Daytime, Not Nighttime, Elevated Atmospheric Carbon Dioxide Exposure Improves Plant Growth and Leaf Quality of Mulberry (Morus alba L.) Seedlings

Abstract

Almost all elevated atmospheric CO₂ concentrations (eCO₂) studies have not addressed the potential responses of plant growth to different CO₂ in daytime and nighttime. The present study was to determine the impact of daytime and/or nighttime eCO₂ on growth and quality of mulberry (Morus alba L.), a perennial multipurpose cash plant. Six-month-old mulberry seedlings were hence grown in environmentally auto-controlled growth chambers under four CO₂ concentrations: (1) ambient CO₂ (ACO₂, 410 μmol mol^-1 daytime/460 μmol mol^-1 nighttime), (2) sole daytime elevated CO₂ (DeCO₂, 710 μmol mol^-1/460 μmol mol^-1), (3) sole nighttime elevated CO₂ (NeCO₂, 410 μmol mol^-1/760 μmol mol^-1), and (4) continuous daytime and nighttime elevated CO₂ (D + NeCO₂, 710 μmol mol^-1/760 μmol mol^-1). Plant growth characteristics, nutrient uptake, and leaf quality were then examined after 120 days of CO₂ exposure. Compared to control, DeCO₂ and (D + N)eCO₂ increased plant biomass production and thus the harvest of nutrients and accumulation of leaf carbohydrates (starch, soluble sugar, and fatty acid) and N-containing compounds (free amino acid and protein), though there were some decreases in the concentration of leaf N, P, Mg, Fe, and Zn. NeCO₂ had no significant effects on leaf yield but an extent positive effect on leaf nutritional quality due to their concentration increase in leaf B, Cu, starch, and soluble sugar. Meanwhile, (D + N)eCO₂ decreased mulberry leaf yield and harvest of nutritious compounds for silkworm when compared with DeCO₂. The reason may be associated to N, P, Mg, Fe, and Zn that are closely related to leaf pigment and N metabolism. Therefore, the rational application of mineral nutrient (especially N, P, Fe, Mg, and Zn) fertilizers is important for a sustainable mulberry production under future atmosphere CO₂ concentrations.

Keywords: CO2 enrichment; biomass production; free amino acid; mineral elements; nutrient use efficiency.

FIGURE 1

Mean temperature (A), relative humidity (B), photosynthetic active radiation (PAR) (C), and CO₂ concentration (D) over the experimental period in the growth chambers.

FIGURE 2

Plant biomass production (A) and carbon accumulation (B) of mulberry seedlings grown for 120 days under four different daytime and nighttime CO₂ concentrations inside environmentally controlled glass growth chambers. Data (means ± SE, n = 3) followed by different letters above the bars are significant differences between CO₂ treatments at P < 0.05. Abbreviations: ACO₂, ambient CO₂ (410 μmol mol^–1 daytime + 460 μmol mol^–1 nighttime); DeCO₂, sole daytime elevated CO₂ (710 μmol mol^–1 daytime + 460 μmol mol^–1 nighttime); NeCO₂, sole nighttime eCO₂ (410 μmol mol^–1 daytime + 760 μmol mol^–1 nighttime); (D + N)eCO₂, continuous daytime/nighttime eCO₂ (710 μmol mol^–1 daytime + 760 μmol mol^–1 nighttime). All CO₂ concentrations had a variation of ± 30 μmol mol^–1 in inside environmentally controlled growth chambers. Daytime: 07:30 a.m.–19:30 p.m. and nighttime: 19:30 p.m.–07:30 a.m.

FIGURE 3

Concentrations and accumulations of leaf macro–micro elements in mulberry seedlings grown for 120 days under four different daytime and nighttime CO₂ concentrations inside environmentally controlled glass growth chambers. Different letters indicate significant differences among CO₂ treatments at P < 0.05. See treatment abbreviations in Figure 1.

FIGURE 4

Nitrogen (N, A), phosphorus (P, B), potassium (K, C), calcium (Ca, D), magnesium (Mg, E), manganese (Mn, F), boron (B, G), copper (Cu, H), zinc (Zn, I) and iron (Fe, J) use efficiency in the leaves of mulberry seedlings grown for 120 days under different daytime and nighttime CO₂ concentrations inside environmentally controlled glass growth chambers. Different letters indicate significant differences among CO₂ treatments at P < 0.05. See treatment abbreviations in Figure 1; UE, use efficiency.

FIGURE 5

Concentrations and accumulations of leaf soluble sugar (A,F), starch (B,G), fatty acid (C,H), total free amino acid (D,I), and protein (E,J) of mulberry seedlings grown for 120 days under different daytime and nighttime CO₂ concentrations inside environmentally controlled glass growth chambers. Different letters indicate significant differencesamong CO₂ treatments at P < 0.05.