Canopy Light Interception Characteristics For Winter Wheat And Summer Maize Under Different Water Stress

In an agricultural ecosystem, solar radiation is the total final energy source. The photosynthetically active radiation(PAR) is the energy source for the process of plant photosynthesis, which plays a vital role in crop growth and formation of crop yields. To improve the light energy utilization of crop groups, the first measure is to ensure a high light interception in crop canopy. In practice, however, leaves within the canopy are actually often in the "light hunger" state. So analysis of the canopy light(especially PAR) transmission and light distribution has become an important issue in the research of winter wheat and summer maize production. Beer’s law has been widely applied to calculate interception and transmission of light in crop canopy. Researches about canopy light interception and transmission traditionally focused on crop growing with sufficient soil moister and fertilizer, while few were done under drought conditions, especially droughts at different periods. Light transmission in the canopy can be influenced by multiple reflections and transmission of leaves, which gives solar radiation a higher heterogeneity in canopy, or different from one position to another. And furthermore, variability among leaves makes intercepted light quantity different in each layer of canopy, which was particularly obvious in the canopy of summer maize.To analyze the PAR transmission and interception characteristics in the canopies of winter wheat and summer maize under different drought conditions, four drought scenarios were setup for winter wheat and summer maize under a rain shelter, including no drought for whole stage(WT1、MT1), drought at vegetative stage(WT2、MT2), drought at reproductive stage(WT3、MT3), and drought at all stages(WT4、MT4). Therefore there were a total of four treatments accordingly for each crop. PAR sensors were installed at different height to automatically monitor the light transmission and interception characteristics within the canopy. Then, the ability of light interception was simulated and validated for each canopy layer during the grain filling stage. However, leaf area index(LAI) is needed to calculate the light interception for each layer. Since there was a high variation in leaf shape within the canopy of summer maize, a study on temporal and spatial variations of leaf shape coefficients( ?) was carried out to obtain the shape coefficients for leaves at different leaf positions and growth stages, and thus to provide a guidance for the calculation of LAI of each canopy layer. Some main conclusions are drawn as follows:(1) Leaf shape coefficient of summer maize showed temporal and spatial variations during the whole growth season. To improve the prediction accuracy of leaf area and LAI with the leaf shape coefficient model, different leaf shape coefficients should be used for different growth stages, leaf types, and leaf positions. Based on linear regression analysis of leaf areas and products of leaf length and width of 760 leaf samples of six different maize cultivars, the general average value of ? were about 0.78, which is recommended to be applied in maize leaf area estimation and replace the commonly used value of 0.75. The accuracy of leaf shape coefficient model was the highest among the five different models investigated for estimation of maize leaf area. Thus, this model was recommended for the estimation of leaf area and LAI in future field studies.(2) The routine of PAR transmission in the canopy was affected by canopy state. When the canopy was thin and low, internal PAR transmission mainly fluctuated with the solar altitude angle. Variation of PAR on canopy top was consistent with the changes of solar radiation, which followed a single-peak curve. At the jointing stage of winter wheat and seedling stage of summer maize, canopy leaked much light due to short plant height. With the increase of growth period and canopy height, the difference of PAR increased between the inside canopy layer and top layer. At ga given noon, PAR inside the canopy was sometimes higher than that of the top, which was against the Beer’s law or the rule of canopy light transmission. This was probably caused by the reflection and transmission of light by plant leaves during light transportation in canopy. An area of high PAR value was formed within canopy. Therefore, the Beer’s law is only feasible for calculating the whole canopy light interception, while not very accurate when calculating canopy light interception at a give time of some day. The analysis of canopy PAR transmission of winter wheat and summer maize under different drought scenarios showed that canopy PAR transmission decreased first and then increased with the growth period. The minimum PAR interception values appeared at 20 and 14 days after filling stage for wheat and maize, respectively. For wheat and maize without any water application in the whole stages, the premature aging led to serious canopy light leak.(3) Because of the low canopy height of winter wheat, difference of light interception in different canopy layers was not as obvious as maize. PAR interception in winter wheat canopy clearly changed with the weather. There was larger interception in sunny days and less in rainy days. The difference could reach 1.2 10 mol m-2. For light interception of different canopy layers at grain filling period of summer maize, the quantities of intercepted light were also different among layers, although total quantity of canopy intercepted PAR was almost the same at the two adjacent days. When the quantities of light interception were equal under different drought scenarios, the difference of PAR interception among different layers were even more obvious. The light interception ability of different canopy layers was simulated with the light interception model based on Beer’s law. The treatments of irrigation in whole stages acquired better simulation results with values of relative root mean square error(RRMSE) and absolute relative error(ARE) of 0.18 and 26.4%, respectively. There were always some errors in the simulation results of PAR interception of treatment MT2, the RRMSE and ARE were 0.05 and 58.8%, respectively. For the treatment of drought at the grain filling stage(MT3), the simulation values of PAR interception of each canopy layer were relatively high as a whole, with a RRMSE of 0.16 and ARE of 483.2%. For the treatment with drought during the whole growth season, simulation values of PAR interception at different canopy layers were also overall high and the values of RRMSE and ARE were respectively 0.4 and 706.2%, respectively, which was the worst among the four treatments. The results suggested that the Beer’s law was more applicable for light interception estimation by canopies with homogeneous leaves distribution, but it might overestimate the canopy light interception of crops under drought conditions.