Effects Of Stomatal Development On Stomatal Limitation Of Photosynthesis And Leaf Temperature During Leaf Growth

It was considered that vertical CO2 diffusion was very limited between leaf adaxial and abaxial side in maize (Zea mays), a C4 monocotyledonous species. Theorically, large CO2 pressure between the lower and upper sub-stomatal will cause vertical CO2 diffusion between two sides of leaves which may further affect photosynthesis. To clarify the question, anatomical properties, gas exchange and the chlorophyll a fluorescence were studied in leaves of maize and sorghum by blocking stomata. The results showed that abaxial stomata play more important role in photosynthesis relative to abaxial stomata regardless of light orientations, as reflected by the marked decrease in photosynthetic rate and stomatal conductance when the abaxial stomata were blocked. Blocking abaxial stomata caused non-stomatal limitation at the beginning of photosynthetic induction and a significant delay in photosynthetic induction under adaxial illumination. Therefore, it is easily deduced that vertical CO2 diffusion from adaxial to abaxial side of leaves in maize and sorghum was very effective and the extended route of CO2 transportation within leaf caused the delay of photosynthetic induction. The results also showed that ΦPSⅡ measured on illuminated adaxial side with adaxial stomata blocked was slightly reduced, but ΦPSⅡ measured on illuminated adaxial side with abaxial stomata blocked was considerably lowered when compared with ΦPSⅡ measured on illuminated adaxial side with non-stomata-blocked on both sides of leaves in maize and sorghum. It suggested that CO2 diffused vertically from abaxial side of leaves to support photosynthesis in adaxial side in two C4 monocotyledonous species under adaxial illumination with adaxial stomata blocked. According to the above studies, it concluded that CO2 may diffuse vertically between adaxial and abaxial side in leaves of maize and sorghum, and the extended route of vertical CO2 transportation within leaf may cause the delay of photosynthetic induction. Therefore, the significantly difference in stomatal distribution and sensitivity of leaf abaxial and adaxial side to environment stimulation may affect vertical CO2 diffusion.The stomatal development also affected stomatal conductance and photosynthesis except the significantly difference in stomatal distribution and sensitivity of leaf abaxial and adaxial side to environment stimulation. It has been confirmed that stomatal limitation of photosynthesis occurred during leaf growth. Stomata, as an important leaf structure that controls the uptake of CO2 for photosynthesis, however, whether the stomatal limitation was caused by the slower stomatal development remaining unknown. To test this hypothesis, leaf structure, stomatal development, stomatal conductance and photosynthesis were carefully studied in both Syringa oblata (normal greening species) and Euonymus japonicus Thunb (delayed greening species). The results show that the size of stomata increased gradually with leaf expansion, resulting in increased stomatal conductance up to the time of full leaf expansion. During this process, photosynthesis also increased steadily. Compared to that in S. oblata, the development of chloroplasts in E. japonicus Thunb was obviously delayed, leading to a delay in the improvement of photosynthetic capacity. Further analysis revealed that before full leaf expansion, stomatal limitation increased rapidly in both S. oblata and E. japonicus Thunb; after full leaf expansion, stomatal limitation continually increased in E. japonicus Thunb. Accordingly, it suggested that the enhancement of photosynthetic capacity is the main factor leading to stomatal limitation during leaf development but that stomatal development can alleviate stomatal limitation with the increase of photosynthesis by controlling gas exchange. The changes of photosynthetic properties with the increase of leaf area in different leaf position of Euonymus bungeanus and Schefflera octophylla was similar to that during leaf development in both E. japonicus and S. oblata.It has been proved that juvenile leaves had higher temperatures than mature leaves in the field. Considering the crucial influences of stomatal development on stomatal conductance and transpiration, it speculated that stomatal development may affect the regulation of leaf temperature. However, juvenile leaves located at the top of the canopy compared with mature leaves, so it is more likely to be exposed to stronger light and higher air temperature. The effects of environment on leaf temperature have not been ruled out in previous studies. To clarify this problem, stomatal development, stomatal conductance, transpiration and the relationship between transpiration and leaf temperature were carefully studied in both Euonymus japonicus and Syringa oblata under controlled light intensity and air temperature. The results showed that the size of stomata increased gradually with leaf expansionin both E. japonicus and S. oblata, resulting in increased stomatal conductance and associated transpiration up to the time of full leaf expansion. Stomatal development helped to reduce leaf temperature through transpirationally driven leaf cooling. Although the differences in density and size of stomata were small between leaves in E. japonicus and S. oblata, there were large differences in stomatal distribution between the two species. Stomata distributed on both sides of leaves in S. oblata, while there were stomata only on abaxial side of leaves in E. japonicus. Under the same conditions of light intensity and air temperature, the stomatal conductance and transpiration of S. oblata were higher than that of E. japonicus, but the leaf temperature in S. oblata was lower than E. japonicus. It proposed that adaxial stomata may mainly contribute to reducing leaf temperature by transpiration cooling.