Alteration in root morphological and physiological traits of two maize cultivars in response to phosphorus deficiency

Phosphorus-efficient cultivars have an increased capacity to exploit the soil and/or to convert non-available forms of phosphorus (P) into available ones for root uptake. Preliminary field observations showed that nitrogen (N) inefficient maize cultivar ZD958 exhibited high tolerance to low P soil in the shoot performance than N-efficient cultivar XY335. However, it remains unknown whether ZD958 had a higher potential in acquiring sparsely soil P than XY335. In this study, three separate glasshouse experiments were carried out to explore the capacity of both cultivars in accessing soil P. The two cultivars grew for 40 days to evaluate functional root traits in soil columns (Expt.1) and for 20 days to examine root hair length in soil filled rhizoboxes (Expt.3) in a calcareous loamy soil amended with P (high P) or background soil (low P). The third experiment (Expt.2) cultured hydroponically the both cultivars for 15 days to determine the correlation between P uptake rate and P supply intensity. Results from Expt.1 showed that shoot P content of ZD958 was significantly higher than XY335. Higher capacity of the ZD958 to access soil P were not correlated with biomass allocation and root morphological traits, since ZD958 had a lower root mass ratio at two P supplies and a lower degree of total root length and root surface area at high P than XY335. Results from Expt.3 indicated that average root hair length was not associated with the improved P uptake by ZD958, since their root hair length remained the same under low P or high P supply. Therefore, changes in root morphological traits did not explain the improved P acquisition by ZD958 roots. Rhizosphere pH, malic acid concentration and acid phosphatase activity were similar between the two cultivars, while citric acid concentration was obviously higher in ZD958 than that in XY335 (Expt.1). These results suggested that the higher capacity of P acquisition by ZD958 might be attributed to the release more citric acids from roots. P uptake rate of ZD958 was significantly higher than XY335 with the solution P ranging from 12.5 to 250 μM (Expt. 2), indicating that the changes in P uptake rate largely explained the greater P acquisition by ZD958 than XY335. Taken together, ZD958 was identified as a P-efficient cultivar, with a better adaptation to low P environment by altering the exudation of citric acid and P uptake rate. The research findings highlight the importance of plant strategies in altering the root morphological and physiological traits for a better adaptation to the low P environment, and enhance our understanding of physiological mechanisms of plant P acquisition.