EFFECTS OF ENHANCED ULTRAVIOLET-B RADIATION ON GROWTH, PHOTOSYNTHESIS AND RESPIRATION OF CUCUMIS SATIVUS LEAVES ON DIFFERENT POSITIONS

The growth and physiological variation of cucumber grown under UV B radiation for 60 d in plant growth chamber were monitored. The results showed that the leaf emergence was delayed, the reduced magnitudes of leaf area and dry weight were positively correlative to leaf position. The reduced magnitude of water content in leaf, varying like a parabola line, was higher in both the oldest leaf (the first leaf in low position) and young leaf (the fifth leaf in high position), and was the lowest in the third leaf (leaf in middle position) because of UV B radiation. Decrease in leaf extension rate, leaf number and area per leaf together resulted in the reduce of total leaf area. Variation of leaf growth played an important role in the relatively stable correlative growth among root, stem and leaf under the condition of enhanced UV B radiation. The decrease in the internode length lead to the reduce of plant height. The inhabited degree of net photosynthetic rate and apparent quantum efficiency by UV B radiation were positively correlative to leaf position. The photorespiration was significantly enhanced by UV B radiation and the increment was not obviously in relation to leaf position. UV B radiation had no effect on the dark respiration of the first leaf, slightly decreased the respiration of the second and third leaf, but sharply raised the dark respiration of the fourth leaf. It was suggested that the reduce of both plant height and leaf area was advantageous for plant to adapt to UV B radiation, decrease in the water content and photosynthesis and increase in photorespiration and dark respiration may be the physiological bases of growth inhibition. Fig 2, Tab 2, Ref 18