Carbon Dioxide and Water Vapour Exchange Parameters of Photosynthesis in a Crassulacean Plant, Kalanchoe daigremontiana

Abstract

In 16-h days and under a 26/19°C day/night temperature regime, leaves of K. daigvemontiana took up CO₂ in the daytime as well as at night. Daytime net CO₂ uptake was in two periods: an initial one about 5 h long, in which net uptake rose to a maximum and then fell to near zero; and, after a period in which stomata were closed and CO₂ exchange did not take place, a second period of net uptake. In this second period net uptake rose rapidly to a steady rate which was maintained for about 8 h until the end of the day. In this period, mean rate of light-saturated photosynthesis in normal air was 25 ng cm^-2 s^-1; maximum obtainable photosynthesis rate averaged 87 ng cm^-2 s^-1 at this time. These values are considerably lower than those to be expected for C₃ plants. The internal resistance to CO₂ uptake averaged about 8 s cm-l. In normal air, leaf resistance to water vapour diffusion averaged about 8.5 s cm^-1 in the steady period of photosynthesis. Both these resistances are higher than those often reported for C₃ plants, and may explain the low photosynthetic rates. The CO₂ compensation point during the period of steady photosynthesis was about 51 μ 1/1 (92 ng cm^-3). This value, together with some biochemical evidence from pulse-chase experiments, suggests that photosynthesis in the last 8 h of the light period follows a C₃-type pathway.