Effects of Periodical Soil Drying and Leaf Water Potential on the Sensitivity of Stomatal Response to Xylem ABA

The study on the changes of stomatal sensitivity in relation to xylem ABA during periodical soil drying and the effect of leaf water status on the stomatal sensitivity has confirmed that xylem ABA concentration is a good indicator of soil water status around roots and the relation between xylem ABA concentration and predawn leaf water potential remained constant during the three consecutive soil drying cycles based on the slopes of the fitted lines. The sensitivity of stomata to xylem ABA increased substantially as the soil drying cycles progressed, and the xylem ABA concentration needed to cause a 50% decrease of stomatal conductance was as low as 550 nmol/L in the next two soil drying cycle, as compared with the 750 nmol/L ABA in the first cycle of soil drying. The results using the split_root system showed that leaf water deficit significantly enhanced the stomatal response to xylem ABA and the xylem ABA concentration needed to cause a 50% decrease in stomatal conductance was 2 to 4 times smaller in the whole_root_drying treatment than those in the semi_root_drying treatment. These results suggested that the sensitivity of stomata to xylem ABA concentration is not a fixed characteristic.

Leaf/shoot level ecophysiology in two broadleaf and two needle-leaf species under representative cloud regimes at alpine treeline

Aims The effects of clouds are now recognized as critically important to the understanding of climate change impacts on ecosystems.Regardless,few studies have focused specifically on the ecophysiological responses of plants to clouds.Most continental mountain ranges are characterized by common convective cloud formation in the afternoons,yet little is known regarding this influence on plant water and carbon relations.Here we compare the ecophysiology of two contrasting,yet ubiquitous growth forms,needle-leaf and broadleaf,under representative cloud regimes of the Snowy Range,Medicine Bow Mountains,southeastern Wyoming,USA.Methods Photosynthetic gas exchange,water use efficiency,xylem water potentials and micrometeorological data were measured on representative clear,overcast and partly cloudy days during the summers of 2012 and 2013 for two indigenous broadleaf(Caltha leptosepala and Arnica parryi)and two needle-leaf species(Picea engelmannii and Abies lasiocarpa)that co-occur contiguously.Important Findings Reductions in sunlight with cloud cover resulted in more dramatic declines in photosynthesis for the two broadleaf species(ca.50-70%reduction)versus the two conifers(no significant difference).In addition,the presence of clouds corresponded with lower leaf conductance,transpiration and plant water status in all species.However,the more constant photosynthesis in conifers under all cloud conditions,coupled with reduced transpiration,resulted in greater water use efficiency(ca.25%higher)than the broadleaf species.These differences appear to implicate the potential importance of natural cloud patterns in the adaptive ecophysiology of these two contrasting,but common,plant growth forms.