Influence of drought on foliar water uptake capacity of temperate tree species

Foliar water uptake (FWU) has been investigated in an increasing number of species from a variety of areas but has remained largely understudied in deciduous, temperate tree species from non- foggy regions. As leaf wetting events frequently occur in temperate regions, FWU might be more important than previously thought and should be investigated. As climate change progresses, the number of drought events is expected to increase, basically resulting in a decreasing number of leaf wetting events, which might make FWU a seemingly less important mechanism. However, the impact of drought on FWU might not be that unidirectional because drought will also cause a more negative tree water potential, which is expected to result in more FWU. It yet remains unclear whether drought results in a general increase or decrease in the amount of water absorbed by leaves. The main objectives of this study are, therefore: (i) to assess FWU- capacity in nine widely distributed key tree species from temperate regions, and (ii) to investigate the e ff ect of drought on FWU in these species. Based on measurements of leaf and soil water potential and FWU- capacity, the e ff ect of drought on FWU in temperate tree species was assessed. Eight out of nine temperate tree species were able to absorb water via their leaves. The amount of water absorbed by leaves and the response of this plant trait to drought were species- dependent, with a general increase in the amount of water absorbed as leaf water potential decreased. This relationship was less pronounced when using soil water potential as an independent variable. We were able to classify species according to their response in FWU to drought at the leaf level, but this classification changed when using drought at the soil level, and was driven by iso- and anisohydric behavior. FWU hence occurred in several key tree species from temperate regions, be it with some variability, which potentially allows these species to partly reduce the e ff ects of drought stress. We recommend including this mechanism in future research regarding plant- water relations and to investigate the impact of di ff erent pathways used for FWU

Foliar water uptake in trees : negligible or necessary?

Foliar water uptake (FWU) has been identified as a mechanism commonly used by trees and other plants originating from various biomes. However, many questions regarding the pathways and the implications of FWU remain, including its ability to mitigate climate change-driven drought. Therefore, answering these questions is of primary importance to adequately address and comprehend drought stress responses and associated growth. In this review, we discuss the occurrence, pathways, and consequences of FWU, with a focus predominantly on tree species. Subsequently, we highlight the tight coupling between FWU and foliar fertilizer applications, discuss FWU in a changing climate, and conclude with the importance of including FWU in mechanistic vegetation models

Variation in nocturnal stomatal conductance and development of predawn disequilibrium between soil and leaf water potentials in nine temperate deciduous tree species

It is widely acknowledged that many plant species can keep stomata open during night. We examined how nocturnal stomatal conductance differs among potted saplings of nine temperate tree species from diverse native habitats in wet and dry soil conditions, and how it affects plant predawn water status. Nocturnal stomatal conductance in dry soil conditions was low in all the species (with a maximum value of 14.6 mmol m(-2) s(-1)); in wet conditions, it was the highest in Populus tremula L., a fast-growing and anisohydric pioneer species, and the lowest in Quercus robur L., a late-successional and isohydric species. Relatively high nocturnal stomatal conductance in wet conditions in P. tremula compared with the other species resulted in the highest difference in water potential values between the leaves and soil at predawn. As drought progressed, different species tended to keep stomata almost closed at night, and the observed differences between anisohydric and isohydric species disappeared. At an ample soil water supply, nocturnal stomatal behaviour was species dependent and varied according to both the water-use and the life strategies of the species. Keeping that in mind, one should therefore be careful when using predawn leaf water potential as a proxy for soil water potential, sampling different species

Identifying the pathways for foliar water uptake in beech (Fagus sylvatica L.) : a major role for trichomes

Foliar water uptake (FWU), the direct uptake of water into leaves, is a global phenomenon, having been observed in an increasing number of plant species. Despite the growing recognition of its functional relevance, our understanding of how FWU occurs and which foliar surface structures are implicated, is limited. In the present study, fluorescent and ionic tracers, as well as microcomputed tomography, were used to assess potential pathways for water entry in leaves of beech, a widely distributed tree species from European temperate regions. Although none of the tracers entered the leaf through the stomatal pores, small amounts of silver precipitation were observed in some epidermal cells, indicating moderate cuticular uptake. Trichomes, however, were shown to absorb and redistribute considerable amounts of ionic and fluorescent tracers. Moreover, microcomputed tomography indicated that 72% of empty trichomes refilled during leaf surface wetting and microscopic investigations revealed that trichomes do not have a cuticle but are covered with a pectin-rich cell wall layer. Taken together, our findings demonstrate that foliar trichomes, which exhibit strong hygroscopic properties as a result of their structural and chemical design, constitute a major FWU pathway in beech

Identifying the pathways for foliar water uptake in beech ( Fagus sylvatica

Foliar water uptake (FWU), the direct uptake of water into leaves, is a global phenomenon, having been observed in an increasing number of plant species. Despite the growing recognition of its functional relevance, our understanding of how FWU occurs and which foliar surface structures are implicated, is limited. In the present study, fluorescent and ionic tracers, as well as microcomputed tomography, were used to assess potential pathways for water entry in leaves of beech, a widely distributed tree species from European temperate regions. Although none of the tracers entered the leaf through the stomatal pores, small amounts of silver precipitation were observed in some epidermal cells, indicating moderate cuticular uptake. Trichomes, however, were shown to absorb and redistribute considerable amounts of ionic and fluorescent tracers. Moreover, microcomputed tomography indicated that 72% of empty trichomes refilled during leaf surface wetting and microscopic investigations revealed that trichomes do not have a cuticle but are covered with a pectin-rich cell wall layer. Taken together, our findings demonstrate that foliar trichomes, which exhibit strong hygroscopic properties as a result of their structural and chemical design, constitute a major FWU pathway in beech

Hydraulic redistribution of foliar absorbed water causes turgor-driven growth in mangrove seedlings

Although foliar water uptake (FWU) has been shown in mature Avicennia marina trees, the importance for its seedlings remains largely unknown. A series of experiments were therefore performed using artificial rainfall events in a greenhouse environment to assess the ecological implications of FWU in A. marina seedlings. One-hour artificial rainfall events resulted in an increased leaf water potential, a reversed sap flow, and a rapid diameter increment signifying a turgor-driven growth of up to 30.1 +/- 5.4 mu m. Furthermore, the application of an artificial rainfall event with deuterated water showed that the amount of water absorbed by the leaves and transported to the stem was directly and univocally correlated to the observed growth spurts. The observations in this process-based study show that FWU is an important water acquisition mechanism under certain circumstances and might be of ecological importance for the establishment of A. marina seedlings. Distribution of mangrove trees might hence be more significantly disturbed by climate change-driven changes in rainfall patterns than previously assumed