Differing responses of root morphology and physiology to nitrogen application rates and their relationships with grain yield in rice

Root morphology and physiology influence aboveground growth and yield formation in rice.However,root morphological and physiological differences among rice varieties with differing nitrogen(N)sensitivities and their relationship with grain yield are still unclear.In this study,rice varieties differing in N sensitivity over many years of experiments were used.A field experiment with multiple N rates(0,90,180,270,and 360 kg ha^(-1))was conducted to elucidate the effects of N application on root morphology,root physiology,and grain yield.A pot experiment with root excision and exogenous application of 6-benzyladenine(6-BA)at heading stage was used to further verify the above effects.The findings revealed that(1)under the same N application rate,N-insensitive varieties(NIV)had relatively large root biomass(root dry weight,length,and number).Grain yield was associated with root biomass in NIV.The oxidation activity and zeatin(Z)+zeatin riboside(ZR)contents in roots obviously and positively correlated with grain yield in N-sensitive varieties(NSV),and accounted for its higher grain yield than that of NIV at lower N application rates(90 and 180 kg ha^(-1)).(2)The root dry weight required for equal grain yield of NIV was greater than that of NSV.Excision of 1/10 and 1/8 of roots at heading stage had no discernible effect on the yield of Liangyoupeijiu(NIV),and it significantly reduced yield by 11.5%and 21.3%in Tianyouhuazhan(NSV),respectively,compared to the treatment without root excision.The decrease of filled kernels and grain weight after root excision was the primary cause for the yield reduction.Root excision and exogenous 6-BA application after root excision had little influence on the root activity of NIV.The oxidation activity and Z+ZR contents in roots of NSV decreased under root excision,and the increase in the proportion of excised roots aggravated these effects.The application of exogenous 6-BA increased the root activity of NSV and increased filled kernels and grain weight,thereby reducing yield loss after root excision.Thus,the root biomass of NIV was large,and there may be a phenomenon of"root growth redundancy."Vigorous root activity was an essential feature of NSV.Selecting rice varieties with high root activity or increasing root activity by cultivation measures could lead to higher grain yield under lower N application rates.

Root Morphology and Physiology in Relation to the Yield Formation of Rice

Root system is a vital part of plant and regulates many aspects of shoot growth and development. This paper reviews how some traits of root morphology and physiology are related to the formation of grain yield in rice （Oryza sativa L.）. Higher root biomass, root oxidation activity, and cytokinin contents in roots are required for achieving more panicle number, more spikelets per panicle, greater grain-filling percentage, and higher grain yield. However, these root traits are not linearly correlated with yield components. When these traits reach very high levels, grain filling and grain yield are not necessarily enhanced. High numbers of mitochondria, Golgi bodies, and amyloplasts in root tip cells benefit root and shoot growth and yield formation. Proper crop management, such as an alternate wetting and moderate soil drying irrigation, can significantly improve ultra-structure of root tip cells, increase root length density and concentration of cytokinins in root bleedings, and consequently, increase grain-filling percentage, grain yield, and water use efficiency. Further studies are needed to investigate the mechanism underlying root-shoot and root-soil interactions for high grain yield, the roles of root-sourced hormones in regulating crop growth and development and the effects of soil moisture and nutrient management on the root architecture and physiology.