Photosynthetic activity and water use efficiency of dune species: the influence of air temperature on functioning

Differences in leaf traits among the dune species developing along the Latium coast were analysed. Cakile maritima Scop. subsp. maritima, Elymus farctus (Viv.) Runemark ex Melderis subsp. farctus, Ammophila arenaria (L.) Link subsp. australis (Mabille) Lainz, Ononis variegata L., Pancratium maritimum L., Eryngium maritimum L., and Anthemis maritima L. were considered. The considered species showed a similar net photosynthetic rate (P (N)) and chlorophyll content (Chl) during the year, with a peak from the end of April to the middle of May [13.0 +/- 3.6 mu mol (CO(2)) m(-2) s(-1) and 0.63 +/- 0.21 mg g(-1), respectively, mean values of the considered species], favoured by air temperature in the range 13.3-17.5A degrees C, and 6% of soil water availability. In June-July, the increase of air temperature (T(max) = 28.4A degrees C), associated with a lower water availability (42 mm, total rainfall of the period) and a 1% of soil water availability determined a significant decrease of P (N) (59%, mean of the considered species) and Chl (38%), and an increase of the carotenoid (Car)/Chl ratio (59%). The significant correlation between P (N) and stomatal conductance (g (s)) (p < 0.05) explained 67% of P (N) variations. Moreover, the correlation between P (N) and leaf temperature (T (l)) underlined that the favourable T (l) enabling 90-100% of the highest P (N) for the considered species was within the range 23.4 to 26.6A degrees C. P (N) decreased below half of its maximum value when T (l) was over 35.8 and 37.4A degrees C for E. farctus subsp. farctus and A. arenaria subsp. australis, respectively and over 32.2A degrees C for the other considered species (mean value). Leaf mass area (LMA) varied from 6.8 +/- 0.7 mg cm(-2) (O. variegata) to 30.6 +/- 1.6 mg cm(-2) (A. arenaria). PCA (principal component analysis) carried out using the considered morphological and physiological leaf traits underlined that the co-occurring species were characterised by different adaptive strategies: E. farctus and A. arenaria photosynthesized for a long period also when air temperature was over 35.8 and 37.4A degrees C, respectively, because of their lower transpiration rates [E, 1.4 +/- 0.1 mmol (H(2)O) m(-2) s(-1)], which seemed to be controlled by the highest LMA. On the contrary, A. maritima and C. maritima subsp. maritima had a higher P (N) (on an average 52% higher than the others) in the favourable period, allowed by the highest succulence index (SI, 85.7 +/- 9 mg cm(-2)) and the lower LMA. The results allowed us to hypothesize that A. arenaria and E. farctus might be at a competitive advantage relative to the other considered species with respect to the increase of air temperature, by their ability to photosynthesize at sufficient rates also during summer.