Simulation Study On Plant Type And Light Utilization In Rice

The plant type of rice affects the distribution and interception of photosynthetically active radiation (PAR) within rice (Oryza sativa L.) canopy and consequently influences canopy photosynthesis and biomass production. Quantitative analysis of the relationships between plant type and light distribution/photosynthesis within rice canopy is of significant importance for improving population light utilization efficiency and unit area grain yield. In the present study, the characteristics of plant morphology, spatial-temporal distribution of photosynthetically active radiation in rice canopy and dry matter accumulation in aboveground were elucidated based on the field experiments with different cultivar types and nitrogen rates. Also, the dynamic models of relative dry matter accumulation and canopy light distribution in rice were developed, and the relationships between plant type and canopy photosynthesis under different circumstances were quantitatively analyzed. The results help to lay a theoretical foundation for high yield cultivation and design of optimal plant type in rice. The main results of this study can be briefly summarized as follows.Field experiments were conducted in Nanjing in 2007 and 2008, involving two rice cultivars and three nitrogen rates. The plant morphology and spatial-temporal distribution of PAR at different canopy heights were measured at filling stage with SunScan canopy analysis system. Nitrogen rates had significant effects on the plant height, panicle curvature, and leaf angle of rice. The leaf area was greater in mid canopy than in upper and basal canopy. The maximum leaf area index (LAI) layer appeared at 0.60 of relative canopy height. The average PAR transmittance decreased from top to bottom in rice canopy, exhibiting a quick attenuation within the upper and mid canopy and a slow reduction within the basal canopy. In addition, the average PAR transmittance at different canopy heights decreased with increasing nitrogen application rate. The diurnal variation of average PAR transmittance was greater at noon than in the morning and afternoon. With increasing LAI accumulation, the average PAR transmittance at different canopy heights decreased in an exponential pattern. The diurnal variation of population extinction coefficient (K) was smaller at noon than in the morning and afternoon, and the K value was in the range of 0.35-0.50. The 3-dimensional distribution of PAR in rice canopy indicated that the PAR transmittance on horizontal plane was greater, and exhibited much more light flecks in upper and mid canopy than in basal canopy. The PAR density distribution at the same canopy height was non-uniform on the horizontal plane. Higher PAR transmittance, greater diurnal variation of PAR transmittance, and smaller population extinction coefficient were observed for the erect plant type rice cultivar.In the same field experiments involving two rice cultivars and three nitrogen rates, the amount of PAR interception and its distribution at different canopy heights, leaf area and yield of two cultivars were measured from jointing to maturity. The results indicated that the vertical distribution of upward cumulative leaf area index (LAI) follows a sigmoid curve, which could be well described with Logistic equation (R2>0.99). The maximum leaf area density in heading,17 days after heading and maturity stages appeared at 0.53,0.56 and 0.60 of relative canopy height, respectively. The relative leaf area density of upper and mid canopy increased with rice development progress, but the relative leaf area density of basal canopy decreased with rice development progress. The relationship between fraction of PAR interception (FIPAR) and downward cumulative LAI could be well described with a negative exponential equation, i.e., FIPAR=α×(1-e-K×LAI) (R2>0.86). The value of extinction coefficient (K) decreased with rice development progress. The amount of PAR interception (AIPAR) showed multi-peaks distribution during rice development period, and the maximum peak appeared at 58～70 days after transplanting, around booting to heading stages. The AIPAR increased with increasing nitrogen rates. Diurnal variation of AIPAR showed one peak distribution on the clear days, and the maximum AIPAR appeared at 11:00～13:00 in rice canopy. The vertical distribution of leaf area affected PAR interception and distribution within rice canopy. The grain yield of rice was positively related to the PAR use efficiency, while PAR conversion efficiency first increased and then decreased with increasing PAR interception.A model of canopy light distribution in rice was developed by integrating certain advantages in existing models, and then a process-based photosynthetic production model was developed by integrating the canopy light distribution model in rice. The canopy was divided into five layers according to leaf area index(LAI). Solar radiant intensity on horizontal plane at each layer was calculated by Monsi-Saeki exponential model. The direct and diffuse radiations were calculated from the local meteorological data of sunshine duration, while the diurnal variation of PAR and the influence of direct radiation extinction coefficient on both canopy structure and solar position were also considered. Preliminary validation of canopy light distribution model with independent field experiment datasets showed that the model could accurately predict canopy light distribution under different growthing environments. The RMSE of predicting in rice dry weight for canopy-light-distribution-based model and existing model were 0.74 t·hm-2 and 1.26 t·hm-2, respectively, which indicated that the canopy-light-distribution-based model developed in this study had a better performance.Another field experiment with four cultivar types under different nitrogen rates was conducted, and the aboveground dry matter accumulation (DMA) was measured at the main growth stages. Based on the experiment data, a dynamic model of relative dry matter accumulation (RDMA) was established with normalized DMA and TEP (product of thermal effectiveness and PAR) from emergence to maturity, and the temporal characteristics of DMA changes was quantitatively analyzed using the RDMA model. It was seen that the time-course changes of the RDMA in rice could be well described with Richards equation, i.e., RDMA=1.0157/(1+e3.6329-7.5907×RTEP)1/0.5574 (r=0.9938). The model was then validated with independent field experiment datasets, involving different eco-sites, cultivars and nitrogen rates. The RMSE (root mean square error) between the simulated and observed values of DMA at varied RTEP was 0.86 t·hm-2. According to two inflexion points of the equation for dry matter accumulation rate, the whole process of dry matter accumulation in rice could be divided into early, middle, and late phases. The maximum dry matter accumulation rate (ARmax), relative TEP at ARmax (ARTEP) and relative dry matter accumulation at ARmax (ARDMA) were found to be 2.24,0.56 and 0.46, respectively.By using canopy-light-distribution-based photosynthetic model and designing various combinations of model input parameters for erect and flat plant types of rice, quantitative analyses were further made on the diurnal variation in extinction coefficient of canopy direct radiation with different plant types, the vertical distribution characteristics of direct radiation at noon and photosynthetic rate within canopy, the characteristics of canopy photosynthetic rate with the changed LAI, as well as the diurnal variation of canopy photosynthetic rate under different radiant intensities. The results showed that the high yield potential of erect plant type in rice relied on great LAI, leaf photosynthetic efficiency, solar altitude and the intensity of solar radiation etc.