Interacting determinants of specific leaf area in 22 herbaceous species: effects of irradiance and nutrient availability

We measured specific leaf area (SLA) and six of its determinants (the thickness of lamina, mesophyll, epidermis, mid-vein and mid-vein support tissues and leaf water content) in a collection of 22 herbaceous species grown in factorial combinations of high μ 1100 (mol m^–2 s^–1) and low (200) irradiance crossed with high (1 : 1) and low (1 : 6 dilution) concentrations of a modified Hoagland hydroponic solution. SLA increased with both decreasing irradiance and with increasing nutrient availability but there was a strong interaction between the two. Lamina and mesophyll thickness both increased with increasing irradiance and nutrient availability without any interaction. The experimental treatments had complicated effects on mid-vein thickness and its support tissues. Leaf water content (a measure of leaf tissue density) decreased with increasing irradiance levels and with decreasing nutrient supply, but with an interaction between the two treatments. Changes in nutrient supply had no effect on SLA at high irradiance because leaf thickness and leaf tissue density changed in a compensatory way. A path analysis revealed that each of the components affected SLA when the others were statistically controlled but the strengths of the effects of mesophyll thickness, mid-vein thickness and water content differed between treatment groups. The effect of epidermal thickness on SLA was constant across environments and it showed no significant covariation with the other determinants. There was significant covariation between mesophyll thickness, mid-vein thickness and water content and this covariation was constant across the treatment groups.