Molecular mechanisms of foliar water uptake in a desert tree

Abstract

Water deficits severely affect growth, particularly for the plants in arid and semiarid regions of the world. In addition to precipitation, other subsidiary water, such as dew, fog, clouds and small rain showers, may also be absorbed by leaves in a process known as foliar water uptake. With the severe scarcity of water in desert regions, this process is increasingly becoming a necessity. Studies have reported on physical and physiological processes of foliar water uptake. However, the molecular mechanisms remain less understood. As major channels for water regulation and transport, aquaporins (AQPs) are involved in this process. However, due to the regulatory complexity and functional diversity of AQPs, their molecular mechanism for foliar water uptake remains unclear. In this study, Tamarix ramosissima, a tree species widely distributed in desert regions, was investigated for gene expression patterns of AQPs and for sap flow velocity. Our results suggest that the foliar water uptake of T. ramosissima occurs in natural fields at night when the humidity is over a threshold of 85 %. The diurnal gene expression pattern of AQPs suggests that most AQP gene expressions display a circadian rhythm, and this could affect both photosynthesis and transpiration. At night, the PIP2-1 gene is also upregulated with increased relative air humidity. This gene expression pattern may allow desert plants to regulate foliar water uptake to adapt to extreme drought. This study suggests a molecular basis of foliar water uptake in desert plants.

Keywords: Aquaporins; PIP2-1; Tamarix ramosissima; drought adaptation; foliar water uptake.

Figure 1.

Diurnal variation in meteorological factors at the field experiment site on 25 June 2013. The diurnal change in (A) PAR (mmol), (B) AirRh (%), (C) SoilTm (°C), (D) wind speed (m s⁻¹), (E) AirTm (°C) and (F) AirP (hPa).

Figure 2.

Diurnal variation in sap velocity at different positions of the sapwood of T. ramosissima. (A) For the main stem with a diameter of ∼30 cm (MS), (B) for the stem with a diameter of ∼20 cm (S20), (C) for the stem with a diameter of ∼10 cm (S10), (D) for the main root with a diameter of ∼30 cm (MR) and (E) for the root with a diameter of ∼20 cm (R20). The null sap flow velocity is marked with a dashed dotted line, which was used as a transition criterion to determine whether foliar water uptake occurred.

Figure 3.

Correlation between sap flow velocity and PAR simulated by two-order line regression for each curve with 95 % CI as generated sigma plot 12.0. The scatter dot plot shows observed data from the field experiment site. The curve represented fit curve with the significant equation. (A) For the main stem with a diameter of ∼30 cm (MS), (B) for the stem with a diameter of ∼20 cm (S20), (C) for the stem with a diameter of ∼10 cm (S10), (D) for the main root with a diameter of ∼30 cm (MR) and (E) for the root with a diameter of ∼20 cm (R20).

Figure 4.

Water content in leaves and small stems from 20:00 to 22:00 on 25 June 2013.

Figure 5.

Diurnal relative conductivity of leaves on 25 June 2013.

Figure 6.

Water gradient potential in leaves and stems in Tamarix from 16:00 to 22:00 on 25 June 2013.

Figure 7.

Phylogenetic relationship of the six AQPs in the T. ramosissima. The phylogenetic tree was constructed with maximum likelihood using the Jones–Taylor–Thornton (JTT) model in MEGA 6.0. At, Arabidopsis thaliana; Th, T. ramosissima. The distance scale is 0.5.

Figure 8.

Diurnal expression of six AQPs in T. ramosissima from 6:00 to 22:00 on 25 June 2013.

Figure 9.

Comparison of the response of PIP1-2 with different air moisture levels under natural conditions for 2 days. The black bars and dotted line show the expression level of PIP2-1 and AirRh, respectively, on 25 June 2013. The grey bars and long dashed line show the expression level of PIP2-1 and AirRh, respectively, for 28 June 2013.

Figure 10.

Impact of air moisture and root pressure on expression level changes in PIP2-1 in natural field and control experiments. The humidity inside the chamber was over 85 %, and it was ∼50 % outside the chamber.