Response of Soybean Bulk Leaf Water Relations to Ultraviolet-B Irradiation

Abstract

Summary Erosion of the stratospheric ozone layer by anthropogenic emission of halogenated compounds may lead to increased UV-B radiation at ground level. In 1990 soybeans ( Glycine max (L.) Merr. cv. Essex) were grown in open-top field chambers with 3 levels of UV-B replicated 3 times. UV-B treatments corresponded to changes in the total column ozone thickness of15% (low), -20% (medium) and -35% (high). Leaves were sampled during four intervals of the growing season and subjected to pressure-volume (P-V) analysis to determine symplastic volume (V o ); maximum turgor pressure ( p MAX), symplastic solute content (N s ), tissue elasticity coefficient (z), and the potential at turgor loss (TLP). Leaf conductance, average specific leaf weight (SLW), and area per leaf were measured several times during the season. During the second sampling period at 58 days after planting (DAP), V o was significantly decreased at the highest UV-B level but was not affected by UV-B treatments again. Also during the second sampling period the elasticity coefficient, z, in the medium and high UV-B treatments was significantly less than in the low UV-B treatment. In the fourth sampling period (100 DAP), z again was significantly affected by UV-B treatment; in this case, however, while z was lower in the high than the medium UV-B treatment, it did not diffet from the low UV-B treatment. The relative symplastic volume at the turgor loss potential (RSV TLP ) was the only other parameter to show a significant UV-B effect, but only at the highest treatment level and only near the end of the season. Generally, the significant changes in all P-V parameters with plant age were much larger than any treatment effects. There were no significant differences in leaf conductance, leaf area or SLW, indicating that, although the leaf P-V relationships were noticeably altered, these sporadic treatment effects had little real influence on leaf water balance.