Phosphorus application and elevated CO2 enhance drought tolerance in field pea grown in a phosphorus-deficient vertisol

Abstract

Background and aims: Benefits to crop productivity arising from increasing CO2 fertilization may be offset by detrimental effects of global climate change, such as an increasing frequency of drought. Phosphorus (P) nutrition plays an important role in crop responses to water stress, but how elevated CO2 (eCO2) and P nutrition interact, especially in legumes, is unclear. This study aimed to elucidate whether P supply improves plant drought tolerance under eCO2.

Methods: A soil-column experiment was conducted in a free air CO2 enrichment (SoilFACE) system. Field pea (Pisum sativum) was grown in a P-deficient vertisol, supplied with 15 mg P kg(-1) (deficient) or 60 mg P kg(-1) (adequate for crop growth) and exposed to ambient CO2 (aCO2; 380-400 ppm) or eCO2 (550-580 ppm). Drought treatments commenced at flowering. Measurements were taken of soil and leaf water content, photosynthesis, stomatal conductance, total soluble sugars and inorganic P content (Pi).

Key results: Water-use efficiency was greatest under eCO2 when the plants were supplied with adequate P compared with other treatments irrespective of drought treatment. Elevated CO2 decreased stomatal conductance and transpiration rate, and increased the concentration of soluble sugars and relative water contents in leaves. Adequate P supply increased concentrations of soluble sugars and Pi in drought-stressed plants. Adequate P supply but not eCO2 increased root length distribution in deeper soil layers.

Conclusions: Phosphorus application and eCO2 interactively enhanced periodic drought tolerance in field pea as a result of decreased stomatal conductance, deeper rooting and high Pi availability for carbon assimilation in leaves.

Keywords: Climate change; FACE; P nutrition; Pisum sativum; crop nutrition; drought tolerance; free air CO2 enrichment; pea; root length distribution; stomatal conductance; water-use efficiency.

Fig. 1.

Daily rainfall, solar radiation, and minimal (T_min) and maximal (T_max) temperatures during the experimental period from 15 June to 15 October 2012 near the experimental site. Three rounds of measurement on water status were at the initial-phase drought (63–70 % FWC) (Day 107 after sowing), mid-phase drought (52–57 % FWC) (Day 114) and final-phase drought (43–46 % FWC) (Day 122), respectively.

Fig. 2.

The effects of CO₂, P and water regime on dry weight (DW) of shoots (A) and roots in the 0–20, 20–40 and 40–60 cm soil layers (B), and water-use efficiency (WUE) (C) of field pea. Plants were exposed to ambient (aCO₂) or elevated CO₂ (eCO₂) treatments for 123 d in a P-deficient vertisol soil supplied with 15 (P15) or 60 mg P kg⁻¹ (P15) soil, and drought-stressed plants had water withheld until the soil reached the permanent wilting point in the last 3 weeks of the experiment. Columns are means of four replicates ± s.e. The vertical bars indicate the the LSD (P = 0.05).

Fig. 3.

The effects of CO₂ and P water regime on the stress tolerance index (STI) of field pea. Plants were exposed to ambient (aCO₂) or elevated CO₂ (eCO₂) treatments for 123 d in a P-deficient vertisol soil supplied with 15 (P15) or 60 mg P kg⁻¹ (P60) soil, and drought-stressed plants had water withheld until the soil reached the permanent wilting point in the last 3 weeks of the experiment. Columns are means of four replicates ± s.e. The vertical bars indicate the the LSD (P = 0.05).

Fig. 4.

The effects of CO₂, P and water regime on root length (A) and root length distribution of field pea in the 0–20 cm (20), 20–40 cm (40) and 40–60 cm (60) soil layers (B). Plants were exposed to ambient (aCO₂) or elevated CO₂ (eCO₂) treatments for 123 d in a P-deficient vertisol supplied with 15 (P15) or 60 mg P kg⁻¹ (P60) soil, and drought-stressed plants had water withheld until the soil reached the permanent wilting point in the last 3 weeks of the experiment. Columns are means of four replicates ± s.e. The vertical bars indicate the the LSD (P = 0.05).

Fig. 5.

The effects of CO₂, P and water regime on root nodule number (A), nodule dry weight (B), plant N concentration (C) and total N uptake (d) of field pea. Plants were exposed to ambient (aCO₂) or elevated CO₂ (eCO₂) treatments for 123 d in a P-deficient vertisol supplied with 15 (P15) or 60 mg P kg⁻¹ (P60) soil, and drought-stressed plants had water withheld until the soil reached the permanent wilting point in the last 3 weeks of the experiment. Columns are means of four replicates ± s.e. The vertical bars indicate the the LSD (P = 0.05).

Fig. 6.

The effects of CO₂, P and water regime on P concentration in shoot (A) and roots (B), and total P uptake (C) of field pea. Plants were exposed to ambient (aCO₂) or elevated CO₂ (eCO₂) treatments for 123 d in a P-deficient vertisol supplied with 15 (P15) or 60 mg P kg⁻¹ (P60) soil, and drought-stressed plants had water withheld until the soil reached the permanent wilting point in the last 3 weeks of the experiment. Columns are means of four replicates ± s.e. The vertical bars indicate the the LSD (P = 0.05).

Fig. 7.

The effects of CO₂, P and water regime on stomatal conductance (g_s) (A) and instantaneous transpiration efficiency (ITE) (B) of field pea at the flowering stage. Plants were exposed to ambient (aCO₂) or elevated CO₂ (eCO₂) treatments for 123 d in a P-deficient vertisol supplied with 15 (P15) or 60 mg P kg⁻¹ (P60) soil, and drought-stressed plants had water withheld to generate 63–70 % of field water capacity (FWC) (initial-phase drought at Day 107), 52–57 % of FWC (mid-phase drought at Day 114) and 43–46 % of FWC (final-phase drought at Day 122) during the last 3 weeks of the experiment. Columns are means of four replicates ± s.e. The vertical bars indicate the LSD (P = 0.05).

Fig. 8.

The effects of CO₂, P and water regime on relative water content of leaf (RWC) (A), and concentrations of total soluble sugars (B) and inorganic P (Pi) (C) in leaves of field pea at the flowering stage. Plants were exposed to ambient (aCO₂) or elevated CO₂ (eCO₂) treatments for 123 d in a P-deficient vertisol supplied with 15 (P15) or 60 mg P kg⁻¹ (P60) soil, and drought-stressed plants had water withheld to generate 63–70 % of field water capacity (FWC) (initial-phase drought at Day 107), 52–57 % of FWC (mid-phase drought at Day 114) and 43–46 % of FWC (final-phase drought at Day 122) during the last 3 weeks of the experiment. Columns are means of four replicates ± s.e. The vertical bars indicate the LSD (P = 0.05).