GROWTH ANALYSIS OF UV-B-IRRADIATED CUCUMBER SEEDLINGS AS INFLUENCED BY PHOTOSYNTHETIC PHOTON FLUX SOURCE AND CULTIVAR

A growth analysis was made of ultraviolet-B (UV-B)-sensitive (Poinsett) and insensitive (Ashley) cultivars of Cucumis sativus L. grown in growth chambers at 600 mumol m-2 s-1 of photosynthetic photon flux (PPF) provided by red- and far-red-deficient metal halide (MH) or blue- and UV-A-deficient high pressure sodium/deluxe (HPS/DX) lamps. Plants were irradiated 6 h daily with 0.2 (-UV-B) or 18.2 (+UV-B) kJ m-2 day-1 of biologically effective UV-B for 8 or 15 days from time of seeding. In general, plants given supplemental UV-B for 15 days showed lower leaf area ratio (LAR), and higher specific leaf mass (SLM), mean relative growth rate (MRGR) and net assimilation rate (NAR) than that of control plants, but they showed no difference in leaf mass ratio (LMR). Plants grown under HPS/DX lamps vs MH lamps showed higher SLM and NAR, lower LAR and LMR, but no difference in MRGR. LMR was the only growth parameter affected by cultivar; at 15 days, it was slightly greater in Poinsett than in Ashley. There were no interactive effects of UV-B, PPF source or cultivar on any of the growth parameters determined, indicating that the choice of either HPS/DX or MH lamps should not affect growth response to UV-B radiation. This was true even though leaves of UV-B-irradiated plants grown under HPS/DX lamps have been shown to have greater chlorosis than those grown under MH lamps.