Combined effect of boron and salinity on water transport

Abstract

Boron toxicity is an important disorder that can limit plant growth on soils of arid and semi arid environments throughout the world. Although there are several reports about the combined effect of salinity and boron toxicity on plant growth and yield, there is no consensus about the experimental results. A general antagonistic relationship between boron excess and salinity has been observed, however the mechanisms for this interaction is not clear and several options can be discussed. In addition, there is no information, concerning the interaction between boron toxicity and salinity with respect to water transport and aquaporins function in the plants. We recently documented in the highly boron- and salt-tolerant the ecotype of Zea mays L. amylacea from Lluta valley in Northern Chile that under salt stress, the activity of specific membrane components can be influenced directly by boron, regulating the water uptake and water transport through the functions of certain aquaporin isoforms.

High concentrations of boron are often associated to saline soils in semi arid and/or arid climates and frequently crops are exposed to both stresses simultaneously.¹ As there is no a unique plant response to combination of salinity and boron toxicity, several mechanisms has been proposed to explain the experimental results. Some reports showed no additive effects of boron and salinity on shoot weight of different cultivars suggesting independent of the interaction.^2–5 However, additive effects^6–9 have been also proposed and the interaction of boron and salinity declined the rate of germination and limited growth in maize and sorghum plants.¹⁰ No explanation is currently available for these contradictory observations. Recently, the Abbot method has been applied to characterize the combined effect of boron and salinity at toxic levels in pepper plants, observing mainly an antagonistic relationship regarding growth and yield.¹¹ Antagonism between salinity and boron may be the result of decreased toxicity of boron in the presence of NaCl, reduced toxicity of NaCl in the presence of boron, or both together. Letey et al.,¹² have reported that increased soil salinity may also reduce boron movement to the broccoli plants and hence result in a reduction of boron toxicity symptoms. Reduction of boron accumulation in leaves in the presence of salinity has been also reported for melon,⁵ tomato⁸ jack pine¹³ and grapesvines¹⁴ and could be the result of the reduced rates of transpiration in plants where boron is transported via xylem as consequence of the osmotic effect of the salt. On the other hand, it has been observed that concentration of Na⁺ in leaves decreased with increasing addition of boron to the soil, probably due to the inhibition in root growth and reduction in root density caused by the boron treatment.¹⁵ Grieve and Poss⁷ found in wheat plants that the Cl⁻ content in the leaves was reduced when boron was increased. Similar results were reported in pepper plants suggesting that boron could reduce Cl⁻ toxicity.¹¹ Also, in our recent report although a nutrient imbalance resulted from the effect of salinity or boron alone, a general optimisation was observed when both treatments were applied together.¹⁶

Under saline conditions, an optimal water balance is important in order to maintain the plant homeostasis and aquaporins may be one of the mechanisms involved under environmental and developmental changes.^17–19 However, there is no information concerning plant water uptake and transport in response to combined excess boron and salinity.

It has been reported that, at high external B concentrations, considerable B transport occurs through the plasma membrane aquaporins, and a specific membrane intrinsic protein (MIP) has been described.²⁰ Thus boron uptake across the plasma membrane, by permeation through the lipid membrane and aquaporins, may be greatly influenced by the plant tolerance to salinity, through the associated changes in root hydraulic conductivity. Wimmer et al.,²¹ showed that salinity could interact with boron toxicity by a combined effect on boron and water uptake. In addition, we reported that the reduction of aquaporin functionality in NaCl-exposed plants could induce the reduction of plant boron concentration, producing a beneficial effect.²²

Recently, we showed in a tolerant ecotype of maize a different pattern for PIP1 and PIP2 protein content under the application of excess of boron in combination with salinity, suggesting a differential aquaporin response in this cultivar and pointed out the complexity of the interaction.¹⁶ These results were in consonance with the previous observation that different aquaporin isoforms may represent a response to environmental changes.^18,19,²³ Thus, we concluded that the activity of specific membrane components can be influenced by boron under salt stress regulating the functions of certain aquaporin isoforms as possible components of the salinity tolerance mechanism. However, although a fine water transport control through the aquaporins could be necessary in order to reduce the accumulation of toxic boron levels in the tissues, the contribution of each isoform to water transport through the plasma membrane under boron-salinity combination must be elucidated.

Addendum to: Martinez-Ballesta Mdel C, Bastías E, Zhu C, Schäffner AR, González-Moro B, González-Murua C, Carvajal M. Boric acid and salinity effects on maize roots. Response of aquaporins ZmPIP1 and ZmPIP2, and plasma membrane H+-ATPase, in relation to water and nutrient uptake. Physiol Plant. 2008;132:479–490. doi: 10.1111/j.1399-3054.2007.01045.x.