Chlorophyll fluorescence is red light emitted from chlorophyll a pigment and hence accurate measurement of this emission allows us to assess photosynthetic functions of the plant. The chlorophyll fluorescence induction imaging measures a chlorophyll fluorescence induction phenomenon. This phenomenon is a dynamic change in chlorophyll fluorescence intensity induced by illuminating plant body with an excitation light at a stable intensity under dark condition. The time course of the chlorophyll fluorescence intensity during this phenomenon is called induction curve. In this study, we developed a chlorophyll fluorescence induction imagining system for whole tomato plants as a first prototype for plant diagnosis in greenhouse. By using this system, we assessed the effects of sunlight exposure treatment, i.e. PPFD 1500 μmol m⁻²s⁻¹ for 2.5 h, on the photosynthetic functions of a whole tomato plant of 1.1 m high. A substantial transformation of induction curve was observed between before and just after the treatment, but it was recovered in 6 h under dark condition. During the recovery process, the inflection points of P and M, which are the characteristic inflections of an induction curve, showed different behaviour. The M kept lower values for 1.5 h after the sunlight exposure treatment even though the P had been almost recovered. This result suggests that the chlorophyll fluorescence induction imaging, especially concurrent monitoring of the images of P and M inflection points, is useful to detect invisible photosynthetic dysfunctions at whole plant level.

View full abstract

Supplemental red light irradiation at the beginning of the dark period or blue light irradiation at the end of the dark period at a low level (0.6% of the irradiance of the light period) compared to no supplemental irradiation (control) promoted biomass production and was accompanied by an increase in the leaf area in spinach. Photosynthetic rate and stomatal conductance measured under white light in the leaves of plants grown under supplemental lighting during the dark period were the same as those of the control plants. Microscopic analysis of the transverse and longitudinal directions of the leaves revealed that red light irradiation at the beginning of the dark period increased leaf cell size, while leaf cell size was not affected by blue light irradiation at the end of the dark period. These results suggest that red light irradiation at the beginning of the dark period accelerates leaf expansion by an increase in leaf cell size and blue light irradiation at the end of the dark period accelerates leaf expansion by an increase in leaf cell number. The increase in leaf area by supplemental lighting leads to an increase in photosynthesis at the whole-plant level, which contributes to the enhancement of biomass production under suboptimal light irradiation.

View full abstract

We investigated fluctuations in the fruit maturation period (from flowering to harvest), anthocyanin content, and reduced ascorbic acid content in strawberry (Fragaria×ananassa Duch.) depending on the differences in harvest time, and discussed their relationships with the cultivation environment (solar radiation, temperature). The fruit maturation period of ‘Nyoho’ and ‘Toyonoka’ decreased significantly as cultivation progressed. In both cultivars, the fruit maturation period had a high correlation with solar radiation and temperature, and in particular, a complete proportional relationship was observed between integrated maximum temperature and fruit maturation period, suggesting the possibility of predicting harvest dates using the regression formula. Although the anthocyanin content of ‘Nyoho’ was higher than that of ‘Toyonoka’, a difference in harvest time was scarcely observed between the two cultivars. Furthermore, the correlation between the anthocyanin content and solar radiation or temperature was significantly low compared with the fruit maturation period, but that in ‘Toyonoka’ tended to be higher than that in ‘Nyoho’, indicating the characteristics of both cultivars. The reduced ascorbic acid content was the same in both cultivars, significantly decreasing in the middle of the cultivation period, and its correlation with solar radiation and temperature fluctuated irrespective of their variations.

View full abstract

To study the effects of ultraviolet (UV) light on the essential oil concentration in Japanese mint (Mentha arvensis L. var. piperascens), we exposed the plants to different combinations of irradiation with a white fluorescent lamp (W), UV-A fluorescent lamp (UVA; peak wavelength, 360 nm), and UV-B fluorescent lamp (UVB; peak wavelength, 306 nm). Japanese mint transplants hydroponically grown from a rhizome in a controlled environment were used as the plant material. Young plants were cultivated in growth chambers [air temperature, 25/23°C; photosynthetic photon flux, 250 μmol·m⁻²·s⁻¹; CO₂ concentration, 1,000 μmol·mol⁻¹] under the following light conditions: W, W+ UVA (2.0 mW·m⁻², 315–400 nm), W+UVB (0.5 mW·m⁻², 280–350 nm), and W+UVA+ UVB (2.5 mW·m⁻², 280–400 nm). The UV irradiation period was 2 h per day during the light period (12 h). After 7 days of irradiation, the plants grown in different light conditions showed no difference in the number of leaves and leaf area. The l-menthol and limonene concentrations and the total antioxidant capacity (TAC) in the upper leaves of plants grown under the W+UVB and W+UVA+UVB conditions were significantly higher than those in the upper leaves of plants grown under the W condition. The upper leaves unfolded after the initiation of UV irradiation; further, supplemental UV-B irradiation seemed to increase the essential oil concentration and the TAC of the leaves. These results suggest that longer supplemental UV-B irradiation of Japanese mint plants may increase the yield of essential oils per plant by increasing the number of leaves that contain high concentrations of essential oils.

View full abstract