Heat Shock Increases the Tolerance of Plants to UV-B Radiation: 1. Growth,Development, and Water Supply to Tissues

The effects of heat shock (HS); UV-B irradiation; and the consecutive action of these factors on the growth, development, and water supply of seven-day-old melon (Melo sativusSager., cv. Torpeda) seedlings were investigated. Depending on the HS severity, we observed growth stimulation (after treatment at 45°C for 1 h), growth retardation (after treatment at 45°C for 2 h or at 48°C for 1 h), or complete growth inhibition and cell death (after treatment at 45°C for 3 h or at 55°C for 1 h). UV-B irradiation (18.3 kJ/(m²h)), depending on its duration, stimulated (5–10 min), retarded (60 min), or resulted in complete growth inhibition and plant death (90 min). HS treatment (at 45°C for 1 h) prior to UV-B irradiation (for 1 h) favorably affected both the growth and water balance of seedlings. Apparently, the HS pretreatment increases the tolerance of seedlings to high doses of UV-B radiation.     

Heat Shock Increases the Tolerance of Plants to UV-B Radiation: 2. Ethylene and Carbon Dioxide Evolution

The effects of heat shock (HS, 45°C) and UV-B radiation (280–320 nm, 18.3 kJ/(m²h)) and the consecutive action of the combination of these factors on ethylene production, gas exchange, and the growth of intact melon (Melo sativusSager.) seedlings were investigated. The changes in ethylene production and carbon dioxide exchange were described by a single-peaked curve. In the course of UV-B irradiation, the time of maximum ethylene and CO₂evolution coincided (the first 5 min) and comprised 0.36 nl/(seedling h) for ethylene and 146.2 l/(seedling h) for carbon dioxide. After HS, the maximum of ethylene production (0.37 nl/(seedling h)) was reached within 10 min, and that of carbon dioxide production (313 l/(seedling h)), within 45 min. The rate of ethylene production (0.22 nl/(seedling h)), carbon dioxide production (97.7 l/(seedling h)), and oxygen consumption (162.5 l/(seedling h)) in the control seedlings did not change in the course of experiment. Throughout the experiment, the respiratory quotient of seedlings was ca. 0.6 regardless on the nature and duration of the acting factor. Preliminary heating at 45°C for 1 h increased the tolerance of seedlings to the subsequent UV-B radiation for 1 h. The protective effect of HS manifests itself in alleviating the inhibiting action of UV-B radiation on seedling growth and development, and this effect was preceded by an increase in ethylene production and respiration. The possible mechanisms of cross-tolerance of plants to overheating and UV-B radiation are discussed.