It is of great significance to study the root characteristics of rice to improve water and nitrogen (N) use efficiency and reduce environmental pollution.  This study investigated whether root traits and architecture of rice influence grain yield, as well as water and N utilization efficiency.  An experiment was conducted using the upland rice cultivar Zhonghan 3 (a japonica cultivar) and paddy rice cultivar Huaidao 5 (also a japonica cultivar) using three N levels, namely, 2 g urea/pot (low amount, LN), 3 g urea/pot (normal amount, NN), and 4 g urea/pot (high amount, HN), and three soil water potentials (SWPs, namely, well-watered (0 kPa), mildly dried (–20 kPa) and severely dried (–40 kPa).  The results showed that with decreasing SWP, the percentage of upland rice roots increased in the 0–5 cm tillage layer, and decreased in the 5–10 and 10–20 cm tillage layers, whereas paddy rice roots showed the opposite trend.  With increasing amounts of N, the yield of upland and paddy rice increased, and the percentage of root volume ratios of the two rice cultivars in the 0–5 and 5–10 cm tillage layers increased, whereas that in the 10–20 cm tillage layer decreased.  The roots of upland rice are mainly distributed in the 10–20 cm tillage layer, whereas most paddy rice roots are in the 0–5 cm tillage layer.  These results indicate that the combination of −20 kPa SWP and NN in upland rice and 0 kPa SWP and LN in paddy rice promotes the growth of the root system during the middle and late stages, which in turn may decrease the requirements for water and N fertilizer and increase rice yield. 

 

Rice (Oryza sativa L.) quality depends mainly on the characteristics of starch stored in kernels.  Understanding the changes in starch characteristics in kernels during variety improvement would have great significance to improve rice quality.  This study was designed to investigate the starch characteristics in the kernels and associated physiological traits of indica rice varieties in the lower reaches of the Yangtze River in China in last 80 years.  Eight representative mid-season indica rice varieties were grown in the field.  The results showed that the grain yield was significantly increased with the improvement of varieties and such an increase was mainly attributed to the increase in total number of spikelets.  The tall varieties applied in the 1940s–1950s had higher protein content, relative crystallinity and infrared (IR) ratio of 1 045/1 022 cm^–1.  The semi-dwarf varieties applied in the 1980s–1990s had higher gel consistency, amylopectin content, IR ratio of 1 022/995 cm^–1, and breakdown value.  With the improvement of varieties, the amylose content, large-sized starch granule number and volume distribution, onset and peak of gelatinization temperature, gelatinization and retrogradation enthalpy, setback value, pasting temperature, viscosity of peak, hot and final, and 1-aminocycopropane-1-carboxylic acid (ACC) concentrations in panicles and root bleeding were gradually decreased, whereas the medium-sized starch granule number and volume distribution, activities of key enzymes in grains, and zeatin (Z)+zeatin riboside (ZR) contents in panicles and root bleeding at grain filling stage were gradually increased.  Correlation analysis showed that starch thermodynamic characteristics were closely related to starch structure and components, key enzymes and hormones.  The results suggest that starch quality was enhanced through the optimization of starch components, structure, thermodynamics, and the regulation of key enzymes in grains and hormones in panicles and root bleedings at grain filling stage during the improvement of mid-season indica rice.

A major challenge in rice (Oryza sativa L.) production is to cope with increasing grain yield and fertilizer use efficiency without compromising grain quality.  This study was designed to determine if optimizing integrative cultivation management in rice could improve grain quality while increase yield and nitrogen use efficiency (NUE).  An indica-japonica hybrid rice cultivar and a japonica rice cultivar were grown in the field, with five cultivation managements including no N application (0 N), local farmer’s practice (LFP), and three optimizing integrative cultivation managements, reducing N rate and increasing plant density (ND), ND+alternate wetting and moderate soil drying irrigation (NDW), and NDW+applying rapeseed cake fertilizer (NDWR).  The results showed that the optimizing integrative cultivation managements could not only increase grain yield, but also enhance NUE compared to LFP.  Compared to LFP, NDWR significantly increased brown, milled, head milled rice rate, ratio of the kernel length to breadth and breakdown value of starch, whereas decreased amylose content, gel consistency, prolamin content, setback value, percentage of chalky kernels, and chalkiness.  The three optimizing integrative cultivation managements increased contents of total proteins, albumin and glutelin, activities of the key enzymes involved in the sucrose-starch conversion in grains, root oxidation activity, and malic and succinic acid concentrations in root exudates during the grain-filling period.  The results suggested that optimizing integrative cultivation managements could improve grain quality meanwhile increase grain yield and NUE by enhancing physiological activities of rice plants.

The phenomenon of degenerated spikelets is very common in cereals, and considered as a serious physiological defect and a main constraint to grain production.  Understanding the physiological mechanism in which spikelet degeneration occurs would have great significance in enhancing yield potential in grain crops.  Taking rice as an example, the paper reviewed the physiological mechanism underlying spikelet degeneration, with focus on the roles of phytohormones in regulating the process.  There are several hypotheses for the spikelet degeneration, such as resource limitation, self-organization, and primigenic dominance.  However, convincing evidences are not enough to support the assumptions.  Phytohormones including auxins, cytokinins, gibberellins, abscisic acid, and ethylene are involved in regulating spikelet degeneration in cereals.  The new phytohormones of brassinosteroids and polyamines have been observed to suppress spikelet degeneration in rice.  The interactions among or between plant hormones may play a more important role in regulating spikelet degeneration.  However, the information on such interactions is very limited.  Some agronomic practices, especially proper water and nitrogen management, could reduce spikelet degeneration but the mechanism underlying remains unclear.  Further research is needed to understand the cross-talk among/between phytohormones on spikelet degeneration, to reveal the physiological and molecular mechanism in which phytohormones and their interactions regulate the degeneration of spikelets, to exploit approaches to decrease spikelet degeneration and to elucidate their mechanism.