Root morphology and rhizosheath acid phosphatase activity in legume and graminoid species respond differently to low phosphorus supply

Given the worldwide effort to improve crop phosphorus (P) efficiency, it is important to understand the adaptive strategies of crops with different root types and properties under limited P availability. The present study examined root morphological and physiological responses of two graminoids (wheat and maize), and three legumes (soybean, white lupin and narrow-leafed lupin) to low P supply under two difference growth conditions. Two experiments using non-destructive root observation systems (Expt.1, semi-hydroponic system; Expt.2, rhizoboxes) were conducted under low P (Expt.1: 2 μM; Expt.2: 9 mg kg⁻¹ soil) and optimal P (Expt.1: 200 μM; Expt. 2: 60 mg kg⁻¹ soil). Plants were assessed 40 days after transplanting (Expt. 1) and 35 days after sowing (Expt. 2). Results showed that low P stress generally suppressed plant shoot and root growths of all crops and significantly increased root-shoot ratio in graminoid plants in both experiments. Root length vertical distribution in each 10 cm layer was relatively lower in low P than in optimal P treatments with the majority roots distributed in 10–40 cm layers in both experiments. Under low P, all plants accumulated significantly less P in plant tissues. Maize, soybean and white lupin plants had larger dry weights and P contents than wheat and narrow-leaf lupin under respective P treatment under both experiments except high P content in wheat cv Zhongyu 6 under optimal P in Expt. 2. Low P stress significantly increased plant P-use efficiency in both experiments with more profound increase in maize and white lupin in Expt. 1. Low P stress stimulated cluster roots formation in white lupin. Both lupin species had significantly higher rhizosheath acid phosphatase activity than other crops under low P supply. In conclusion, five crop species differed in their adaptive strategies to low P availability. Graminoid species had strong response by modifying their root morphology while legumes especially lupin species used the limited P more efficiently through enhancing rhizosheath acid phosphatase activity to cope with soil P deficit.