Above and Below-ground Morphological Responses of Cucumber Seedlings (Cucumis sativus) to Ultraviolet-b Radiation

Abstract

The reduction in stratospheric ozone has resulted in an increase in ultraviolet radiation, specifically, UV-B (280-320 ran), incident at the Earth’s surface. This increase in UV-B has potential damaging effects on biological organisms. In this study controlled conditions were employed using growth chambers to investigate early morphological responses and timing of these responses on cucumber seedlings (Cucumis sativus cv. Burpee Pickier) exposed to UV-B radiation simulating ambient springtime UV-B levels for clear sky conditions in San Marcos, Texas. Specifically, above-ground and below-ground morphology, biomass allocation, and growth responses were investigated in cucumber seedlings exposed to UV-B radiation. The timing of below-ground responses was then compared to above-ground plant responses. Hypocotyl length (p< 0.001) and cotyledon area (p< 0.001) were both significantly reduced by UV-B exposure relative to plants that received no UV-B. Primary root length (p= 0.446), projected root area (p= 0.787), and cotyledon area/weight ratio (p= 0.367) were not significantly different between treatments. The projected root area/weight ratio (p= 0.033) was significantly greater in plants exposed to UV-B. Thus, the roots of plants exposed to UV-B were apparently thinner or less dense than those of the control (no UV-B) plants. UV-B treated plants had higher early (days 1-2) relative growth rates for root parameters compared to the control plants. However, these early growth rate responses for the UV-B treated plants changed to slower or equal growth rates later (days 4-5) in the experiment. The morphological responses corresponded with a reduction in biomass for all plant parts. However, there was no significant change in the root/shoot ratio between treatments. The below-ground changes in response to UV-B occurred on the third day of UV-B exposure and occurred on the same day as above-ground changes. These findings suggest that root responses to UV-B were not the result of reduction in shoot growth but a consequence of signal transduction between shoots and roots.