Effect of Temperature on the Translocation of Photosynthates in Cucumber Plants

II. Effect of Night-temperature on the Translocation of ¹⁴C-photosynthates in Cucumber Seedlings

Abstract

In the previous paper (6), it was reported that the temperature of 25°C in day and 14°C at night was the most suitable for raising healthy seedlings of cucumber. The purpose of the present study was to examine the effect of night-temperature on the translocation of ¹⁴C-photosynthates after ¹⁴CO₂ feeding by using tracer techniques.
Seeds of cucumber, variety“Natu-Sairaku No. 3”were sown on April 17, 1971, and transplanted into plastic pots of 15cm in diameter on April 21 at the stage of cotyledon expansion. From April 24, the pots were placed in a phytotron programmed by“the gradient temperature control system”(6). The day-temperature was set from 10:00AM to 14:00PM at 25°C and the night-one from 2:00AM to 6:00AM at 14°C. The shift between day- and night-temperatures was made as moderate as possible.
On May 30, uniformly raised seedlings were exposed to ¹⁴CO₂ in an assimilation chamber for 60 minutes under the natural light condition. Then the distribution of ¹⁴C-radioactivities was determined immediately after ¹⁴CO₂ feeding (Exp. A), and two days (Exp. B), and three days (Exp. C) later. The temperature after ¹⁴CO₂ feeding was kept at 25°C in day and at 20°C (high-night; H) or 14°C (low-night; L) at night by the horizontal temperature control system.
Results obtained were summarized as follows:
1. Percentage of dry matter to fresh weight decreased at the high night-temperature after ¹⁴CO₂ feeding, while it remained constant in the low night-temperature. It is considered that the plant exhausted much more photosynthates at the high night-temperature (20°C) than at the low one (14°C) (Fig.3).
2. Specific activity of respective plant part, shown in Table 2 was the highest in the blade of the tenth leaf from the base, and very low in the old- (1st-2nd) and adult-leaves (3rd-7th), when it was determined immediately after ¹⁴CO₂ feeding. After ¹⁴CO₂ feeding, specific activity of the young leaves except stem apex (8th-12th) decreased remarkably, and those of the stem apex (T), the stem, and the root increased, at high night-temperature. In other hand, only a little decrease in specific activity occurred at the low night-temperature (Fig.4).
3. After the dried sample of each plant part was extracted with hot ethyl alcohol, the alcohol extract was fractionated into four fractions by the procedure described in Figure 1. Radioactivities of five fractions including the alcohol insoluble fraction were compared for each plant part. Radioactivity of the alcohol insoluble fraction containing starch, protein and cellulose increased in the low night-temperature lot, but not in the high night-temperature lot (Fig. 11).
4. The translocation of photosynthates was faster and their exhaustion by respiration was higher at the high night-temperature (20°C) than at the low night-one (14°C). Consequently, the final reservation of photosynthates was more at the low night-temperature than at the high night-one.
On the efficient temperature management in the cultivation under growing structures, it is reasonably recommended from the point of reservation of photosynthates that the air temperature should be kept low at night.