The Effects of Elevated Atmospheric Carbon Dioxide and Water Stress on Ligth Interception, Dry Matter Production and Yield in Stands of Groundnut (Arachis hypogaeaL.

Abstract

Stands of groundnut (Arachis hypogaea L.), a C₃ legume, were grown in controlled-environment glasshouses at 28 °C (±5°C) under two levels of atmospheric CO₂ (350 ppmv or 700 ppmv) and two levels of soil moisture (irrigated weekly or no water from 35 d after sowing). Elevated CO₂ increased the maximum rate of net photosynthesis by up to 40%, with an increase in conversion coefficient for intercepted radiation of 30% (from 1–66 to 2–16 g MJ⁻¹) in well-irrigated conditions, and 94% (from 0–64 to 1·24 g MJ⁻¹) on a drying soil profile. In plants well supplied with water, elevated CO₂ increased dry matter accumulation by 16% (from 13·79 to 16·03 t ⁻¹ ) and pod yield by 25% (from 2·7 to 3·4t ha⁻¹). However, the harvest index (total poddry weight/above-ground dry weight) was unaffected by CO₂ treatment. The beneficial effects of elevated CO₂ were enhanced under severe water stress, dry matter production increased by 112% (from 4·13 to 8·87 t ha⁻¹) and a pod yield of 1·34t ha⁻¹ was obtained in elevated CO₂, whereas comparable plotsat 350 ppmv CO₂ only yielded 0·22 t ha^-1. There was a corresponding decrease in harvest index from 0·15 to 0·05. Following the withholding of irrigation, plants growing on a stored soil water profile in elevated CO₂ could maintain significantly less negative leaf water potentials (P2, allowing prolonged plant activity during drought. In plants which were well supplied with water, allocation of dry matter between leaves, stems, roots, and pods was similar in both CO₂ treatments. On a drying soil profile, allocation in plants grown in 350 ppmv CO₂ changed in favour of root development far earlier in the season than plants grown at 700 ppmv CO₂, indicating that severe waterstress was reached earlier at 350 ppmv CO₂. The primary effects of elevated CO₂ on growth and yield of groundnut stands weremediated by an increase in the conversion coefficient for intercepted radiation and the prolonged maintenance of higher leaf water potentials during increasing drought stress.