Recovery of tomato (Solanum lycopersicum L.) leaves from continuous light induced injury

Impact of streetlights on physiology, biochemistry and diversity of urban bryophyte: a case study on moss Semibarbula orientalis

The use of artificial light at night is a very basic symbol of urbanization and has distorted many ecological, biochemical and physiological phenomena in plants, which have settled for millions of years in the biological system. Continuous illumination of light significantly alters the circadian rhythm of all organisms. The present study was focused to understand the effects of continuous light (CL) on the biochemistry and physiology of moss Semibarbula orientalis. It was observed that H2O2 accumulation and activities of chlorophyllase, phenylalanine ammonia-lyase, superoxide dismutase and catalase enzymes significantly enhanced in plants growing under streetlights. Similarly, plants under CL showed a marked reduction in photosynthetic performance. Specific fluxes (ABS/RC, TR/RC, ET/RC), phenomenological fluxes (ABS/CS, TR/CS, ET/CS), density of photosystem-II, quantum yield of photosynthesis and chlorophyll concentration markedly declined in plants growing under streetlights. Depletion in performance indices (PIcs and PIabs) and primary and secondary photochemistry [PHIO/(1 − PHIO) and PSIO/(1 − PSIO)] were also noticed, which indicated failure of adaptive strategies of photosystem-II, resulting in the loss of biomass of S. orientalis. Biomass decline is also shown by a decrease in coverage, which reduces the bryophyte species richness of the chosen locations. Present studies clearly indicate that artificial light at night drastically affects the moss population. The reduction in the dominating species, S. orientalis, improves species evenness and results in a slow growth rate.

Leaf chlorosis, epinasty, carbohydrate contents and growth of tomato show different responses to the red/blue wavelength ratio under continuous light

The induction of leaf injuries, including leaf chlorosis and epinasty, by continuous light in tomato plants is one of the most interesting and mysterious phenomena regarding plant interactions with light, the mechanism of which has not yet been revealed. To gain further insights into this particular response of tomato plants, we cultivated tomato seedlings (Solanum lycopersicum cv. Momotaro) for 14 days under continuous light with different ratios of red and blue light and compared their performance to those grown under continuous or 14/10-h photoperiodic white light using novel methods to quantitatively evaluate the level of leaf chlorosis and epinasty. Continuous monochromatic blue light induced severe chlorosis but almost completely alleviated epinasty in tomato leaf. In contrast, continuous monochromatic red light caused a lower level of leaf chlorosis but very severe epinasty. The combination of red and blue light at different ratios significantly reduced both leaf chlorosis and epinasty under continuous light condition. Carbohydrate contents showed no correlation with leaf chlorosis, while glucose and fructose contents showed correlations with the petiole and leaflet curvatures. Histochemical staining with 3,3'-diaminobenzidine and nitro blue tetrazodium chloride also did not reveal any significant buildup of hydrogen peroxide and superoxide anion in monochromatic blue light treatment. Taken together, these results suggest that chlorosis and epinasty are two distinctive leaf injuries caused by continuous light that may follow very different mechanisms, and an overaccumulation of carbohydrates in the leaf may not be the main cause of continuous light-induced leaf chlorosis in tomato.

Temperature Variation under Continuous Light Restores Tomato Leaf Photosynthesis and Maintains the Diurnal Pattern in Stomatal Conductance

The response of tomato plants (Solanum lycopersicum L. cv. Aromata) to continuous light (CL) in relation to photosynthesis, abscisic acid (ABA) and reactive oxygen species (ROS) was investigated to improve the understanding of the development and/or alleviation of CL-induced leaf injury in constant and diurnal temperature fluctuations with similar daily light integral and daily mean temperature. The plants were grown in three photoperiodic treatments for 15 days; One treatment with a 16/8 h light/dark period and a light/dark temperature of 27/17°C (Control), two CL treatments with 24 h photoperiods, one with a constant temperature of 24°C (CLCT) and the other one with variable temperature of 27/17°C for 16/8 ho, respectively (CLVT). A diurnal pattern of stomatal conductance (gs ) and [ABA] was observed in the plants grown in the control and CLVT conditions, while the plants in CLCT conditions experienced a significant decrease in stomatal conductance aligned with an increase in ABA. The net photosynthesis (A) was significantly reduced in CLCT, aligned with a significant decrease in the maximum rate of Rubisco carboxylation (Vcmax), the maximum rate of electron transport (Jmax) and mesophyll diffusion conductance to CO2 (gm) in comparison to the control and CLVT. An increased production of H2O2 and O2•- linked with increased activities of antioxidative enzymes was seen in both CL treatments, but despite of this, leaf injuries were only observed in the CLCT treatment. The results suggest that the diurnal temperature fluctuations alleviated the CL injury symptoms, probably because the diurnal cycles of cellular mechanisms were maintained. The ROS were shown not to be directly involved in CL-induced leaf injury, since both ROS production and scavenging was highest in CLVT without leaf chlorotic symptoms.