Analysis of gas exchange of Merkus pine populations by the optimality approach

We hypothesized that northern and northeastern Thai populations of Merkus p ine (Pinus merkusii Jungh. et de Vriese), which differ in the duration of t heir grass stage and which originate from slightly different climates in te rms of water availability differ in their gas exchange characteristics. We compared CO2 exchange response to irradiance, diurnal regulation of leaf co nductance within a 10-day period and structural properties among the popula tions. We used a model, which is based on the concept of optimal stomatal r egulation, to analyze CO2 exchange and transpiration rates. The two geographical groups did not differ in CO2 exchange response to irra diance, diurnal transpiration, or water use. Mean stomatal length was signi ficantly greater in the northeastern population than in the northern popula tions, but stomatal frequency did not differ among populations. First-year shoot growth and dry matter production, allocation of nitrogen to needles a nd root:shoot ratios were similar in the two geographical groups. Genotypic variation in the duration of the grass stage was not reflected in variatio n in gas exchange, indicating that the grass stage is an adaptation to more site-specific conditions. The modeled response of CO2 exchange rates to ir radiance fitted well the rates measured under laboratory conditions. The tr anspiration model, which utilized maximum leaf conductance and other parame ters derived from the CO2 exchange measurements, also fitted well the trans piration rates measured in a greenhouse under changing environmental condit ions.