E. G. Sherudilo

Role of phytochrome B in the development of cold tolerance in cucumber plants under light and in darkness

Cold tolerance of cucumber (Cucumis sativus L.) seedlings was investigated using wild-type plants and the phytochrome B-deficient mutant (lh-mutant). Plants were subjected for 6 days to intermittent short-term cooling (12°C for 2 h per day) and to continuous chilling under conditions of 16-h photoperiod (day/night = 16/8 h) and permanent illumination. “Dehardening” process was initiated by the transfer of plants to either light or dark conditions at 23°C. It was concluded that phytochrome B participates in the development of cold tolerance in cucumber plants under stress conditions, i.e., under short-term intermittent chilling at nights and during dehardening in continuous darkness.

Stomatal conductance in Cucumis sativus upon short-term and long-term exposures to low temperatures

We studied stomatal conductance (gs) in leaves of cucumber plants (Cucumis sativus L., cv. Zozulya) subjected at early developmental stages to either short-term daily cooling (2 h at the end of night periods) or continuous chilling (12°C throughout the day and night). Irrespective of the irradiance during measurements, continuous chilling either lowered gs or had no effect, as compared to gs of control plants (23°C). In plants subjected to periodic short-term cooling, the gs was found to increase at low temperatures both at moderate and high irradiance; it also increased at high temperature (33°C) but only at photosynthetically active irradiance of 800 μmol/(m2 s). It is supposed that heat-loving plants, subjected to different types of low-temperature treatment, mobilize different mechanisms of stomatal regulation and employ different strategies of adaptation in response to low- and high-temperature treatments. The unusual behavior of stomata, manifested in stomatal opening at both low and high temperatures, extends the adaptive potential of plants subjected daily to short-term low-temperature treatments. This leads to a high level of photosynthesis, biomass accumulation, and supports high physiological activity in plants.

Immediate and delayed effects of diurnal temperature drops on growth and reproductive development of tomato plants grown under continuous lighting

Growing tomato plants (Lycopersicon esculentum Mill) under 24-h irradiation at constant temperature results in photoinhibition of the photosynthetic apparatus and light-induced injury of leaves, which is manifested in interveinal chlorosis and has a negative effect on growth and development. At later stages of plant development, the unfavorable influence of continuous lighting (CL) applied during the pre-reproductive period was evident in the delayed flowering, reduced fruit set and yield. In order to eliminate negative consequences of CL, the plants were exposed daily during the pre-reproductive period to temperature drops to 10℃ for 2 h (DROP). Control plants were grown under 16-h photoperiod at 26/20°C (day/night) temperature. By the end of the pre-reproductive period (day 37), DROP-treated and control plants were transplanted to a greenhouse and grown under identical conditions of natural photoperiod throughout the spring-summer season. The application of DROP under CL throughout the pre-reproductive period prevented the appearance of leaf chlorosis, enlarged leaf area, and increased the plant biomass. Furthermore, in DROP-treated plants, no adverse aftereffects of CL were observed in subsequent development. The DROP-treated plants did not differ from control plants in terms of time to flowering, the fruit set and total yield but were superior to control plants with regard to the early fruit yield. It is concluded that the DROP-treatment helps to employ the potential advantages of CL by mitigating its negative effects on plants.

Response of Сucumber (Cucumis sativus L.) Plants to Prolonged Permanent and Short-Term Daily Exposures to Chilling Temperature

We investigated the response of chilling-sensitive plants of cucumber (Cucumis sativus L.) to prolonged permanent (6 days) (PLT) and short-term (3 h) daily exposures to low non-freezing temperatures (9 and 12°С) (DROP) lying on different sides of the critical value (10°С) corresponding to a phase transition of membrane lipids in chilling-sensitive plants from a liquid-crystalline to a solid gel structure. Effects and aftereffects of DROP treatments at temperatures of 9 and 12°С (DROP9 and DROP12, respectively) were identical. They caused a reduction in linear dimensions of plants (as compared with control plants) and enhanced chilling tolerance of leaves but did not influence photosynthetic activity and water relations. However, when exposure to these temperatures was permanent (PLT9 and PLT12), their effects on plants were different. PLT9 almost entirely suppressed plant growth and development, inactivated photosynthetic apparatus (PSA), increased relative electrolite leakage (REL), and boosted lipid peroxidation (LPO). PLT12 also considerably reduced plant height and leaf area and suppressed (but did not inactivate) PSA; it did not boost POL or increased REL. It is important that, after the termination of chilling, PLT9 plants (unlike PLT12 plants) could not quickly resume growth and restore photosynthetic activity. Thus, considerable differences in plant responses to various patterns of chilling (long permanent or short-term daily) are probably due to the fact that, in the case of DROP treatments, relatively short-term (3 h) chilling of plants is followed in the diurnal cycle by a prolonged period (21 h) of optimal temperature when possible deviations and/or disturbances of PSA are restored and toxic substances that could accumulate in the course of chilling metabolized and/or neutralized. Pronounced differences in plant response to permanent exposure to temperatures of 9 and 12°С probably depend on the fact that these temperatures lie on different sides of a critical value (10°С) below which chilling-sensitive plants suffer grave physiological disturbances due to cooling. In addition, we showed that different responses of plants to PLT and DROP treatments are not determined by a usual dose-effect relationship but depend in many respects on the pattern of temperature influence (prolonged or short-term, single or recurring). As a result, the number of DROP spans experienced by plants in the experiments proved to be more important than their duration (at least, within a time interval from 2 to 12 h).

Влияние ДРОП-воздействия на эффективность использования световой энергии в процессе фотосинтеза у растений огурца

We studied the apparent quantum yield of photosynthesis in the leaves of cucumber ( Cucumis sativus L.) plants exposed to daily short-term (2 h) temperature drops to 12 ° C in the darkness (in the end of the night) or light (at the beginning of the day). In order to assess the effect of leaf age on their response to DROP, plants were exposed to DROP treatments when the first leaf was non-mature (in the exponential growth phase) or mature, or during the entire period of leaf expansion. It was shown that DROP causes a decrease in the efficiency with which light energy is utilized in photosynthesis regardless of leaf age. It is suggested that changes in the apparent quantum yield under the influence of DROP treatments are associated with changes in the pigment composition. Unlike leaf age, the presence of light during DROP treatments significantly affected the degree of changes in apparent quantum yield of photosynthesis. In particular, DROP in the light caused significantly greater reduction in the apparent quantum yield than DROP in the darkness. Moreover, DROP in the light reduced not only the apparent quantum yield, but also the rate of photosynthesis. DROP in the darkness did not affect the rate of photosynthesis. It is assumed that the decrease in the light use efficiency in DROPtreated plants results from the initiation of plant defense response to low temperatures that allow avoiding photosynthetic apparatus damage.

Control of continuous irradiation injury on tomato plants with a temperature drop: Effectiveness evaluation

Effects of daily short-term low temperature treatment (DROP) applied during different prereproductive developmental stages on growth and development of tomato plants grown under continuous light (CL) has been studied. It was shown that DROP treatment (daily decrease of temperature from 26 to 10 C for 2 h) applied to plants of different age is not equally effective in preventing or moderating CL injury in tomatoes grown under CL. DROP treatment of seedlings and juvenile plants proved to be ineffective in preventing leaf chlorosis in the aftereffect. DROP treatment of immature and virginile plants of different age had moderate protective effect, reducing the degree of leaf chlorosis. The maximum effect was obtained when plants were treated by DROP throughout the entire prereproductive period.