Studies on Photosynthesis in Sugar Beet Plant. : II. The difference in photosynthetic and respiratory abilities and response to temperature and light between leaves of different leaf positions.

Abstract

The abilities and some characters in photosynthesis and respiration of single leaf of sugar beet plant were studied with leaves of different positions. The intensity of photosynthesis was measured under a condition of almost constant CO₂ concentration of about 300 ppm. The results are summarized as follows. (1) Total photosynthetic and respiratory abilities of top parts of a plant increase at a little lower rate than that of the increment of total leaf area and total dry weight, respectively. (2) Photosynthetic ability per unit leaf area is highest in leaves of middle position, and is lower in leaves of upper and lower positions. On the other hand, respiratory ability per unit leaf dry weight is highest in leaves of upper position, and is lowest in leaves of lower position. As for the photosynthetic ability per leaf, the leaves of middle position have the highest photosynthetic ability because of their large leaf area as well as their high photosynthetic ability per unit leaf area. The respiratory ability in the leaves of middle position is higher than that of any other leaves because of their heavier dry weight. (3) The temperature coefficient (Q₁₀) of respiration is about 1.8 at the temperature ranging from 20°C to 35°C. This value is a little smaller at the temperature ranging from 10°C to 20°C. (4) Photosynthesis is hardly affected by temperature ranging from 10°C to 35°C, which is an ordinary temperature condition in culture field of sugar beet plant. (5) Light response curve of photosynthesis is saturation curve, having a saturation point at the light intensity of about 30 Klx. (6) These characters of leaves mentioned in (3), (4), and (5), are not appreciably different from leaves of different positions and of different growth stages of plant during about five months after sowing.