The influence of ambient solar UV on the growth and morphology of plants differing in diurnal UV sunscreen protection

Background/Question/Methods

Diurnal variation in UV sunscreen protection is widespread among plants and species vary considerably in the magnitude of these changes.  However, the functional and ecological significance of this phenomenon remains unclear.  In this study we compared the influence of ambient solar UV radiation on the growth and morphology of two cultivated species that differ in their degree of diurnal change in UV protection to test whether any relationship exists between UV sensitivity and the capacity for rapid modulation of UV protection.  Plants of okra (Abelmoschus esculentus; large diurnal change in UV protection) and bell pepper (Capsicum annuum; small diurnal change in UV protection) were grown from seed in pots under three UV exclusion treatments (+UVB+UVA controls [Aclar film], -UVB+UVA [Mylar film], and –UVB-UVA [Llumar film]) in late summer/early fall in New Orleans, Louisiana.  After 24 days of growth, plants were harvested and measurements of shoot morphology (shoot height, internode length, leaf size and area), shoot biomass (leaf and stem dry biomass, leaf mass:area) and epidermal UV transmittance (T_UV; nondestructively measured using a UVA-PAM fluorometer) were obtained from 15 plants/species/treatment.  Data were analyzed using ANOVA for a completely randomized design.

Results/Conclusions

We detected significant UV-induced reductions in shoot height growth rate (ANOVA, P=0.03) and stem internode length (P=0.04) in pepper but not okra (P=0.08 and 0.44 for shoot growth and internode length, respectively).  No significant (P>0.05) UV-induced reductions were detected for leaf size/area and shoot biomass in either species, and no consistent effects on growth or morphology were observed between plants in the –UVB+UVA vs. –UVB-UVA treatments.  Diurnal (dawn-midday) changes in T_UV were more pronounced in okra (ΔT_UV = 20.0%) than pepper (ΔT_UV = 1.4%), but pepper maintained higher levels of overall UV protection (time+treatment average T_UV= 27.7% vs. 88.1% for pepper and okra, respectively). 

These results indicate that these two species differ in their sensitivities to ambient solar UV with the more sensitive species exhibiting limited diurnal changes in UV sunscreen protection (pepper).   These findings suggest that growth and morphological sensitivity to solar UV is related to the degree of flexibility in UV sunscreen protection.