Stomatal conductance tracks soil-to-leaf hydraulic conductance in faba bean and maize during soil drying

Abstract

Although regulation of stomatal conductance is widely assumed to be the most important plant response to soil drying, the picture is incomplete when hydraulic conductance from soil to the leaf, upstream of the stomata, is not considered. Here, we investigated to what extent soil drying reduces the conductance between soil and leaf, whether this reduction differs between species, how it affects stomatal regulation, and where in the hydraulic pathway it occurs. To this end, we noninvasively and continuously measured the total root water uptake rate, soil water potential, leaf water potential, and stomatal conductance of 4-week-old, pot-grown maize (Zea mays) and faba bean (Vicia faba) plants during 4 days of water restriction. In both species, the soil-plant conductance, excluding stomatal conductance, declined exponentially with soil drying and was reduced to 50% above a soil water potential of -0.1 MPa, which is far from the permanent wilting point. This loss of conductance has immediate consequences for leaf water potential and the associated stomatal regulation. Both stomatal conductance and soil-plant conductance declined at a higher rate in faba bean than in maize. Estimations of the water potential at the root surface and an incomplete recovery 22 h after rewatering indicate that the loss of conductance, at least partly, occurred inside the plants, for example, through root suberization or altered aquaporin gene expression. Our findings suggest that differences in the stomatal sensitivity among plant species are partly explained by the sensitivity of root hydraulic conductance to soil drying.

Figure 1

Experimental setup and hydraulic model used in this study. A, Scheme of the experimental setup and the different water potentials used for the data analysis. The equivalent water potential in the bulk soil (Ψ_seq), at the root surface (Ψ_{seq, sr}), and the total root water uptake rate (U_tot) were derived from the SWaP measurements. The leaf water potential (Ψ_leaf) was measured with psychrometers, the stomatal conductance with a LICOR 6400. Root length was determined with MRI. B, Scheme of the hydraulic network from the bulk soil to the leaf. The overall conductance from bulk soil to leaf (K_SL) can be separated into the conductance from bulk soil to the root surface (K_SR) and from root surface to the leaf (K_RL).

Figure 2

Reduction of the equivalent soil water potential (Ψ_seq), total root water uptake rate (U_tot), and leaf water potential (Ψ_leaf) during 4 days of soil drying. A, C, and E, Data of all replicates (faba bean n = 12, maize n = 10) as boxplots at selected time points. Different colors in (C) and (E) refer to the two different light levels. Horizontal lines are medians, boxes reach from the first to the third quartile. Whiskers mark the minimal (lower whisker) and maximal (upper whisker) data points within 1.5 times the interquartile range from the first and third quartile, respectively. Circles are outliers beyond the whiskers. We tested for significant differences in Ψ_seqU_tot, and Ψ_leaf between faba bean and maize at each selected time point. P-values were derived with a Mann–Whitney U test. P-values ˂0.05 are indicated by *, ˂0.01 by **, and ˂0.005 by ***. B, D, and F, The continuous time courses of the three parameters during the 4 days of soil drying for one example faba bean plant. Fluctuations of the observed parameters are caused by the fluctuating light intensity. Red dashed lines mark those points used for the boxplots. Shaded areas indicate the nonilluminated periods.

Figure 3

K_SL and g_s drop exponentially during soil drying at higher rates in faba bean than in maize. A, Example data of K_SL (black) and g_s (orange) at reducing Ψ_seq for one faba bean plant. Per light period we considered four measured data points of K_SL (black dots) and one measured data points of g_s (orange dots). Dashed lines follow an exponential fit of the form $K_{\mathrm{SL}}\left({\mathrm{\Psi }}_{\mathrm{seq}}\right)=A·e^{\mathrm{\lambda }({\mathrm{\Psi }}_{\mathrm{seq}}+0.025)}$ or $g_{\mathrm{s}}\left({\mathrm{\Psi }}_{\mathrm{seq}}\right)=A·e^{\mathrm{\lambda }({\mathrm{\Psi }}_{\mathrm{seq}}+0.025)}$ starting at a Ψ_seq of −0.025 MPa. Vertical dotted lines mark the values of Ψ_seq at which K_SL or g_s were reduced to 50% of their initial values at a Ψ_seq of −0.025 MPa. B, Variation of the rate constants λ obtained from the exponential fits of K_SL and g_s among faba bean and maize replicates. λ is a measure for the sensitivity of the conductance to soil drying. Characteristics of the boxplots are similar to Figure 2, A, C, and E. Numbers on the boxes indicate the median values. The inserted figure includes all outliers which are only partly shown in the main panel. g_s was only measured for six replicates while K_SL was measured for 12 (faba bean) and 10 (maize) replicates. Asterisks indicate significant differences (*P-value < 0.05, **P-value < 0.01, ***P-value < 0.005) between faba bean and maize, tested with a Mann–Whitney U test.

Figure 4

Estimated water potential at the root surface (Ψ_{seq, sr}) as a function of the bulk soil water potential (Ψ_seq). Data are an example shown for one faba bean (A) and one maize plant (B). Water potential at the root surface was calculated for the full measured root length and half of the measured root length. Dotted line is the 1:1 line.

Figure 5

Comparison between the hydraulic conductance between root surface and leaf (K_RL) and K_SL throughout the soil drying period. A, K_RL compared with K_SL, as a function of Ψ_seq, example shown for one faba bean plant (same plant as in Figure 3A). K_RL was determined using the calculated water potential at the root surface for the full root length (cyan) or half of the root length (blue). We exponentially fitted the data (dashed lines) to determine the rate constants λ starting at a Ψ_seq of −0.025 MPa. For reasons of clarity, we plotted only one data point per light period here but derived the exponential fit using four data points per light period, analogously to Figure 3. B, Boxplots of the rate constants λ obtained from the exponential fits of K_SL (black, same data as in Figure 3B) and K_RL considering the full root length (cyan) or half of the full root length (blue) for faba bean (n = 12) and maize (n = 10). Characteristics of the boxplots are similar to Figure 2, A, C, and E. The main panel does not include all outliers which are shown in the inserted figure.

Figure 6

Recovery of different hydraulic parameters upon rewatering after 4 days of water restriction for one faba bean plant. Parameters are Ψ_seq (A), Ψ_leaf (B), U_tot (C), and K_SL (D). Vertical dashed lines mark the time point of rewatering. Horizontal dashed line in (D) marks the initial value of K_SL at a Ψ_seq of −0.025 MPa (K_SL,0). Nights are indicated by the shaded areas.