キュウリの光合成産物の動態に及ぼす温度の影響 (第1報)

Abstract

The purpose of the present study was to know diurnal changes of air temperature favourable for the raising of cucumber seedlings. Activities of assimilation and distribution of photosynthates in plant parts were measured by a G.M. counter using tracer techniques.Seeds of cucumber, variety “Natsu-Sairaku No.3” were sown on Jan. 16, 1971, and transplanted into plastic pots, 12cm in diameter, on Jan. 28 at the stage of cotyledon expansion. Then the pots were placed in the phytotron which day and night temperatures were programmed with consideration of the diurnal change of natural temperature. The day temperature was set from 10:00AM to 2:00PM and the night one from 2:00AM to 6:00AM. The shift between day and night temperatures were made as moderate as possible. This temperature regulating system was called “the gradient temperature control system”.To clarify the effect of day or night temperature, temperatures were maintained at 28°C for the day temperature and 14°C for the night temperature (abbr. 28°-14°), 25°-14°, 20°-14°, 15°-14°, and 25°-18°, 25°-14°, 25°-10° (Fig.1). Seedlings raised under the condition of 25°-14° were taken as control.On Feb. 25, whole plants were exposed to 14CO2 for 60 minutes in the artificial light, 18, 000lux at the plant level, and the distribution of 14C-radioactivities in the plant is determined immediately after 14CO2 feeding (Exp. A) or after being kept for one night at the corresponding night temperature (Exp. B). The experiments of A and B were done separately.When the plants were raised at 14°C for the night temperature, the effect of day temperature on growth was as follows; the growth shown in dry weight at 28°-14° was almost the same as that of control (25°-14°), but at 20°-14° and 15°-14°, those were 80 and 70% of control, respectively (Table 2). Senescence of lower leaves of plants grown at 28°C during the day and 14°C at night was accelerated in comparison with control (Fig. 2).When the plants were raised at 25°C for the day temperature, the effect of night temperature was as follows; at 25°-18°, plants grew heavier and broader in the leaf area (Table 2), and higher (Table 1) than control (25°-14°), and senescence of lower leaves was accelerated (Fig.2). Their photosynthetic activity (14C-radioactivity per dry weight) was lower than control, and dry weight per unit leaf area (cm2) was the lowest (Fig.3). The translocation of photosynthates in the plants grown at 28°-14°, 25°-14°, and 20°-14° was greater than in those grown at 15°-14°. The translocation of photosynthates in the plants grown at 25°-18°, 25°-14° was greater than in those grown at 25°-10°. However, the acceleration of respiration rates in plants grown at 25°-18° resulted in a release of CO2 from 14C-photosynthates outside of the plant.In so far as seedlings were raisen under the gradient temperature control system, the best possible growth of seedlings from the view point of 14CO2-assimilation and translocation of photosynthates was given when they were grown at 20°-28°C for day- and 14°C for night-temperature. Among them, when seedlings raised at 28°C day temperature, the senescence of lower leaves was promoted and at 20°C day temperature, the production of dry matter was lower than at 25°C or 28°C day temperature. Therefore, the alternation of temperature at 25°C for the day temperature and 14°C for the night temperature was most suitable for the healthful growth of seedlings.