E. F. Markovskaya

Temperature drop, dry matter accumulation and cold resistance of young cucumber plants

The study aimed to provide information on the involvement of plant cold resistance in a whole cucumber (Cucumis sativus L.) plant response to temperature drop treatments. The main objective was to explain the effects of a temperature drop on dry matter production and plant morphogenesis. The experimental design included temperature drops to 12°C for 0, 1, 2, 4 and 6 hours at the beginning, in the middle and at the end of the night. The overall average temperature for all treatments was 20°C. A 2-hour temperature drop resulted in the most significant reduction in plant dry weight and leaf petiole length, but the highest degree of cold resistance. With the extension of a temperature drop duration to 4--6 hours, dry matter production was similar to that in the control plants, leaf petiole length increased, but not up to the control level and cold resistance was reduced, but was still higher compared to that of the control plants. Changes in dry matter production, leaf petiole length and cold resistance, depending on the duration of the temperature drop, were interrelated and their patterns varied little with the timing (position effect) of the temperature drop. It is concluded that plant cold resistance mechanisms play an important role in a whole plant response to temperature drop treatments. Thus, plant response to temperature drop treatments can be considered as a response to stress.

Optimal temperature drop for the growth and development of young cucumber plants

The thermomorphological responses of young cucumber plants to a temperature drop lasting 2, 4 and 6 hours applied at the beginning, in the middle and at the end of the night were examined. Plant height, leaf petiole length and plant dry weight were measured and, based on the experimental data, the regression dependences of growth variables on the temperature drop were fitted. A method to determine the optimal temperature drop (the duration and time of exposure) which results in shortened plant height and leaf petiole length without a decrease in plant dry weight is proposed.

The effect of daily exposure to low hardening temperature on plant vital activity

Phenomenological responses of plants to daily short-term exposure to low hardening temperature was studied under chamber and field conditions. Experiments were carried out on cucumber (Cucumis sativus L.), barley (Hordeum vulgare L.), marigolds (Tagetes L.), and petunia (Petunia × hybrida) plants. The obtained data demonstrated a similar pattern of response in all studied plant species to different variants of exposure to low hardening temperature. The main features of plant response to daily short-term exposure to low hardening temperature include: a higher increment in cold tolerance (cf. two-or threefold increase relative to constant low hardening temperature) that peaked on day 5 (cf. day 2 at constant low hardening temperature) and was maintained for 2 weeks (cf. 3–4 days at constant low hardening temperature); a simultaneous increase in heat tolerance (cf. twofold relative to constant low hardening temperature) maintained over a long period (cf. only in the beginning of the exposure to constant low hardening temperature); a sharp drop in the subsequent cold tolerance after plant incubation in the dark (cf. a very low decrease in cold tolerance following the exposure to constant low hardening temperature); a combination of high cold tolerance and high photochemical activity of the photosynthetic apparatus (cf. a low non-photochemical quenching at constant low hardening temperature); and the capacity to increase cold tolerance in response to repeated short-term exposures to low hardening temperature in plants grown outdoors (cf. a gradual increase after repeated exposure to constant low hardening temperature). Possible mechanisms underlying the plant response to daily short-term exposure to low temperature are proposed.

Influence of Thermoperiod on Growth and Development in Cucumber

We studied the influence of gradient temperature regimes on various parameters of the formation of shoots and roots of cucumber plants, such as rate of leaf appearance, rate of growth, duration of growth and length of leaves, and the rate of growth shoots organs and roots. The plants were grown under the controlled conditions: at different combinations of day and night temperature, illumination 100 W/m2, and 12 h photoperiod. The comparison of constant and fluctuating diurnal temperature regimes has shown that in the optimal area for all studied indices, the highest values were recorded at the constant daily temperature (25°C for all growth indices of shoots and 20°C for growth of roots), while all gradient regimes either did not affect, or exerted inhibitory effects on the plant. Outside the optimum area, the effects of gradient temperatures differed. The main acting fluctuating temperatures, that exerted stimulating effects, combined low hardening (15°C) and optimal temperatures (25°C), which was earlier described for animals. The 15/35 and 35/15°C combinations were unambiguously inhibitory, since both temperatures are hardening for the cucumber. A lesser stimulating effect of gradient temperatures on the developmental rate in a plant, as compared to poikilothermic animals, could be due to a greater autonomy of plant ontogenesis because of autotrophy and, correspondingly, a greater degree of homeostasis. The mechanisms accounting for the responses to temperature gradients are similar in different groups of ectotherms.

Photothermal model of plant development

The development of plants depends on the photoperiod length, light intensity, temperature, and length of light day integral. The reaction of a plant to the day length or daily light integral can depend on both the range of studied light intensities and photoperiod. Based on the data concerning the effects of light and thermal integrals on the developmental rate of plants of different photoperiodic groups, a photothermal model of plant development was proposed. The model was used to calculate the lengths of optimal photoperiods and ranges of daily temperature gradients ensuring the highest developmental rate of some plants, such as soybean, wheat, cucumber, and barley.

The role of carbohydrates in the responses of chilling-sensitive plants to short- and long-term low-temperature treatments

The content of particular components of water-soluble carbohydrates and cold tolerance of cucumber (Cucumis sativus L.) cotyledonary leaves were studied at early developmental stages in dynamics during 6-day-long treatment with a temperature reduced to 12°C at two regimes: short-term cooling (2 h in the end of the night period, DROP) or permanent low-temperature treatment (PLT). PLT cucumber plants were characterized by the accumulation of oligosaccharides, whereas DROP plants contained increased amounts of glucose, fructose, and raffinose, indicating their higher metabolic status. When changes in carbohydrate fractions were compared with the dynamics of cold tolerance, it was found that these changes were synchronous in PLT plants but asynchronous for glucose and oligosaccharides in DROP plants. We suppose that, in cotyledonary leaves of DROP plants, two pools of sugars are produced; one of them used for tolerance development and another one — in active metabolism. This provides for the combination of activated metabolism and high cold tolerance of these plants. In PLT plants, all components of water-soluble carbohydrates are involved in cold tolerance development.

Differential Gene Expression in Cucumber Plants in Response to Brief Daily Cold Treatments

The mechanisms of plant responses to short-term cold treatments applied daily in the period of active growth remain unknown. Cucumber (Cucumis sativus L.) plants were subjected to brief drops of temperature (2 h, 12°C) at the end of each night over a 6-day period (DROP treatment) and to prolonged (6 days) cooling at 12°C (permanent low-temperature treatment, PLT). The plants exposed to cold treatments and control plants grown at 20°C were compared in terms of cold resistance and changes in gene expression. Cold resistance of plants was determined on the basis of LT50 temperature. The response of cucumber genetic machinery was assessed by means of a differential display method based on polymerase chain reaction (PCR). The changes in mRNA pool in cells of cucumber plants subjected to permanent and periodic chilling were assessed after comparing the populations of PCR fragments of cDNA. In both types of chilling protocols, the cold resistance started to increase from the 2nd day of low temperature treatment. At the end of the experiment (on the 6th day), the increment in cold resistance was three times larger for DROP compared to PLT treatment. Analysis of mRNA pool showed that the numbers of amplified fragments were nearly identical in both types of low-temperature treatment. The higher level of cold resistance under DROP conditions was assumed to depend on features of metabolism.

On the Formalization of Temperature Dependence of Plant Developmental Rate

Based on the data obtained in the experiments with constant and alternating diurnal temperatures, the temperature dependence of developmental rate was calculated for different photoperiodic groups of plants using the square regression equations. These equations made it possible to calculate the temperature regions of maximum developmental rates of plants under different photoperiods.

Pre-sowing seed treatment with a temperature-drop influences plant growth and flowering in marigold and pansy

Summary The effects of a pre-sowing seed treatment with a temperature drop (i.e., seed thermo-priming) on the growth and development of two popular bedding plant species, Tagetes patula nana L. (marigold) and Viola wittrockiana Gams. (pansy), were studied. A temperature drop from 22°C to 10°C was applied for 2 h each day during seed germination, over the 6 d before sowing. This seed priming increased plant dry mass, hastened flowering in both species, and increased flower numbers in marigold, but not in pansy. An involvement of certain phytochromes in these temperature-dependent effects on time-to-flowering is suggested.