Sodium–potassium synergism inTheobroma cacao: stimulation of photosynthesis, water‐use efficiency and mineral nutrition

Abstract

In ecological setting, sodium (Na+) can be beneficial or toxic, depending on plant species and the Na+ level in the soil. While its effects are more frequently studied at high saline levels, Na+ has also been shown to be of potential benefit to some species at lower levels of supply, especially in C4 species. Here, clonal plants of the major tropical C3 crop Theobroma cacao (cacao) were grown in soil where potassium (K+) was partially replaced (at six levels, up to 50% replacement) by Na+, at two concentrations (2.5 and 4.0 mmolc dm−3). At both concentrations, net photosynthesis per unit leaf area (A) increased more than twofold with increasing substitution of K+ by Na+. Concomitantly, instantaneous (A/E) and intrinsic (A/gs) water‐use efficiency (WUE) more than doubled. Stomatal conductance (gs) and transpiration rate (E) exhibited a decline at 2.5 mmol dm−3, but remained unchanged at 4 mmol dm−3. Leaf nitrogen content was not impacted by Na+ supplementation, whereas sulfur (S), calcium (Ca2+), magnesium (Mg2+) and zinc (Zn2+) contents were maximized at 2.5 mmol dm−3 and intermediate (30–40%) replacement levels. Leaf K+ did not decline significantly. In contrast, leaf Na+ content increased steadily. The resultant elevated Na+/K+ ratios in tissue correlated with increased, not decreased, plant performance. The results show that Na+ can partially replace K+ in the nutrition of clonal cacao, with significant beneficial effects on photosynthesis, WUE and mineral nutrition in this major perennial C3 crop.

Sodium as nutrient and toxicant

“…If Na + can be readily beneficial in so many plant species and, associated with this, accumulate to significant concentrations in plant organelles and organs to levels similar to those of K + (Gattward et al 2012;Schulze et al 2012), there must be efficient pathways for its entry across root plasma membranes. Interestingly, despite considerable effort, entry paths for Na + into roots have not as yet been successfully identified at the molecular level across taxonomic groups (Munns and Tester 2008;Zhang et al 2010;Kronzucker and Britto 2011;Cheeseman 2013), while a strong body of evidence has shown, at least in grasses, that one family of genes, HKT2 (formerly referred to as HKT1, but the latter designation is now reserved for a group of Na + transporters believed to be predominantly involved in intra-plant Na + transfer from root to shoot; Sunarpi et al 2005;Møller et al 2009), encodes transporters that can transport Na + at substantial rates across root plasma membranes, especially when K + is limiting (Horie et al 2001(Horie et al , 2011Laurie et al 2002;Munns and Tester 2008;Hauser and Horie 2010).…”

“…For instance, in beet, a replacement of 95 % of the plant's leaf-tissue K + by Na + resulted in no measurable negative impact on osmotic potential (Subbarao et al 1999b). It has, thus, been concluded that a near-complete replacement of K + by Na + in its osmotic function is possible (Shabala and Mackay 2011a, b;Gattward et al 2012). For other, nonosmotic functions of K + , replacement by Na + may, however, not be as easily achieved.…”

“…It is instructive that, in some cases, authors have reported superior stomatal performance in plants cultivated on high Na + /K + ratios than on K + alone (Marschner 1995), and improved water-use efficiency (Gattward et al 2012), suggesting that Na + may not only fill in for K + in the specific context of stomatal guard cell function, but may indeed be capable of outperforming it. We believe, in the light of the established model for stomatal function shown in Fig.…”

“…From beet to chocolate: the classic literature-Na + benefits are common It has long been known that Na + can be of benefit to the growth of algae and cyanobacteria (Allen and Arnon 1955;Simonis and Urbach 1963;Brownell and Nicholas 1968), but, for higher plants, the reputation of the ion as a toxic one has held sway (Maathuis 2007;Munns and Tester 2008;Kronzucker and Britto 2011;Cheeseman 2013), and the vast majority of higher-plant literature on the ion has focused on this aspect, even though studies in a wide variety of species, including such important cultivated ones as tomato, potato, carrot, cacao, and cereals, have demonstrated the potential benefit of the ion for higher-plant growth as well (Wheeler and Adams 1905;Lehr 1941;Lehr and Wybenga 1955;Woolley 1957;Williams 1960;Brownell 1965;Brownell and Jackman 1966;Montasir et al 1966;El-Sheikh et al 1967;Hylton et al 1967;Draycott and Durrant 1976;Galeev 1990;Takahashi and Maejima 1998;Gattward et al 2012). It is important to emphasize that every substance has a threshold below which it is not toxic, in accordance with the "sola dosis facit venemum" (only the dose makes the poison) principle, famously attributed to Theophrastus Bombastus von Hohenheim (Paracelsus), but, for Na + , beneficial effects are seen well into the range of concentrations that would be considered high for ordinary nutrient ions, such as NO 3 − , NH 4 + , or K + , and, in the cases of halophytes, go far beyond that (Flowers and Colmer 2008).…”

Growth and yield responses in wheat and barley to potassium supply under drought or moderately saline conditions in the south-west of Western Australia

“…For example, low to moderate Na may improve crop growth, depending upon soil K levels and genotypic difference in salt tolerance (Ma et al 2011;Kronzucker et al 2013), and there is a strong substitution of K by Na in barley (Ma et al 2011). It is suggested that Na can substitute for non-specific biophysical functions of K by maintaining cell turgor especially in stomatal guard cells and ionic balance (Marschner 1995;Subbarao et al 2003;Gattward et al 2012;Kronzucker et al 2013). In this study, the findings of positive response of leaf gas exchange to soil K supply and small differences in growth and grain yield of barley among the treatments of 20, 40 and 120 kg K/ha suggested at least partial substitution of K by Na under low K and moderate salinity.…”