Spinach Plants Favor the Absorption of K+ over Na+ Regardless of Salinity, and May Benefit from Na+ When K+ is Deficient in the Soil

Abstract

Two spinach (Spinacea oleracea L.) cultivars were evaluated for their response to deficient (0.25 mmol c L −1 or 0.25 K) and sufficient (5.0 mmol c L −1 or 5.0 K) potassium (K) levels combined with salinities of 5, 30, 60, 90, and 120 mmol c L −1 NaCl. Plants substituted K for Na proportionally with salinity within each K dose. Plants favored K + over Na + , regardless of salinity, accumulating significantly less Na at 5.0 K than at 0.25 K. Salinity had no effect on N, P, and K shoot accumulation, suggesting that spinach plants can maintain NPK homeostasis even at low soil K. Ca and Mg decreased with salinity, but plants showed no deficiency. There was no Na + to K + or Cl − to NO 3 − competition, and shoot biomass decrease was attributed to excessive NaCl accumulation. Overall, 'Raccoon' and 'Gazelle' biomasses were similar regardless of K dose but 'Raccoon' outproduced 'Gazelle' at 5.0 K at the two highest salinity levels, indicating that 'Raccoon' may outperform 'Gazelle' at higher NaCl concentrations. At low K, Na may be required by 'Raccoon', but not 'Gazelle'. This study suggested that spinach can be cultivated with recycled waters of moderate salinity, and less potassium than recommended, leading to savings on crop input and decreasing crop environmental footprint.

Germination and Growth of Spinach under Potassium Deficiency and Irrigation with High-Salinity Water

“…However, the increase from 0.3 to 3.0 mmol c L −1 in K was enough to show that K tissue accumulation was antagonistic to Na tissue accumulation. These results agree with a previous report that concluded that a 20-fold increase in K from 0.25 to 5.0 mmol c L −1 decreased Na tissue accumulation [ 8 ].…”

“…The steady and stepwise increase in tissue Cl, which surpassed the increase in tissue Na ( Figure 3 ), indicates that spinach is more efficient in controlling Na than Cl. Increases in both tissue Na and Cl of similar magnitudes have been reported recently when irrigation water contained 90 and 120 mmol c L −1 of NaCl [ 8 ]. Interestingly, the use of saline water containing 120 and 160 mmol c L −1 in this study resulted in Na shoot accumulation of approximately 50–60 g kg −1 , smaller than the 60–70 g kg −1 of shoot Na reported previously [ 8 ] for the same cultivars under 90 and 120 mmol c L −1 of Na.…”

“…Plants maintained adequate concentrations of N, P, and K in roots, regardless of salinity or K dose. Although Ca, Mg, and S decreased significantly with salinity, regardless of K dose, plants maintained a minimum concentration of these macronutrients needed for growth [ 8 , 47 ]. Root-P increase in both cultivars as salinity increased ( Table 3 ) could have been an attempt for the plant to balance ions using HPO 4 2− to compensate for a constant concentration of NO 3 − in the leaf (interpreted through the constant tissue levels of mineral N), and in response to a steady increase of Na + in saline irrigation waters, as suggested by others [ 48 ].…”

“…Depending on water or soil salinity, Na and Cl can accumulate in spinach shoots at levels as high as the macronutrients N and K [ 6 , 8 ], indicating the existence of an efficient vacuolar sequestration of potentially toxic Na + and Cl − ions in this species. On the contrary, many glycophytes, including strawberry and Jerusalem artichoke, have mechanisms that prevent Na + toxicity by reducing Na + transport to leaves [ 5 , 9 ].…”

“…Considering that spinach is a moderately salt-tolerant glycophyte [ 6 , 8 , 40 ], we hypothesized that irrigation waters providing elevated NaCl-induced salinity combined with potassium deficiency would mimic similar conditions found in soil solutions, thus providing a model system to study spinach response to these stresses at both germination and late growth stage. Thus, the objective of this research was to examine the salt tolerance of two cultivars of spinach at different developmental stages (germination, seedling establishment, and vegetative growth) under cultivation with elevated salinity combined with K deficiency applied through the irrigation water.…”