LAD with increasing cumulative thermal time took the shape of an ‘S’ type curve; whereas shoot N concentration decreased with increasing LAD, according to a power function. Both LAD and shoot N concentration increased with increasing N application. The new LAD based Nc dilution curve was determined and described as Nc=1.6774LAD-0.37 when LAD＞0.13. However, when LAD ≤ 0.13, Nc was constant and can be calculated by the equation when LAD=0.13. The validation of Nc dilution curve with dataset acquired from independent experiments confirmed that N nutrition index (NNI) predictions based on the newly established Nc dilution curve could precisely diagnose N deficiency at different plant growth stages. The integrated N nutrition index (NNIinte), which was obtained by the weighted mean of NNI, was used to estimate shoot N concentration, shoot dry matter, LAD, and yield using regression functions. The linear relationships between NNIinte and these growth variables were well correlated. These results provided enough evidence that the new LAD–based Nc dilution curve could effectively and precisely diagnoses N deficiency in winter wheat crops.

Extreme heat-stress events are becoming more frequent under anticipated global warming, which is having devastating effect on grain yield, as well as quality, of rice (Oryza sativa L.). The effects of heat stress at booting stage on grain quality of two japonica varieties, Nanjing41 and Wuyunjing24, were investigated in phytotrons during 2014 and 2015. Rice plants were subjected to four mean temperature regimes 27°C, 31°C, 35°C and 39°C of 2, 4 and 6 days’ duration. The results showed that high temperatures of 35°C and 39°C for 4 and 6 days significantly reduced panicle size, seed-setting rate, grain size, chalky grain rate, milling characteristics and amylose content, but increased protein content. Severe heat stress decreased values of peak viscosity and breakdown, and increased pasting temperature. An increase in heat degree-days decreased the percentage of chalky grains exponentially, and decreased amylose content and increased protein content linearly. Sensitivity of grain quality to heat stress in the two varieties differed among quality traits and with heat stress intensity. This study indicates that rice-grain quality had some resistance to mild heat stress, but it could not withstand severe heat stress at booting. Short-term heat stress at booting stage deteriorates most grain-quality traits, posing a potential risk to rice quality. The impacts on grain quality could be well quantified by the combined effects of the intensity and duration of heat stress at booting stage.