CARBON-DIOXIDE EFFLUX FROM ROOTS OF CALAMODIN AND APPLE

The specific rate of CO2 efflux (respiration) from roots of intact fruiting calamodin plants (Citrus madurensis Lour.) showed no diel trend, and did not respond significantly to short-term (2 day) changes in shoot irradiance. Mean root respiration rate was about 8.4 nmol CO2 g-1 s-1 at 20-degrees-C, and increased with temperature with a Q10 of about 2. In calamodin plants, the proportion of total root length that was white averaged 6.0 mm m-1. Respiration of roots of apple plants (Malus domestica Borkh.), planted in spring as rootstocks and grown at high irradiance and N supply, declined from about 5.3 to 2.8 nmol CO2 g-1 s-1 between 46 and 138 days after bud burst. At 50% irradiance, root respiration was reduced more than 25% at 46 and 92 days after bud burst, but was not significantly affected later in the season. Reducing shoot irradiance reduced the proportion of total root length that was white, e.g., from 217 to 146 mm m-1 at 46 days after bud burst. For plants previously grown at low irradiance, increasing shoot irradiance for 6 days increased the rate of root respiration by 5 to 10%. For plants previously grown at high irradiance, reducing shoot irradiance for 6 days reduced root respiration by about 20% early in the season, but had no significant effect later in the season. For plants grown with low-N supply (5% of the high-N), root respiration was reduced early in the season, but was not significantly affected later. Reducing the N supply increased slightly the proportion of total root length that was white. For plants previously grown with low-N, increasing the N supply for 6 days reduced further the rate of root respiration. For plants previously grown with high-N, reducing the N supply for 6 days did not significantly affect the rate of root respiration. Specific respiration rates of root systems excised from mature trees growing outdoors peaked in June, at about 2.4 nmol CO2 g-1 s-1, and then declined for the remainder of the growing season.