Seasonal fluctuations in the concentration of UV-absorbing compounds in the leaves of some Mediterranean plants under field conditions

Leaves of 14 representative Mediterranean plant species were collected on a monthly basis and assayed for UV-absorbing compounds concentration, either on an area or a dry mass basis, from 1995 to 1997, Strong seasonal fluctua tions were observed in eight species tall evergreens, two phrygana, one dec iduous, one summer perennial and one winter perennial). Two different patte rns of changing concentrations of UV-absorbing compounds were observed. In the first, concentration of these compounds was higher in young developing leaves and concentration declined during maturation, whereas in other plant s, the opposite trend was observed. These differences could be attributed t o the particular leaf surface morphology of each plant. The observed season al fluctuations of UV-absorbing compounds seem to be more correlated to dev elopmental processes, than to seasonal fluctuations of the naturally occurr ing UV-B radiation. Most of the winter perennials did not show strong fluct uations during the period of development. The concentration of these compou nds varied not only on a seasonal basis among the examined plants, but betw een different life forms as well: during winter, examination of the leaves of 13 species showed that evergreen sclerophylls and phrygana had at least two-fold higher concentration of UV-B-absorbing compounds on a leaf area ba sis than winter perennials. In addition, during the same season and irrespe ctive of life form and species, the absorbance at 300 nm per unit of mature leaf area followed an asymptotic exponential decrease when specific leaf a rea increased. The UV-B radiation screening capacity of the leaves of these plants is discussed in relation to each adaptive strategy.