Abstract Adaptive values of plasticity in Iris pumila leaf traits (morphological: SLA, specific leaf area; anatomical: SD, stomatal density; LT, leaf thickness; VBN, vascular bundle number; SW, sclerenchyma width; CW, cuticle width, and physiological: ChlT, total chlorophyll concentra- tion; ChlA/B, chlorophyll a/b ratio) were tested at three irradiance levels in a growth-room. Siblings from 28 full-sib families from an open dune site and a woodland understory responded similarly to variation in light availability: SLA gradually increased, while anatomical and physiological traits decreased with light reduction. In the Dune population, standardized linear selection gradi- ents were significant for SLA and ChlT at high light, VBN along the entire light gradient, SW at high- and low-, and ChlA/B at low-irradiance. In the Woods population, the significant standard- ized linear selection gradients were observed for SLA and LT at low- and VBN at both high- and low-irradiance. A significant nonlinear selection gradient was recorded for SD and LT at medium irradiance. Comparisons of the plastic responses to each light quantity with the phenotypes favored by selection in that environments revealed that only an increased SLA value at low light in the Woods population was ecologically significant (adaptive). In the Dune population, SD and VBN entailed plasticity costs at low irradiance, while a cost of homeostasis was recognized for ChlT and ChlA/B at medium light, SD and CW at high- and low-, and SLA at high- and medium-light level. In the shaded population, CW and ChlA/B incurred plasticity costs at high irradiance, while for ChlT plasticity costs appeared under medium- and low-light condi- tions. In all leaf traits, genetic variation for plas- ticity was statistically undetectable. Genetic correlations between these traits were mostly insignificant, implying that they possess a capa- bility for relatively independent evolution by natural selection across different light environ- ments. Keywords Adaptive plasticity Æ Costs of plasticity/homeostasis Æ Genetic correlations Æ Irradiance responses Æ Multivariate selection gradient analysis Æ Reaction norms Introduction Environmental conditions occupied by plants are highly variable, both spatially and temporally. As a consequence of such environmental heteroge- neity, a remarkable phenotypic variation can be S. Avramov (&) Æ D. Pemac Æ B. Tucic ´ Department of Evolutionary Biology, Institute for Biological Research ‘Sinis ˇa Stankovic ´’, Bul. Despota Stefana 142, 11060 Belgrade, Serbia e-mail: stevan@ibiss.bg.ac.yu Plant Ecol (2007) 190:275–290 DOI 10.1007/s11258-006-9207-3 123 ORIGINAL PAPER Phenotypic plasticity in response to an irradiance gradient in Iris pumila: adaptive value and evolutionary constraints Stevan Avramov Æ Danijela Pemac Æ Branka Tucic ´ Received: 14 January 2005 / Accepted: 2 August 2006 / Published online: 6 September 2006 Ó Springer Science+Business Media B.V. 2006