Genotypic variation of P acquisition efficiency of upland rice using 33P labeling
Authors: Mundschenk Eva , Remus Rainer , Holz Maire 
Published: March 2022

&lt;p&gt;Upland rice is often cultivated on P poor soils with a high P fixing capacity or in areas where P fertilizers are unavailable. Therefore, rice genotypes with a high P acquisition efficiency (PAE) are needed. In order to identify mechanisms driving differences in rice genotypes&amp;#8217; PAE, root and rhizosphere traits such as the root surface area (RSA) or the P acquisition of contrasting soil P pools is of special interest.&lt;/p&gt;&lt;p&gt;The objective of this study was to identify root and rhizosphere traits that can explain differences in PAE of two known (DJ123, Nerica4) and two recently developed promising (AB199, AB67) upland rice genotypes.&lt;/p&gt;&lt;p&gt;A pot experiment with a high P fixing Andosol from Tsukuba, Japan, and four rice genotypes differing in PAE (high PAE: DJ123, AB199, AB67; low PAE: Nerica4) was performed. The genotypes were grown under low (3.5 mg P kg&lt;sup&gt;-1&lt;/sup&gt;, LP) and high (70 mg P kg&lt;sup&gt;-1&lt;/sup&gt;, HP) P conditions. For each pot, 1.5 MBq &lt;sup&gt;33&lt;/sup&gt;P labeled phosphoric acid was diluted in N:P:K (NH&lt;sub&gt;4&lt;/sub&gt;NO&lt;sub&gt;3&lt;/sub&gt;,&amp;#160; NaH&amp;#173;&lt;sub&gt;2&lt;/sub&gt;PO&lt;sub&gt;4&lt;/sub&gt;, K&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;) nutrient solution and homogenized into the soil using a handheld electric mixer. Plants were harvested 10 days after emerging (DAE) (estimated beginning of P uptake) and DAE 31. After each harvest, roots were scanned with an Epson flatbed scanner to measure the RSA. Shoots and roots were dried and their biomass as well as total P and &lt;sup&gt;33&lt;/sup&gt;P uptake was determined. A hedley fractionation (0.5 M NaHCO&lt;sub&gt;3&lt;/sub&gt;, HCO&lt;sub&gt;3&lt;/sub&gt;-P; 0.1 M NaOH, OH-P; 0.5 H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;, H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;-P) was conducted for bulk (BS) and rhizosphere soil (RS) to estimate &lt;sup&gt;33&lt;/sup&gt;P and P in the different soil P pools. Total P in soil was determined after pressure digestion using and ICP-OES.&lt;/p&gt;&lt;p&gt;Plant shoot and root dry weight, P concentration in root and shoot, P uptake in root and shoot and RSA were higher in HP plants compared to LP plants. In the LP treatment, AB199 showed the highest shoot and root dry weight and &lt;sup&gt;33&lt;/sup&gt;P uptake 10 DAE, followed by DJ123, AB67 and Nerica4. Additionally, AB199 showed the highest P uptake per RSA, i.e. the highest PAE among all tested genotypes. This trend was confirmed for the second harvest, 31 DAE. However, differences in P uptake and PAE between DJ123 and AB199 were not significant. Nevertheless, these results indicate that the new variety AB199 might indeed by a promising upland variety for P limiting conditions. The hedley fractionation showed that the largest &lt;sup&gt;33&lt;/sup&gt;P fraction was the OH-P fraction (65-75%) followed by H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;-P (20-30%) and HCO&lt;sub&gt;3&lt;/sub&gt;-P (5-7%). The H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;-P fraction was slightly lower in the rhizosphere soil in DJ123 and AB199, indicating that rhizosphere processes helped these genotypes to solubilize larger amounts of H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;-P, compared to the other two genotypes.&lt;/p&gt;&lt;p&gt;We showed that the coupling of &lt;sup&gt;33&lt;/sup&gt;P labelling with measurements of soil P fractions and plant P uptake can help to identify parameters responsible for increased PAE of upland rice genotypes. This may assist in identifying valuable root and rhizosphere traits for breeding for enhanced PAE in upland rice.&lt;/p&gt;