Onion plants (Allium cepa L.) react differently to salinity levels according to the regulation of aquaporins

Abstract

As salinity is one of the main environmental stresses that reduces the growth and productivity of crops by reducing water uptake and transport, in this work, we associated the physiological tolerance response of onion to increased NaCl concentration (from 25, 50, 75, to 100 mM) with the expression of aquaporins. Measurements of transpiration, gas exchange and nutrients content in leaf, roots and bulb tissues were determined in relation to the expression of PIP2, PIP1, and TIP2 aquaporin genes. The results indicated a significant decrease in growth in leaves, roots and bulbs only when 50 mM NaCl was applied. However, this was not correlated with the rest of the parameters, such as transpiration, number of stomata, osmotic potential, or chlorophyll concentration. In this way, the finding that the decreases in Mn, Zn and B observed in leaves, roots and bulbs at 50 mM NaCl were related to the expression of aquaporins, leaded to propose two phases of responses to salinity depending on level of NaCl. Therefore, the activation of PIP2 at 75 mM, in relation to Zn uptake, is proposed as relevant in the response of onion to high salinity.

Keywords: Allium cepa L.; PIP1; PIP2; Salinity; TIP2.

Fig. 1

Fresh weights of leaves (a) bulbs (b) and roots (c) of onion plants for the control and the salinity treatments (25, 50, 75 and 100 mM). Each value represents the mean of 6 biological replicates ± standard error. Different letters indicate significant differences (p < 0.05) between the control and the treatments according to a post hoc Duncan's test.

Fig. 2

Physiological parameters: Relative water contain (a) osmotic potential in leaves (b) and osmotic potential in roots (c) for the control and the salinity treatments (25, 50, 75 and 100 mM). Each value represents the mean of 6 biological replicates ± standard error. Different letters indicate significant differences (p < 0.05) between the control and the treatments according to a post hoc Duncan's test.

Fig. 3

Transpiration rate (a) and number of stomata per mm2 (b) for the control and the salinity treatments (25, 50, 75 and 100 mM). Each value represents the mean of 6 biological replicates ± standard error. Different letters indicate significant differences (p < 0.05) between the control and the treatments according to a post hoc Duncan's test.

Fig. 4

Chlorophyll A (a), Chlorophyll B (b) and Carotenoid (c) content per gr of leaf fresh weight for the control and the salinity treatments (25, 50, 75 and 100 mM). Each value represents the mean of 6 biological replicates ± standard error. Different letters indicate significant differences (p < 0.05) between the control and the treatments according to a post hoc Duncan's test.

Fig. 5

Concentration of macronutrients K, Ca & Na (columns) in leaves, roots & bulbs (rows) for the control and the salinity treatments (25, 50, 75 and 100 mM) (a–i). Each value represents the mean of 4 biological replicates ± standard error. Different letters indicate significant differences (p < 0.05) between the control and the treatments according to a post hoc Duncan's test.

Fig. 6

Concentration of micronutrients Mn, Zn & B (columns) in leaves, roots & bulbs (rows) for the control and the salinity treatments (25, 50, 75 and 100 mM) (a–i). Each value represents the mean of 4 biological replicates ± standard error. Different letters indicate significant differences (p < 0.05) between the control and the treatments according to a post hoc Duncan's test.

Fig. 7

Relative expression levels of aquaporin genes PIP1, PIP2 and TIP2 (rows) in leaves, roots & bulbs (columns) for the control and the salinity treatments (25, 50, 75 and 100 mM) (a–i). All the analyses were performed using QuantStudio 5 Flex Real-Time qPCR system and transcript levels were calculated using 2−ΔΔCt method (relative units). The values are the means ± S.E. of 3–4 biological replicates with 3 technical replicates of each measurement. Different letters indicate significant differences (p < 0.05) between the control and the treatments according to a post hoc Duncan's test.