Maintenance and Growth Components of Carbon Dioxide Efflux from Growing Pea Fruits

Carbon dioxide efflux from 5- to 20-day-old pea fruits was measured for plants grown in controlled environment at 15 °C and 600 μmol s−1 m−2 photon flux density in a 16 h photoperiod. The rate of CO2 output per fruit increased quickly from 0.005 to 0.018 mg CO2 min−1 during fruit elongation and subsequently more slowly to 0.030 mg CO2 min−1 as the fruits inflated. On a d. wt basis the rate was highest, 0.175 mg CO2 g−1 min−1, in the youngest fruits and declined curvilinearly with increasing fruit weight to 0.02 mg CO2 g−1 min−1. Separation of maintenance and growth components was achieved by starvation methods and by multiple regression analysis. From the latter method estimates of the maintenance coefficient declined hyperbolically from 150±8.7 mg carbohydrate g−1 d. wt day−1 in the very young fruits (0.05 g) to 10.4±0.36 mg carbohydrate g−1 d. wt day−1 in older fruits (2.0 g). On a nitrogen basis maintenance costs decreased from 2240 to 310 mg carbohydrate g−1 nitrogen day−1 while nitrogen concentration fell from 6.7 to 3 per cent d. wt. A simple linear relationship between maintenance cost per unit d. wt and nitrogen concentration was not observed. A growth coefficient of 50±6.7 mg carbohydrate g−1 growth (equivalent to a conversion efficiency, YG, of 0.95) was estimated for all fruits examined. The overall efficiency, Y, increased from a mean of 0.70 to 0.85 during fruit elongation and subsequently declined to 0.80. For a given fruit weight, efficiency increased asymptotically with relative growth rate; both asymptote and slope of the relationship increased as the fruits grew.