Stomatal control partly explains different photosynthetic characteristics in Helianthus laetiflora and H. annuus

Gas exchange characteristics were investigated in the showy sunflower (Helianthus laetiflora) and in the common sunflower (H. annuus) to determine the importance of stomatal control of photosynthesis under glasshouse conditions. Leaf net photosynthetic rate (Pn), stomatal conductance (gs), and intercellular CO(2) concentration (Ci) were measured on a leaf-area basis of pot-grown plants in the glasshouse by a portable photosynthesis measuring system. In response to gradually increasing light, Pn showed a sharp increase and then remained steady for both sunflower species, whereas the response of Ci was the inverse, first decreasing with increasing light before later becoming steady. In the common sunflower, gs increased and then remained constant with increasing light (similar to Pn), whereas in the showy sun flower, gs remained relatively constant except for slight fluctuations evident at the lowest light levels. The responses of Pn and gs to Ci were similar to the light responses in both sunflower species. Similar changes in the diurnal response of Pn and Ci were observed in both sunflowers, both species increased (for Pn) or decreased (for Ci) during the day. In the showy sunflower, gs hardly changed during the day, whereas in the common sunflower it increased and then decreased. Obvious positive correlations between Pn and gs were observed in the common sunflower, more so than in the showy sunflower. The limitation of stomata to photosynthesis (Ls) was greater in the common sunflower but the values of Ls were high in both species. It is suggested that stomatal control plays a role in both species but that there was a more obvious stomatal limitation to photosynthesis in the common sunflower under unstressed conditions. The difference in photosynthetic characteristics may correlate with different physiological specialisations in the two species.