Ionic partitioning and stomatal regulation

Abstract

Vegetable grafting is commonly claimed to improve crop’s tolerance to biotic and abiotic stresses, including salinity. Although the use of inter-specific graftings is relatively common, whether the improved salt tolerance should be attributed to the genotypic background rather than the grafting per se is a matter of discussion among scientists. It is clear that most of published research has to date overlooked the issue, with the mutual presence of self-grafted and non-grafted controls resulting to be quite rare within experimental evidences. It was recently demonstrated that the genotype of the rootstock and grafting per se are responsible respectively for the differential ion accumulation and partitioning as well as to the stomatal adaptation to the stress. The present paper contributes to the ongoing discussion with further data on the differences associated to salinity response in a range of grafted melon combinations.

Keywords: :

• vegetable grafting
• Cucumis melo L.
• salt stress
• NaCl
• ion partitioning
• stomata

Salinity results in plant downregulation of physiological functions, accumulation of ions in vegetative tissues up to toxic concentrations and, more generally, to the overall depletion of the crop performances.¹ Several researches have confirmed the beneficial role played by the adoption of grafting in counteracting the detrimental effect of salinity.^2,3,4 To date, grafting is a common practice in melon (Cucumis melo L.) cultivation, mainly through the adoption of interspecific rootstocks, which are generally selected genotypes of squash (Cucurbita maxima Duch x Cucurbita Moschata Duch.). Although these rootstocks have proven to improve crop’s performances in presence of biotic^5,6 and abiotic⁴ stresses, whether the beneficial effects should be attributed to the rootstock genotype rather than grafting per se is still a controversial matter among scientists. In a recent publication,⁷ the comprehension of the role of grafting in improving the response to salinity in melon was addressed. The manuscript reported on the effects associated to cultivar, grafting, and salinity over 4 experiments in a range of different environmental conditions. As only grafting and salinity presented significant interactions, results from all experiments were jointly discussed, thus offering innovative elements for the comprehension of the effect of grafting on the plant physiological response to the stress. In the present manuscript, additional results obtained from one of the experiments conducted at Bologna University, Italy (experiment 1#)⁷ are discussed, with the aim of further elucidating how the differential stomatal and ionic response observed in self-grafted vs. interspecific grafting may lead to similar performances upon salt stress (0, 40, and 80 mM NaCl, starting from 10 Days After Transplanting, DAT, and lasting 30 d). Similarly to other experiments presented in the manuscript, 2 melon cultivars (namely Brennus, ZKI, Hungary, and London, Nunhems, The Netherlands) were used, altogether with a squash rootstock (Rs841, Monsanto, USA). Every genotype was used either non-grafted, self-grafted, or grafted on the interspecific squash rootstock. However, the peculiarity of this experiment was that also non-grafted and self-grafted Rs841 plants were included in the trial. The ANOVA analysis highlighted that the melon scion genotype did not actually affect the plant’s response to salinity, with significant differences observed only between plants either non-grafted, self-grafted, and interspecific graftings. Fresh biomass production was depleted as a consequence to salinity in all grafting combinations, although to a greater extent⁷ in non-grafted plants undergoing 80 mM NaCl. Upon salinization, an increase in Na⁺:K⁺ ratio in roots was observed in all grafting combinations under study (Fig. 1), confirming that, in both grafted and non-grafted plants, an increase of Na⁺ was to be experienced independently from the genotype of the root system.⁴ An explanation of the highest Na⁺:K⁺ values observed in all salinized root tissue of Rs841 (either non-grafted, self-grafted, or grafted with a melon scion) may be found in the consistent lower Na⁺ loading in the epigeous organs: filtration of Na⁺ at the grafting union level was observed (Fig. 1), although to a limited extent, whereas major differences were observed in stems and leaves. In these organs, although negligible changes could be detected upon salinity in any of the plants with a Rs841 root system, a general increase of the Na⁺:K⁺ ratio was recorded in all self- and non-grafted melons. Despite the observed changes in Na⁺ concentrations, it has been suggested that functional response to salinity in grafted plants may include K⁺ accumulation in aboveground organs.⁸ Nonetheless, while salinity imbalanced the Na⁺:K⁺ ratio in epigeous tissue of all melon plants (either non- or self-grafted), the adaptation to the stress in self-grafted plants was mediated by the tight transpirational regulation operated at stomatal level. This is clearly represented by the response of the Water Use Efficiency (WUE) to salinity, which was scarcely (Brennus) or not (London) affected in self-grafted plants (Fig. 2). Although a similar behavior was observed also in interspecific-graftings, it should be noted that the rootstock (Rs841) had very little to do with it: WUE was extremely responsive to salinity in both self-grafted and non-grafted Rs841 plants (Fig. 2).

Figure 1. Na⁺:K⁺ ratio in plant organs (root, grafting union, stems, and leaves) from grafted and non-grafted plants of melon (cv Brennus and London) and squash (rootstock Rs841), as affected by 0 (black), 40 (gray), and 80 (white) mM NaCl. Mean values ± SE (n = 9). Different letters indicate significant differences within grafting combination at P ≤ 0.05.

Figure 2. Water Use Efficiency (WUE) in grafted and non-grafted plants of melon (cv Brennus and London) and squash (rootstock Rs841), as affected by 0 (black), 40 (gray), and 80 (white) mM NaCl. Mean values ± SE (n = 9). Different letters indicate significant differences within grafting combination at P ≤ 0.05.

Plant preservation of WUE under stressful environments is generally achieved through tight stomatal control, which mainly consists in the reduction of water luxury consumption observable under non-limiting growing conditions.^9,10 The increased CO₂ assimilation per unit water used was a consequence of the changes in the stomatal morphology (mainly through increase in the stomatal index, SI),¹¹ rather than stomatal closure, which, on the other hand, turned out to be the main response to salinity in non-grafted plants.⁷ Clearly, the improved response of grafted plants to salinity followed a binary pathway: signaling (possibly mediated by wound-related hormones, i.e., ABA and ethylene) resulted in a physiological pre-adaptation to the stress, most efficient when rootstock and scion belonged to the same species. On the other hand, the interspecific rootstock efficiently operated as a toxic-ions filter, thus resulting in lower accumulation in the epigeous plant organs.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

This research was conducted in collaboration with Prof Yuksel Tuzel and Dr Golgen Bahar Oztekin (Ege University, Izmir, Turkey) and Prof Noemi Kappel (Curvinus University, Budapest, Hungary).