Submitted to: Photochemistry and Photobiology
Publication Type: Abstract Only
Publication Acceptance Date: July 10, 1999
Publication Date: N/A
Stomata control leaf photosynthesis, transpiration, and adaptation to drought, heat, and air pollutants such as ozone. Several soybean lines make a flavonol glycoside (kaempferol triglucoside, K9) associated with reduced stomatal density and altered stress tolerance. One of the most consistently reported effects of ultraviolet-B radiation (UV-B, 290-315 nm) )in plants is increased synthesis of phenolic compounds, such as flavonoids We hypothesize that solar UV-B affects stomatal development and water-use-efficiency (WUE) in soybean leaves. Soybean lines OX941, OX942, OX921, and OX922 were grown in the field under plastic films that either transmitted (Teflon) or blocked (polyester) UV-B. OX941 and OX922 make K9 as their major flavonoid. OX921, the isoline pair of OX922, makes a normal complement of flavonols. OX942, the isoline pair of OX941, is blocked in flavonol synthesis, but makes other phenolics. In every line, soluble UV-screening phenols (including flavonoids in competent lines) were higher in plants exposed to UV-B. Leaf area was also reduced significantly in 3 out of 4 lines. Plant growth and metabolism were clearly affected by ambient UV-B in the field, although yield was not reduced in any lines. Flavonols are, therefore, not a prerequisite for UV tolerance. However, the type of flavonol may affect responses to UV-B. Thus, leaf stomatal density and conductance (upper epidermis) increased under UV-B in the line (OX921) with "normal" flavonolds. In contrast, ambient solar UV-B significantly reduced stomatal density in soybean isolines expressing K9. Decreased stomatal density was associated with increased season-long WUE and decreased leaf internal C02 concentration (estimated by 13C discrimination).