CO2 and temperature effects on leaf area production in two annual plant species

We studied leaf area production in two annual plant species, Abutilon theophrasti and Amaranthus retroflexus, under three day/night temperature regimes (18 [degrees]/14 [ degrees], 28 [degrees]/22 [degrees], and 38 [degrees]/31 [degrees] C) and two concentrations of carbon dioxide (400 and 700 [mu]L/L). The production of whole-plant leaf area during the first 30 d of growth was analyzed in terms of the leaf initiation rate, leaf expansion, individual leaf area, and, in Amaranthus, production of branch leaves. Temperature and [CO.sub.2] influenced leaf area production through effects on the rate of development, determined by the production of nodes on the main stem (the plastochron index), and through shifts in the relationship between whole-plant leaf area and the number of main stem nodes. In Abutilon, leaf initiation rate was highest at 38 [degrees], but area of individual leaves was greatest at 28 [degrees]. Total leaf area was greatly reduced at 18 [degrees] due to slow leaf initiation rates. Elevated [CO.sub.2] concentration increased leaf initiation rate at 28 [degrees], resulting in an increase in whole-plant leaf area. In Amaranthus, leaf initiation rate increased with temperature, and was increased by elevated [CO.sub.2] at 28 [degrees]. Individual leaf area was greatest at 28 [degrees], and was increased by elevated [CO.sub.2] at 28 [degrees] but decreased at 38 [degrees]. Branch leaf area displayed a similar response to [CO.sub.2], but was greater at 38 [degrees]. Overall, whole-plant leaf area was slightly increased at 38 degrees] relative to 28 [degrees], and elevated [CO.sub.2] levels resulted in increased leaf area at 28 [degrees] but decreased leaf area at 38 [degrees]. The effects on leaf area closely parallel rates of biomass accumulation in the same experiment, suggesting that responses of developmental processes to elevated [CO.sub.2] and interacting factors may play an important role in mediating effects on plant growth.