Radiation use efficiency and biomass partitioning to storage roots in fodder beet crops

Intercepted photosynthetic active radiation (IPAR), radiation use efficiency (RUE) and partitioning of dry matter (DM) to storage roots (proot, %) were quantified for fodder beet crops subjected to contrasting water (irrigated or rain-fed) and nitrogen (N; 0, 25, 50, 100 and 200 kg/ha) supply conditions in Canterbury, New Zealand. The objectives were to enhance the understanding of physiological processes controlling fodder beet response to abiotic stresses and also to estimate parameters for biophysical models that simulate crop growth. Data from three field experiments showed a wide range of fodder beet yield (14-29 t DM/ha) in response to water and N stress. These yield differences were mostly explained by the treatments effect on IPAR (650-1050 MJ PAR/ha), with relatively smaller responses in RUE and proot. For unconstrained (fully irrigated; ≥100 kg N/ha) growth conditions, maximum values were 3.6 g DM/MJ PAR for RUE and 78% for proot for total biomass at final harvest. The RUE for the rain-fed, 0 kg N/ha crops was reduced by up to 25%, while proot was only marginally reduced (∼3% in rain-fed crops) compared with crops under unconstrained growth conditions. While the RUE responded linearly to additional N supply in irrigated crops, rain-fed crops showed a more consistent decline in RUE across N fertiliser rates with 65-70% of unstressed values. High RUE values were observed across a wide range of crop N status (Nitrogen Nutrition Index [NNI] from 0.8 to 1.6) in irrigated crops. In contrast, RUE in rain-fed was low at high NNI estimates because of high N concentration in a smaller canopy area. The lower RUE in rain-fed crops was aligned with reduced leaf photosynthetic rates (Pnleaf) although there was a high variability in Pnleaf measurements. These results give a first quantification of IPAR, RUE, proot and Pnleaf in fodder beet. They provide insights on the relative sensitivity of fodder beet to water and N stresses. These results are valuable for the interpretation of crop responses and for setting parameters for biophysical models to simulate fodder beet growth.