Root exudation patterns of contrasting rice (Oryza sativa L.) lines in response to P limitation

Abstract

Rice exudation patterns changed in response to P deficiency. Higher exudation rates were associated with lower biomass production. Total carboxylate exudation rates mostly decreased under P-limiting conditions. Within the rhizosphere, root exudates are believed to play an important role in plant phosphorus (P) acquisition. This could be particularly beneficial in upland rice production where P is often limited. However, knowledge gaps remain on how P deficiency shapes quality and quantity of root exudation in upland rice genotypes. We therefore investigated growth, plant P uptake, and root exudation patterns of two rice genotypes differing in P efficiency in semi-hydroponics at two P levels (low P = 1 µM, adequate P = 100 µM). Root exudates were collected hydroponically 28 and 40 days after germination to analyze total carbon (C), carbohydrates, amino acids, phenolic compounds spectrophotometrically and carboxylates using a targeted LC-MS approach. Despite their reported role in P solubilization, we observed that carboxylate exudation rates per unit root surface area were not increased under P deficiency. In contrast, exudation rates of total C, carbohydrates, amino acids and phenolics were mostly enhanced in response to low P supply. Overall, higher exudation rates were associated with lower biomass production in the P-inefficient genotype Nerica4, whereas the larger root system with lower C investment (per unit root surface area) in root exudates of the P-efficient DJ123 allowed for better plant growth under P deficiency. Our results reveal new insights into genotype-specific resource allocation in rice under P-limiting conditions that warrant follow-up research including more genotypes.

Keywords: Amino acids; Carbohydrates; Carboxylates; Phenolics; Phosphorus.

Fig. 1

Shoot dry weight (a), root dry weight (b), total P uptake (c) and shoot P concentration (d) of rice genotypes (G) DJ123 (DJ) and Nerica4 (N4) in response to different P treatments (T) including low (LP) and adequate (AP) phosphorus supply at 28 and 40 days after germination (DAG). Values represent means ± SE, n = 5–6. Different letters indicate significant differences within each time point (two-way ANOVA and LSD, P < 0.05)

Fig. 2

Root exudation rates per root surface area of rice genotypes (G) DJ123 (DJ) and Nerica4 (N4) in response to different P treatments (T) including adequate (AP) and low (LP) phosphorus supply at 28 and 40 days after germination (DAG). a Dissolved organic C (= total C). b Total soluble carbohydrates expressed in nmol Glc equivalents. c Total amino acids expressed in nmol Gly equivalents. d Total phenolic compounds expressed in CGA equivalents. Values represent means ± SE, n = 5–6. Different letters indicate significant differences within each time point (two-way ANOVA and LSD, P < 0.05). Asterisks indicate significant differences for each group between the two time points (two-sample unequal variance t-Test, P < 0.05). Glc glucose, Gly glycine, CGA chlorogenic acid, equiv equivalents, AP adequate phosphorus, LP low phosphorus, G genotype, T P treatment

Fig. 3

Root exudation rates of carboxylates per root surface area of rice genotypes (G) DJ123 (DJ) and Nerica4 (N4) in response to different P treatments (T) including adequate (AP) and low (LP) phosphorus supply at 28 and 40 days after germination (DAG). a Absolute exudation rates. Values represent means, n = 5–6. Different letters indicate significant differences across the different treatments and genotypes within each sampling event (two-way ANOVA and LSD, P < 0.05). Asterisks indicate significant differences for each group between the two time points (two-sample unequal variance t-Test, P < 0.05). b Relative contribution of individual organic anions (%) to total organic anions in root exudates. AP adequate phosphorus, LP low phosphorus, DJ DJ123, N4 Nerica4

Fig. 4

Estimated relative contribution of analyzed compound classes to total C exuded by rice genotypes DJ123 (DJ) and Nerica4 (N4) under adequate P supply (AP) and P deficiency (LP). DAG days after germination