Plant Stress ©2010 Global Science Books
Plants under continuous light: A Review
Marina I. Sysoeva • Eugenia F. Markovskaya • Tatjana G. Shibaeva
Institute of Biology, Karelian Research Centre, Russian Academy of Science, 11, Pushkinskaya st., 185910 Petrozavodsk, Russia

In this review an attempt has been made to analyze the results of the studies that explored the changes in the growth and development of plants exposed to continuous light published since the 1930s, including papers that are available in Russian only. Potential benefits of using a 24-h photoperiod for the production of greenhouse crops, transplant production in closed controlled environment systems and the culture of plants in controlled ecological life support systems are reviewed. Continuous lighting is shown to be a useful tool for speeding up the selection of crops. The mechanisms involved in a plant’s response to continuous light and causes of negative effects of continuous light (foliar chlorosis, limited or reduced plant growth and productivity) are discussed. Plant response to continuous light depends on plant tolerance and can be modified by alterations in temperature, light intensity, CO2 level, humidity, mineral nutrition and other environmental factors.

For LD legume chickpea (Cicer arietinum L.), supplemental lighting was recommended under Indian conditions in order to provide continuous illumination, which greatly reduces the vegetation period and allows breeders obtaining four generations of chickpea a year (Sethi et al. 1981).

In pot roses (Rosa x hybrida) (neutral-day plant (NDP)) continuous light decreased the number of days to flowering by 12% and increased the number of flowers by 34% compared to 18-h photoperiod (Pettersen et al. 2006, 2007). In contrast, continuous light had no influence on number of days to flowering for Begonia (LDP) and geranium (Pelargonium x hortorum Bailey) (NDP), while increased the number of buds and flowers in begonia compared to 16-h daylength (Gislerød et al. 1989).

Continuous light is commonly used to accelerate the reproductive cycle in Arabidopsis (Arabidopsis thaliana (L.) Heynh.) (LDP) (Handling Arabidopsis Plants and Seeds 2004; Massa et al. 2007). It was assumed that the 24-hour illuminated Arabidopsis plants may not produce as much seed, having had less time to accumulate carbohydrates by vegetative growth (Massa et al. 2007). However, only one report was found suggesting that lower seed yields results from this treatment, and it involved ‘weak mutants’ (Hirono 1964).

There are some reports in the scientific literature with the examples of the use of continuous light to accelerate trees and shrubs timing. Thus, in experiments of Hohlova et al. (1976) and Moshkov (1987) with black currant (Ribes nigrum L.) they were unable in the first year to stimulate flowering by continuous light, although plants grew vigorously and at the age of 2-2,5 months looked like 2-year-old seedlings. Later Hohlova et al. (1979) found a particular combination of continuous light followed by SD resulted in flowering of 70% of plants at the age of 9 months and fruiting of 58% of plants. Hohlova (1979) also observed that seedlings grown under continuous light followed by SD had compared well with 3-5-year old fruiting seedlings in the field as regards to flowering intensity, disease resistance and winter hardiness. The ability to obtain black currant flowering in the first year allowed breeders intensification of their work as usually it required 3-5 years in Siberia to estimate cultivars by their fruit quality (Lisovskij and Dolgushev 1986). Working with honeysuckle (Lonicera edulis Turcz. Ex Freyn), sea buckthorn (Hippophae rhamnoides L.) and apple trees (Malus domestica Borkh.) Lisovskij and Dolgushev (1986) showed that grown under continuous light for 4-6.5 months plants looked similar to 2-3-year-old field grown seedlings and were suitable for spring transplanting to permanent place. They failed to induce flowering in the first year, nevertheless plants flowered and fruited some years earlier compared to field-grown plants.

There are several indications of the role of continuous light spectrum in plant development. Thus, Kasajima et al. (2007) investigated the developmental rate of wheat (a Japanese spring wheat var. Norin 61 and a winter wheat var. Shun-yo) under continuous light of eight different qualities obtained by combining three out of four different kinds of fluorescent lamps (white, blue, purplish red and ultraviolet- A) at a constant temperature of 20ºC. Results suggested that green and red lights play important roles in the regulation of the developmental rate having a promotive effect, independent of photoperiodism and vernalisation. This research aimed to accelerate heading, which shortens the generation cycle and improves the efficiency of the crossing. Apple trees grown under continuous light developed faster when supplemental red lighting was used (Isaeva 1978), while accelerated development of cucumber plants was observed when they were treated by supplemental blue light (L’vova 1978). Under continuous light, longer period of natural lighting in combination with shorter period of fluorescent lighting resulted in considerably faster generative development of vetch (Vicia spp.) plants (Rzhanova 1978). The great effect of light quality for continuous illuminating at 'night' on floral initiation of wild strawberry (Fragaria chiloensis L.) grown under a 24-h photoperiod was shown by Yanagi et al. (2006).

Due to its ability to accelerate reproductive cycle continuous light was shown to be a useful tool for the breeders of some LD crops as it provides the possibility to obtain several generations of plants in the winter period and ensures more uniform material in respect of developmental rate, time of flowering, leafness etc. compared to shorter photoperiods. For some crops combinations of continuous light followed or preceded by short or long photoperiods depending on plant sensitivity were found to be favorable.

Continuous Lighting