Water-saving Physiological Mechanism And Evaluation Of Water Use Efficiency For Ground Cover Rice Production Systems

Cultivation in a traditional paddy rice production system (TPRPS) is typically characterized by luxurious water consumption and low efficiency, therefore exploration of high-efficient rice production technology is imperative. Among available water-saving rice production technologies, the ground cover rice production system (GCRPS) has attracted much attention because of its contribution to both saving water and increasing yields. In recent years, efforts have been made by many researchers to investigate this novel system regarding plant growth, yield formation, water consumption and nitrogen utilization efficiency, etc. However, the physiological processes and mechanisms driving rice under GCRPS to produce more grain yield along with less transpiration are not clear, and yet there are few studies on evaluating water use efficiency of GCRPS.In this research, TPRPS was compared with GCRPS in a series of hydroponic, soil column and field experiments for two consecutive growth seasons at Fangxian County, Hubei Province. Root growth,morphology, distribution, function, water-saving physiological characteristics, water/nitrogen absorption and their relationships under GCRPS were analyzed. Physiological processes and mechanisms explaining the superiority of GCRPS were investigated. The water production function based on Jensen model and the soil moisture distribution model on water balance method were established and verified for GCRPS.Water consumption, yield and water use efficiency of various irrigation managements, set up according to local conditions, were analyzed using the aforementioned models. The main results were obtained as follows:1. Physiological processes and mechanisms for superior water productivity of GCRPSDuring the early growth period (before mid-tillering in field and early-tillering in column experiments), root zone soil water content under all the treatments was completely or nearly saturated,but soil temperature was higher for GCRPS, leading to enhanced root growth and branch, but lower water uptake rate per root length compared to TPRPS. In subsequent growth stages, when the warming effect under GCRPS weakened and even disappeared,the reduced soil water content caused that the preponderance of root growth under GCRPSsat disappeared in comparison to TPRPS. Under other GCRPS treatments (GCRPSfwc and GCRPS80%, keeping root zone average soil water content higher than 100%and 80% of field water capacity, same below), significant limitation on root growth and branch, but higher root nitrogen content and water uptake rate per root length were found. Although soil temperature and moisture conditions significantly influenced root length density distribution, the normalized root length density distributions were similar and consistent, and could be described by an exponential function with physical significance.During the early growth period, no significant difference was found in leaf transpiration rates between all treatments, while the leaf photosynthetic rates under GCRPS were significantly higher than those under TPRPS due to higher soil temperature. Afterwards, leaf transpiration and photosynthetic rates under GCRPS was significantly limited as a function of the lower average root zone soil water content,with the effect on leaf transpiration rate being much more significant than that on leaf photosynthetic rate.Plant nitrogen content might be responsible for that: Results from hydroponic experiment indicated leaf photosynthetic rates of rice increased in a manner approximately proportional to specific leaf nitrogen (R2=0.39**), and specific leaf nitrogen under GCRPS was consistently higher than under TPRPS, therefore photosynthetic rates under GCRPS was subjected to minor suppression,the trend weakened with plant growth.Compared to TPRPS, leaf area and transpiration before max-tillering under GCRPS significantly increased because of warming effect, and transpiration was much lower during the later period due to the lower average root zone soil water content. Seasonal transpiration decreased an average of 6.1% under GCRPS than that under TPRPS. Similarly to the relationship between photosynthetic rate and specific leaf nitrogen, root water/nitrogen uptake capacity also increased linearly (R~2 = 0.76** and 0.69**,respectively) with specific root nitrogen, regardless of nitrogen form, concentration in nutrient solution,and experimental period. Influenced by the sharp decrease of deep percolation under GCRPS, nitrogen loss through leaching was remarkably limited, furthermore higher soil temperature under GCRPS during the early growth period also accelerate nitrogen transformations and root uptake, thus nitrogen uptake under GCRPS enhanced by up to 96.3% in comparison to TPRPS. Therefore, rice under GCRPS could alleviate the inhibition of photosynthesis by water stress and have greater water use efficiency, maintained the preponderance developed early in the growth season, and ultimately realized saving water and increasing yields.2. Evaluating water use efficiency of GCRPSThe water production function based on Jensen model was established and verified for GCRPS,which could well simulate the relationship between evapotranspiration and grain yield under GCRPS.The water sensitive coefficients at tillering, panicle initiation, anthesis and milk ripe stages were 0.108,0.110, 0.114 and 0.099, respectively. The relative error between the simulated and measured grain yield was only 1.1%.The model of soil moisture distribution could also well describe water movement in root zone under GCRPS and reliably estimate evapotranspiration, with the root mean square error and normalized root mean squared error between simulated and measured soil water content less than 0.039 cm3 cm-3 and 14.7%, respectively.Five irrigation managements including GCRPSsat,GCRPS100%,GCRPS90%,GCRPS80%,and GCRPS70%, set up according to local conditions, were analyzed using the aforementioned models. The simulated results showed grain yields under GCRPS were all significant higher than that under TPRPS.Evaporation, transpiration, deep seepage, runoff and yield under various irrigation schedules of GCRPS decreased with the decrease of irrigation, but water use efficiency increased. Among the five irrigation schedules of GCRPS, grain yield under GCRPSsat was the highest and even 13.8% higher than that under TPRPS. Water use efficiency based on total input water under GCRPS70% was the highest with 121.4%and 27.8% higher than that under TPRPS and GCRPSsat, respectively.