Leaf Dynamics of Panicum maximum under Future Climatic Changes

Abstract

Panicum maximum Jacq. 'Mombaça' (C4) was grown in field conditions with sufficient water and nutrients to examine the effects of warming and elevated CO2 concentrations during the winter. Plants were exposed to either the ambient temperature and regular atmospheric CO2 (Control); elevated CO2 (600 ppm, eC); canopy warming (+2°C above regular canopy temperature, eT); or elevated CO2 and canopy warming (eC+eT). The temperatures and CO2 in the field were controlled by temperature free-air controlled enhancement (T-FACE) and mini free-air CO2 enrichment (miniFACE) facilities. The most green, expanding, and expanded leaves and the highest leaf appearance rate (LAR, leaves day(-1)) and leaf elongation rate (LER, cm day(-1)) were observed under eT. Leaf area and leaf biomass were higher in the eT and eC+eT treatments. The higher LER and LAR without significant differences in the number of senescent leaves could explain why tillers had higher foliage area and leaf biomass in the eT treatment. The eC treatment had the lowest LER and the fewest expanded and green leaves, similar to Control. The inhibitory effect of eC on foliage development in winter was indicated by the fewer green, expanded, and expanding leaves under eC+eT than eT. The stimulatory and inhibitory effects of the eT and eC treatments, respectively, on foliage raised and lowered, respectively, the foliar nitrogen concentration. The inhibition of foliage by eC was confirmed by the eC treatment having the lowest leaf/stem biomass ratio and by the change in leaf biomass-area relationships from linear or exponential growth to rectangular hyperbolic growth under eC. Besides, eC+eT had a synergist effect, speeding up leaf maturation. Therefore, with sufficient water and nutrients in winter, the inhibitory effect of elevated CO2 on foliage could be partially offset by elevated temperatures and relatively high P. maximum foliage production could be achieved under future climatic change.

Fig 1. Daily courses of meteorological conditions and soil temperature during the period of the experiment, from August 22 to September 20, 2013.

A—Total solar radiation. B—Air relative humidity (RH) and air temperature. C—Soil temperature in control and heated plots.

Fig 2. Canopy temperature during the period of the experiment, from August 22 to September 20, 2013.

A—Canopy temperature of heated (solid line) and Control (dashed line) treatments. B—Difference (ΔT canopy) between heated and regular canopy temperatures indicating the deviations from the target elevated temperature of 2°C above the Control.

Fig 3. Number of green (A), expanded (B), expanding (C), and senescent (D) leaves per tiller of Panicum maximum under ambient CO₂ and canopy temperature (Control), under an elevated CO₂ concentration of 600 ppm (eC), under an elevated canopy temperature of +2°C (eT), and under both treatments (eC+eT).

Bars indicate average values and lines at the top of bars indicate the standard error. Different letters above bars indicate significant differences among datasets according to a Mann-Whitney test at p < 0.05.

Fig 4. Leaf appearance rate (A), leaf elongation rate (B), leaf area (C), and leaf biomass (D) per tiller of Panicum maximum under regular CO₂ and canopy temperature (Control), under elevated CO₂ concentration of 600 ppm (eC), under elevated canopy temperature of +2°C (eT), and under both treatments (eC+eT).

Bars show average values and lines at the top of the bars show the standard error. Different letters above bars indicate significant differences among datasets according to a Mann-Whitney test at p < 0.05.

Fig 5. The leaf/stem biomass ratio (A) and leaf nitrogen content (B) of Panicum maximum under regular concentration of CO₂ and canopy temperature (Control), under elevated CO₂ concentration of 600 ppm (eC), under elevated canopy temperature of +2°C (eT), and under both treatments (eC+eT).

Bars show average values and lines at the top of the bars the standard error. Different letters above the bars indicate significant differences among the datasets after the Mann-Whitney test at p < 0.1.

Fig 6. Average values of leaf biomass as a function of leaf area per tiller of Panicum maximum under regular atmospheric CO₂ concentration and canopy temperature (Control, A), under elevated atmospheric CO₂ concentration of 600 ppm (eC, B), under elevated canopy temperature of +2°C (eT, C), and under both treatments (eC+eT, D).

Fig 7. Accumulated number of expanded (A), expanding (B), and senescent (C) leaves; and the mean number of cut-expanded leaves (D) per tiller of Panicum maximum growing under different atmospheric conditions.

A—Regular concentration of CO₂ and canopy temperature (Control). B—Elevated CO₂ concentration of 600 ppm (eC). C—Elevated canopy temperature of +2°C (eT). D—Combination of treatments (eC+eT). The days of measurement were August 22 and 29, and September 3, 9, 12, and 20, 2013.