Seasonal and diurnal variation of photosynthetic parameters in seedling of five Pinus species under different light conditions

Seasonal and diurnal variations in photosynthetic parameters in well-watered, 2-year-old seedlings of five pine species were studied. The photosynthetic parameters were determined using an infra-red gas analyser under three different light conditions, thereafter referred to as sun, shade and illuminated treatments. The five species selected were P. brutia, P. maritima, P. pinea, P. sylvestris and P. nigra. The parameters determined were a) photosynthetic parameters, namely assimilation rate (A), stomatal conductance (gs), transpiration rate (E), and intercellular CO2 (Ci); b) environmental parameters including lighting density (PAR), leaf and air temperature (Tl and Ta), relative humidity (Rh) and vapor pressure deficit (VPD). Total chlorophyll content (Tchl), specific leaf weight (SLW) and height of the plant (h) were also estimated. Photosynthesis recorded positive values during the whole experimental period (Nov. 93 - Oct. 94). Maximal values were reached during summer. Differences in assimilation rates among species were due to genetic variation. Higher assimilation rates, stomatal conductance and transpiration rates were noticed compared to those recorded in literature for unirrigated plants. Environmental conditions were the main factors affecting monthly variation in photosynthetic parameters. P. maritima reformed the best under sun treatment, while P. pinea rated as the worst. However, the latter overlapped the other species under shade, as exemplified by its higher chlorophyll content, specific leaf area and height growth recorded under shade. P. brutia proved the least shade tolerant species among the five. Midday depression in stomatal conductance was noticed during summer; however it was not intense due to water availability. Shade seedlings had lower assimilation rates compared to those subjected to sun treatment. These values increased upon sudden transfer of the seedlings to the normal day light. Multiple regression equation reassured that light intensity, relative humidity, and leaf temperature are positively correlated with photosynthesis (R2=69)