Effect of Nitrogen Fertilizer on the Radiation Interception and Utilization Efficiency of two Spring Rapeseed Cultivars

Crop biomass accumulation is directly related to the amount of photosynthetically active intercepted radiation by the canopy during the crop’s cycle. The slope of this relationship represents the radiation use efficiency (RUE), defined as crop biomass produced per unit of total solar or photosynthetically active intercepted radiation (IPAR). Therefore, above-ground biomass (g m−2) can be expressed as a product of the cumulative IPAR (MJ m−2) during the crop cycle and RUE (g MJ−1). The amount of IPAR depends on the length of the crop cycle over which radiation is accumulated, the fraction of intercepted radiation (FIR) by the canopy each day and the total incident solar radiation. The length of crop cycle is affected by the genotype and/or environmental factors mainly temperature and photoperiod. Indeed, soil constraints such as nutritional deficiencies have shown minor impacts on the crop cycle. FIR depends on the leaf area index (LAI) and the canopy light extinction coefficient (k). Several studies on different crops have shown that IPAR is negatively affected by nitrogen (N) deficiency. RUE is affected by the crop species, environmental conditions and also by management factors such as crop nutritional status. Over the past few decades, one of the aspects of improving crop productivity has been increasing N fertilizer utilization. Nitrogen application is one of the key inputs for crop production that determines the potential of total crops dry matter production through its effect on IPAR, RUE, and/or both. The aim of the present study was to comparatively evaluate in two rapeseed cultivars their responses on IPAR, RUE and related traits under different N availabilities at field conditions. In this study, the effects of N utilization on IPAR and RUE of two spring rapeseed cultivars (Hyola 401 and Dalgan) were investigated. Field experiment was conducted as a split plot in a randomized complete block design with three replications at Agricultural Sciences and Natural Resources University of Khuzestan. N consumption in seven levels of 0, 50, 100, 150, 200, 250 and 300 kg ha-1 was considered as the main factor and the type of rapeseed cultivar in two levels as the sub-factor. The results showed that the response of the above-ground biomass to N supply in both of the studied rapeseed cultivars was related to the change in both IPAR and RUE. The total accumulated IPAR during the growing season under of consumption of 200 kg N ha-1 was about 27 and 35% higher than those of the control treatments in the cultivars Hyola 401 and Dalgan, respectively. In both rapeseed cultivars, IPAR was strongly dependent on LAI and LAD. This means that N utilization with increasing LAI and LAD resulted in an increase in IPAR. Reduction of LAI in response to N deficiency can be explained by (1) changing the phyllochron, (2) decreasing leaf blade area, and (3) the higher rate of loss of old leaves. Utilization of N had no effect on the light extinction coefficient (K) of crop canopy, and this coefficient was estimated to be 0.65 (± 0.02) in the cultivar Hyola 401 and 0.73 (± 0.02) in the cultivar Dalgan. Depending on N level, the above-ground biomass varied from 932 to 2192 g m-2 in the cultivar Hyola 401 and between 853 and 1811 g m-2 in the cultivar Dalgan. Depending on the availability of N, the RUE varied between 2.02 (control treatment) and 3.25 g MJ-1 (200 kg N ha-1) for the cultivar Dalgan and between 1.86 (control treatment) and 3.62 g MJ-1 (300 kg N ha-1) for the cultivar Hyola 401. Reducing RUE in N-limit treatments may be due to reduced leaves nitrogen content, which reduces their photosynthetic capacity. The results of this study showed that the above-ground biomass production by two rapeseed cultivars was related to differences in their growth rates in response to the applying different amounts of N. The physiological traits that explained above-ground biomass responses of both cultivars were the cumulative intercepted radiation during the crop cycle and radiations use efficiency. In both rapeseed cultivars, the K coefficient was not affected by N utilization, which indicates that IPAR responses were the result of changes in LAI and LAD during the crop cycle affecting the fraction of intercepted radiation by the crop.