Leaf waxes of slow-growing alpine and fast-growing lowland Poa species: inherent differences and responses to UV-B radiation

Abstract

We investigated whether alpine and lowland Poa species exhibit inherent differences in leaf cuticular waxes, leaf UV absorbing compounds and/or growth responses to UV-B treatment. All plants were grown hydroponically in a growth cabinet (constant 20°; 14 hr photoperiod; 520 μmol photons m−2 s−1 PAR). Two alpine (P. fawcettiae and P. costiniana), one sub-alpine (P. alpina) and three temperate lowland species (P. pratensis, P. compressa and P. trivialis) were grown under conditions without UV radiation for 36 days. In a subsequent experiment, four Poa species (P. costiniana, P. alpina, P. compressa and P. trivialis) were also exposed for 21 days to UV-B/(UV-A) radiation (`UV-B treatment') that resulted in daily UV-B radiation of 7.5 kJ m−2 day−1, with control plants being grown without UV-B (`UV-A control treatment'). All treatments were carried out in the same growth cabinet. There was no altitudinal trend regarding wax concentrations per unit leaf area, when the six species grown under UV-less conditions, were compared at similar developmental stage (20–30 g shoot fresh mass). However, large differences in cuticular wax chemical composition were observed between the alpine and lowland species grown under UV-less conditions. For example, a single primary alcohol was present in the waxes of the lowland and sub-alpine species (C26H53OH), but was virtually absent in the alpine species. Although alkanes were present in all six species (primarily C29H60 and C31H64), the proportion of total wax present as alkanes was highest in the alpine species. Aldehydes were only present in the waxes of the alpine species. Conversely, substantial amounts of triterpenoids were mainly present in the three lowland species (squalene and lupeol were the dominant forms). The proportion of total wax present as long-chain esters (LCE-s) was similar in all six species grown in the absence of UV radiation. Acetates were observed only in the wax of P. trivialis. Of the four species exposed to UV-B, only P. costiniana and P. compressa showed any differences in wax chemical composition (compared to UV-A control plants). In P. costiniana, the proportion of total waxes present as alkanes was substantially lower in the UV-B grown plants (replaced by an unknown compound with a high retention time value). UV-B grown P. costiniana and P. compressa both exhibited less wax per unit leaf area. In P. compressa, this resulted from a lower absolute amount of alcohol per unit leaf mass in the UV-B treatment. The concentrations of UV absorbing compounds in whole leaf extracts differed between the four investigated species. However, no systematic differences were apparent between the alpine, sub-alpine and lowland species. Exposure to UV-B radiation had no effect on the specific leaf area (ratio of leaf area to leaf dry mass). We conclude that the alpine and lowland Poa species have a cuticular wax composition that is inherently different.