Evaluating Leaf Wettability and Salt Hygroscopicity as Drivers for Foliar Absorption

The objective of this study was to evaluate the rate of foliar absorption of magnesium (Mg) salts with different deliquescence and efflorescence relative humidity values (DRH and ERH, also known as point of deliquescence (POD) and point of efflorescence (POE), respectively) when supplied to leaves of model plants with different wettability properties. For this purpose, a greenhouse pot experiment was conducted with lettuce (very wettable), broccoli (highly unwettable) and leek (highly unwettable). Foliar sprays contained 0.1% surfactant plus 100 mM Mg supplied as MgCl₂·6H₂O, Mg(NO₃)₂·6H₂O or MgSO₄·7H₂O. Leaf Mg concentrations were determined 1 and 7 days after foliar application. Anion concentrations were also measured in lettuce where a significant foliar Mg absorption was detected. Leaf wettability, leaf surface free energy and fertilizer drop deposit appearance onto the foliage were assessed. It is concluded that despite including a surfactant in the spray formulation, leaf wettability plays a major role in foliar Mg absorption.

The 18 O-signal transfer from water vapour to leaf water and assimilates varies among plant species and growth forms

The ¹⁸ O signature of atmospheric water vapour (δ¹⁸ O_V ) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the ¹⁸ O-signal transfer differs among plant species and growth forms. We performed a 9-hr greenhouse fog experiment (relative humidity ≥ 98%) with ¹⁸ O-depleted water vapour (-106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the ¹⁸ O-signal transfer by calculating the mean residence time of O in leaf water (MRT_LW ) and sugars (MRT_Sugars ) and related it to leaf traits and physiological drivers. MRT_LW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRT_Sugars was shorter in C₃ and C₄ plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRT_Sugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs, and herbs, and longest for conifers, epiphytes, and succulents. Sucrose was more sensitive to δ¹⁸ O_V variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ¹⁸ O_V variations, which is important for the interpretation of δ¹⁸ O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.

Diffuse light and wetting differentially affect tropical tree leaf photosynthesis

Most ecosystems experience frequent cloud cover resulting in light that is predominantly diffuse rather than direct. Moreover, these cloudy conditions are often accompanied by rain that results in wet leaf surfaces. Despite this, our understanding of photosynthesis is built upon measurements made on dry leaves experiencing direct light. Using a modified gas exchange setup, we measured the effects of diffuse light and leaf wetting on photosynthesis in canopy species from a tropical montane cloud forest. We demonstrate significant variation in species-level response to light quality independent of light intensity. Some species demonstrated 100% higher rates of photosynthesis in diffuse light, and others had 15% greater photosynthesis in direct light. Even at lower light intensities, diffuse light photosynthesis was equal to that under direct light conditions. Leaf wetting generally led to decreased photosynthesis, particularly when the leaf surface with stomata became wet; however, there was significant variation across species. Ultimately, we demonstrate that ecosystem photosynthesis is significantly altered in response to environmental conditions that are ubiquitous. Our results help to explain the observation that net ecosystem exchange can increase in cloudy conditions and can improve the representation of these processes in Earth systems models under projected scenarios of global climate change.