NONPHOTOSYNTHETIC MECHANISMS OF GROWTH REDUCTION IN PEA (PISUM-SATIVUM L.) EXPOSED TO UV-B RADIATION

Pisum sativum cv, Guide grown under controlled environment conditions was exposed to either low or high UV-B radiation (2.2 or 9.9 W m(-2) d (-1) plant-weighted W-B, respectively), Low or high UV-B was maintaine d throughout growth (LL and HH treatments, respectively) or plants wer e transferred between treatments when 22 d old (giving LH and HL treat ments), High UV-B significantly reduced plant dry weight and significa ntly altered plant morphology, The growth and morphology of plants tra nsferred from low to high UV-B were little affected, when compared wit h those of LL plants, By contrast, plants moved from high to low UV-B showed marked increases in growth when compared with HH plants, This c ontrast between HL and LH appeared to be related to the effect of UV-B on plant development, Exposure to high UV-B throughout development co nsistently reduced leaf areas. In fully expanded leaves there was no s ignificant UV-B effect on cell area and reduced leaf area could be att ributed to reduced cell number, suggesting effects on leaf primordia, Further reductions in the leaf area of younger leaves were the result of the slower development rate of plants grown at high UV-B, which als o resulted in significant reductions in leaf number.