Foliar application of plant nutrients is a common practice adopted by many growers in multiple species. It is usually adopted to prevent situations of limited supply of nutrients through the soil, to quickly revert symptoms of deficiency already appearing in the field, or to supply plants with nutrients required in higher amounts during specific growth stages. However, the efficacy of foliar application to overcome nutritional deficiencies relies on the effective absorption of nutrients and translocation to other organs of the plant. In this issue,Li et al. (2021) explored the absorption process of foliar-applied zinc (Zn) by non-glandular trichomes of sunflower using cutting-edge techniques to reveal that not only can absorb foliar-applied Zn, but also have roles in translocation of the foliar-absorbed Zn.
The principle of nutrient absorption by leaves
Whereas the main function of leaves is related to the capture of light and CO2, the potential of plant leaves to absorb water and nutrients is gaining much attention. Due to particularities in composition and anatomy, the capacity of leaves to absorb foliar-applied nutrients differs greatly among plant species, and the underlying mechanisms of foliar absorption are not fully understood. The comprehensive review of Fernández and Eichert (2009) highlighted possible uptake mechanisms of foliar nutrients by leaves. However, our understanding of the mechanisms controlling the uptake of water and solutes by plant leaves is directly related to the development of more reliable techniques able to monitor the absorption process.
Currently, it is well accepted that the process of nutrient absorption by leaves can occur via the cuticles, trichomes, stomata, veins, and other epidermal structures (Fernandez et al., 2021). The cuticular pathway gained attention first and was believed to be the main process of nutrient uptake by leaves (Schonherr, 1976; Schreiber, 2005). However, emerging evidence in the last few decades suggests that stomata and, more recently, trichomes might make a larger contribution than initially thought.
Despite the existing controversy, the uptake of solutes by the stomatal pathway is becoming more accepted based on the evidence provided by Eichert and Burkhardt (2001) and Eichert et al. (2008). However, from a practical perspective, the contribution of the stomata might be limited when compared with the other pathways. The disparities found in studies dealing with stomatal uptake of nutrients might be related to external processes that can affect the wettability of the guard cell surface of individual stomata, ‘activating’ them for solute transport (Fernández et al., 2021). Much less attention is focused on the role of the trichomes in the uptake process. In this regard, the unprecedented development of methods in the past decades allowed a better clarification of the mechanism of nutrient uptake by glandular and non-glandular trichomes, with a level of detail not seen before.
Li et al. (2018) observed accumulation of foliar-applied Zn in glandular trichomes of soybean (Glycine max). In sunflower (Helianthus annuus), the non-glandular trichomes showed a Zn concentration 1.9 times higher than in the cuticule, demonstrating the role of non-glandular trichomes in the uptake of foliar-applied Zn (Li et al., 2019). Along the same lines, Schreel et al. (2020) found moderate cuticular uptake but considerable absorption and redistribution of tracers in trichomes of beech (Fagus sylvatica) using synchrotron-based microtomography. These new pieces of evidence suggest that trichomes might play an important role in the absorption of foliar-applied nutrients.
New methods available to probe the process of nutrient absorption
X-ray spectrometry is sensitive to most plant mineral nutrients; however, it neither reveals the composition of the cuticle nor shows the biochemistry behind the scene. A wider overview on a complex phenomenon such as the absorption and translocation of foliar-applied nutrients requires a combination of techniques probing both inorganic and organic players. Non-destructive or barely invasive techniques such as X-ray fluorescence spectrometry (Rodrigues et al., 2018), Raman spectroscopy (Zeng et al., 2021), gas exchange analysis (Montanha et al., 2020), and multi-/hyperspectral imaging (Galletti et al., 2020) can be combined to yield high value complementary information. The approach employed by Li et al. (2021) may represent a first step towards the simultaneous combination of these techniques under in vivo conditions at synchrotron beamline stations, for example as shown in Fig. 1. This strategy has been employed by materials scientists for almost 20 years. Likewise, plant scientists and physicists should work together to overcome the challenges of looking at biological phenomena while they are happening.
(A) Schematic diagram showing the integration of non-destructive or barely invasive techniques such as XRF, Raman, and hyperspectral imaging. (B) These tools may be directed to a single spot and employed to simultaneously trace nutrient absorption under in vivo conditions.
If, on the one hand, small X-ray beams provide high lateral resolution at the tissue and even the cell level, on the other hand one has to pay attention to the statistical significance of a local phenomenon. For example, what is variability between trichomes and how many individuals should be analysed to generalize the observed results? Averaging the behaviour under a certain area might represent an alternative to monitoring the behaviour of a single trichome or cell.
Another key feature of X-ray fluorescence spectrometry refers to its multielementary characteristic. If an incoming X-ray beam of, for example, 10 keV hits a plant tissue, it is going to excite the 1s orbital of several plant nutrients such as Zn, Cu, Ni, Fe, Mn, Ca, K, S, P, and Mg. The ability to detect these elements will depend, of course, on the experimental conditions such as the presence of filters in front the detector, the length of the air path between the tissue and the detector, and, last but not least, the concentration of the element in the sample. Frequently it is possible to detect several elements simultaneously, which opens up the possibility of verifying whether a correlation exists between them. This can reveal possible X-ray-induced radiation damage during the experiment (Jones et al., 2020), point out synergistic or antagonistic effects between nutrients (Blamey et al., 2019), or perform simultaneous investigations on a mixture of nutrients which is a common situation faced under field conditions (Rodrigues et al., 2018).
Anatomy and the role of foliar non-glandular trichomes in nutrient absorption
The epidermis is the outermost plant cell layer nevertheless it is a complex tissue constituted by several types of cells. Among them, the trichomes have an important ecophysiological role. The glandular trichomes are responsible for secreting a myriad of secondary metabolites, whilst non-glandular trichomes protect against herbivory and absorb water and solutes (Pina et al., 2016; Li et al., 2019, 2021). The non-glandular trichomes and their basal cells can be either uni- or multicellular and continue living or die at maturity. They are coated by a cuticle and have a wall composition that is associated with protection against plant dissection. Among the various types of diversity in plant species, the non-glandular trichomes and basal cells can present different compositions in the cuticle and cell walls.
Li et al. (2021) demonstrated that the cuticle composition of the non-glandular trichomes basal cells of sunflower is different from that of ordinary leaf epidermal cells and that this might facilitate Zn absorption. Not only the basal cells, but also non-glandular trichomes themselves can present a pectin-rich cell wall (Fig. 2A), associated with the hydrophilic nature that may promote nutrient uptake. The pectin-rich cell wall may act as a hydrophilic spot where the water and nutrients can be absorbed, as observed in other structures, such as leaf emergence in Microlepis oleaefolia that contributes to water absorption (Milanez and Machado, 2008). This seems to be a common strategy in water absorption, being shared with xeric species growing in dry environments. After entering the leaf, the water and nutrients can move from the epidermis to the phloem by two possible pathways: symplastic and apoplastic (Fig. 2B). The apoplastic pathway has been demonstrated previously by the same group (Li et al., 2018), which revealed Zn in the mesophyll cell walls. Now, Li et al. (2021) shed light on the possible symplastic mechanism in which ABC transporters b, HIPP, and DTX could be potentially involved in the absorption and translocation of the foliar-absorbed Zn.
Pathways involved in water and nutrient absorption by the non-glandular trichomes using soybean (Glycine max L. Merrill) as a model. (A) Digital microscopy image of the adaxial surface of a soybean leaf before and after the ruthenium red 0.01% staining method demonstrating the pectin-rich cell wall composition of the non-glandular trichomes above the leaf vein. (B) Proposed model of the nutrient absorption by non-glandular trichomes and non-glandular trichomes basal cells. The nutrients can penetrate through the cuticle of the non-glandular trichomes and pectin-rich cell wall and could be taken to the phloem by the symplastic or apoplastic pathway.
Final remarks
Despite being studied for more than two centuries, the processes underlying the mechanisms of absorption of foliar-applied nutrients, especially those related to the trichome pathway, are still poorly understood. The evidence presented by Li et al. (2021), using cutting edge techniques, reveals that non-glandular trichomes of sunflower play a non-negligible role in the absorption of the foliar-applied Zn and translocation of the foliar-absorbed Zn to other parts of the plant. Such evidence might not only have further implications for understanding the absorption pathways of foliar-applied nutrients, but can also have implications for breeding programmes aiming to deliver varieties (e.g. varieties having leaf trichomes which can absorb and translocate foliar fertilizers) with improved capacity for absorption of foliar-applied nutrients to achieve high yields with improved nutrient content and quality.
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