| The present project investigated the regulation of leaf lamina hydraulic conductance (Klam) in bur oak (Quercus macrocarpa Michx.) and aspen (Populus tremuloides Michx.). The four different studies that were undertaken examined: (1) the diurnal and seasonal variation and the irradiance sensitivity of Klam in bur oak and aspen, (2) the chemical inhibition of the irradiance response of Klam in bur oak, (3) the chemical inhibition of Klam in aspen, and (4) the involvement of aquaporin transcription in Klam irradiance response in bur oak.;In bur oak, Klam increased 4-7 times in response to increased irradiance. The response was present throughout the day, declined in senescing leaves and was absent from the leaves of young seedlings. In aspen, K lam was not sensitive to irradiance. The Klam values declined from morning to late afternoon and decreased drastically before leaf senescence. The irradiance response of bur oak Klam was localized outside of the leaf main vein network. The response was suppressed by several chemical inhibitors such as NO (general metabolic inhibitor), atrazine (inhibitor of photosynthetic electron transport), HgCl2 (aquaporin blocker), fusicoccin (stimulator of plasma membrane H+-ATPase), and beta-mercaptoethanol (reducing agent disrupting quaternary protein structure). The response was saturated in relatively low light intensity and was greater in blue and green light compared with orange and red light. In aspen, Klam was drastically suppressed by NaF, but showed little response to HgCl2 and was not affected by atrazine. For both species, when PTS3 (trisodium 3-hydroxy-5,8,10-pyrenetrisulphonate) apoplastic tracer was pressure-infiltrated inside the leaves, its concentration in the leaf exudates did not change from ambient light to high irradiance treatment, nor with the presence of HgCl 2 in the treatment solution. Five cDNA sequences showing high similarity to members of aquaporin gene family from other plant species were also characterized from bur oak leaves. There was no change in the relative expression level of the four examined putative aquaporin genes in the leaves due to high irradiance. However, in sun leaves that were exposed to different light levels prior to leaf collection (leaves in full sunlight, shade, and covered for 16 h), the relative transcript levels of two of the putative aquaporin genes increased several times in shaded leaves compared to the sun-exposed or covered leaves. |
