Perturbations of the normal UV-B radiation environment alter leaf growth rates in Phlomis fruticosa L. seedlings

Abstract

Phlomis fruticosa L. seedlings were raised in the field under ambient UV-B radiation for 18 months. On 15 March 1994, just before the spring growth burst), seedlings were transferred in a glasshouse under three regimes of artificial UV-B radiation, i.e. 0.06, 5.22 and 8.55 kJ m −2 biologically effective daily doses. The last two doses correspond to UV-B radiation reaching the Patras area (38.3° N, 29.1° E) under clear sky and with normal column ozone thickness at mid-April and mid-July. Leaf demography was monitored for 96 days. Growth rates new leaf formation and total leaf area increase) were significantly higher under 5.22 kJ m −2 day −1, intermediate under 8.55 kJ m −2 day −1 and severely suppressed in the absence of UV-B radiation. In addition, shedding of older leaves after mid-May, was significantly more intense in the absence of UV-B radiation. The observed differences could not be attributed to UV-B radiation effects on CO 2 assimilation, photosynthetic electron flow or leaf water relations, since net photosynthetic rates, photochemical efficiency of photosystem II and leaf relative water contents showed no significant differences between treatments. Total dry matter measured at plant harvest was highest under 5.22 kJ m −2 day −1 and lowest in the absence of UV-B radiation. The results indicate that natural UV-B radiation may be an important factor in normal leaf development of Phlomis fruticosa.