Differential coordination of stomatal conductance,mesophyll conductance,and leaf hydraulic conductance in response to changing light across species

Stomatal conductance (g_s) and mesophyll conductance (g_m) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf) across species, under both steady‐state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf, gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf, and anatomical traits varied widely across species. Under light‐saturated conditions, the A, g_s, g_m, and K_leaf were strongly correlated across species. However, the response patterns of A, g_s, g_m, and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark‐adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light‐adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.     

Diurnal depression of leaf hydraulic conductance in a tropical tree species

Diurnal patterns of hydraulic conductance of the leaf lamina (K_leaf) were monitored in a field‐grown tropical tree species in an attempt to ascertain whether the dynamics of stomatal conductance (g_s) and CO₂ uptake (A_leaf) were associated with short‐term changes in K_leaf. On days of high evaporative demand mid‐day depression of K_leaf to between 40 and 50% of pre‐dawn values was followed by a rapid recovery after 1500 h. Leaf water potential during the recovery stage was less than ?1 MPa implying a refilling mechanism, or that loss of K_leaf was not linked to cavitation. Laboratory measurement of the response of K_leaf to Ψ_leaf confirmed that leaves in the field were operating at water potentials within the depressed region of the leaf ‘vulnerability curve’. Diurnal courses of K_leaf and Ψ_leaf predicted from measured transpiration, xylem water potential and the K_leaf vulnerability function, yielded good agreement with observed trends in both leaf parameters. Close correlation between depression of K_leaf, g_s and A_leaf suggests that xylem dysfunction in the leaf may lead to mid‐day depression of gas exchange in this species.     

Water movement through Quercus rubra I. Leaf water potential and conductance during polycyclic growth

Two experiments examined simultaneous changes in leaf area (A_L), root length (L_r), stomatal conductance (g_s), leaf water potential (Ψ_L), transpiration and hydraulic plant conductance per unit leaf area (G) during the first three shoot cycles of northern red oak (Quercus rubra L.) grown under favourable and controlled conditions. Each shoot cycle consisted of bud swell, stem elongation, leaf expansion and rest; roots grew almost continuously. The g_s of all leaves decreased substantially while leaves of the newest flush were expanding and increased modestly when seedling leaf area remained constant. Overall, g_s decreased. The Ψ_L of mature leaves decreased during leaf expansion and increased by an equivalent amount during intervening periods. Possible explanations for the paired changes in g_s and Ψ_L are considered. Changes in G closely paralleled those of canopy g_s. These parallel changes during polycyclic seedling growth should act to keep seedling Ψ_L relatively constant as plant size increases and thereby help prevent Ψ_L from dropping to levels that would cause runaway embolism.     

Temperature responses of photosynthesis and leaf hydraulic conductance in rice and wheat

Studies on the temperature (T) responses of photosynthesis and leaf hydraulic conductance (K_leaf) are important to plant gas exchange. In this study, the temperature responses of photosynthesis and K_leaf were studied in Shanyou 63 (Oryza sativa) and Yannong 19 (Triticum aestivum). Leaf water potential (Ψ_leaf) was insensitive to T in Shanyou 63, while it significantly decreased with T in Yannong 19. The differential Ψ_leaf − T relationship partially accounted for the differing g_m–T relationships, where g_m was less sensitive to T in Yannong 19 than in Shanyou 63. With different g_m–T and Ψ_leaf–T relationships, the temperature responses of photosynthetic limitations were surprisingly similar between the two lines, and the photosynthetic rate was highly correlated with g_m. With the increasing T, K_leaf increased in Shanyou 63 while it decreased in Yannong 19. The different K_leaf–T relationships were related to different Ψ_leaf–T relationships. When excluding the effects of water viscosity and Ψ_leaf, K_leaf was insensitive to T in both lines. g_m and K_leaf were generally not coordinated across different temperatures. This study highlights the importance of Ψ_leaf on leaf carbon and water exchanges, and the mechanisms for the g_m–T and K_leaf–T relationships were discussed.     

The response of stomatal conductance to seasonal drought in tropical forests

Stomata regulate CO₂ uptake for photosynthesis and water loss through transpiration. The approaches used to represent stomatal conductance (g_s) in models vary. In particular, current understanding of drivers of the variation in a key parameter in those models, the slope parameter (i.e. a measure of intrinsic plant water‐use‐efficiency), is still limited, particularly in the tropics. Here we collected diurnal measurements of leaf gas exchange and leaf water potential (Ψ_leaf), and a suite of plant traits from the upper canopy of 15 tropical trees in two contrasting Panamanian forests throughout the dry season of the 2016 El Niño. The plant traits included wood density, leaf‐mass‐per‐area (LMA), leaf carboxylation capacity (V_c,max), leaf water content, the degree of isohydry, and predawn Ψ_leaf. We first investigated how the choice of four commonly used leaf‐level g_s models with and without the inclusion of Ψ_leaf as an additional predictor variable influence the ability to predict g_s, and then explored the abiotic (i.e. month, site‐month interaction) and biotic (i.e. tree‐species‐specific characteristics) drivers of slope parameter variation. Our results show that the inclusion of Ψ_leaf did not improve model performance and that the models that represent the response of g_s to vapor pressure deficit performed better than corresponding models that respond to relative humidity. Within each g_s model, we found large variation in the slope parameter, and this variation was attributable to the biotic driver, rather than abiotic drivers. We further investigated potential relationships between the slope parameter and the six available plant traits mentioned above, and found that only one trait, LMA, had a significant correlation with the slope parameter (R² = 0.66, n = 15), highlighting a potential path towards improved model parameterization. This study advances understanding of g_s dynamics over seasonal drought, and identifies a practical, trait‐based approach to improve modeling of carbon and water exchange in tropical forests.     

Acclimation of leaf hydraulic conductance and stomatal conductance of Pinus taeda (loblolly pine) to long-term growth in elevated CO2 (free-air CO2 enrichment) and N-fertilization

We investigated how leaf hydraulic conductance (K_leaf) of loblolly pine trees is influenced by soil nitrogen amendment (N) in stands subjected to ambient or elevated CO₂ concentrations (CO₂^a and CO₂^e, respectively). We also examined how K_leaf varies with changes in reference leaf water potential (Ψ_leaf‐ref) and stomatal conductance (g_s‐ref) calculated at vapour pressure deficit, D of 1 kPa. We detected significant reductions in K_leaf caused by N and CO₂^e, but neither treatment affected pre‐dawn or midday Ψ_leaf. We also detected a significant CO₂^e‐induced reduction in g_s‐ref and Ψ_leaf‐ref. Among treatments, the sensitivity of K_leaf to Ψ_leaf was directly related to a reference K_leaf (K_leaf‐ref computed at Ψ_leaf‐ref). This liquid‐phase response was reflected in a similar gas‐phase response, with g_s sensitivity to D proportional to g_s‐ref. Because leaves represented a substantial component of the whole‐tree conductance, reduction in K_leaf under CO₂^e affected whole‐tree water use by inducing a decline in g_s‐ref. The consequences of the acclimation of leaves to the treatments were: (1) trees growing under CO₂^e controlled morning leaf water status less than CO₂^a trees resulting in a higher diurnal loss of K_leaf; (2) the effect of CO₂^e on g_s‐ref was manifested only during times of high soil moisture.     

Aquaporin regulation in roots controls plant hydraulic conductance,stomatal conductance,and leaf water potential in Pinus radiata under water stress

Stomatal regulation is crucial for forest species performance and survival on drought‐prone sites. We investigated the regulation of root and shoot hydraulics in three Pinus radiata clones exposed to drought stress and its coordination with stomatal conductance (g_s) and leaf water potential (Ψ_leaf). All clones experienced a substantial decrease in root‐specific root hydraulic conductance (K_root‐r) in response to the water stress, but leaf‐specific shoot hydraulic conductance (K_shoot‐l) did not change in any of the clones. The reduction in K_root‐r caused a decrease in leaf‐specific whole‐plant hydraulic conductance (K_plant‐l). Among clones, the larger the decrease in K_plant‐l, the more stomata closed in response to drought. Rewatering resulted in a quick recovery of K_root‐r and g_s. Our results demonstrated that the reduction in K_plant‐l, attributed to a down regulation of aquaporin activity in roots, was linked to the isohydric stomatal behaviour, resulting in a nearly constant Ψ_leaf as water stress started. We concluded that higher K_plant‐l is associated with water stress resistance by sustaining a less negative Ψ_leaf and delaying stomatal closure.     

Stomatal responses to changes in vapor pressure deficit reflect tissue‐specific differences in hydraulic conductance

The vapor pressure deficit (D) of the atmosphere can negatively affect plant growth as plants reduce stomatal conductance to water vapor (g_wv) in response to increasing D, limiting the ability of plants to assimilate carbon. The sensitivity of g_wv to changes in D varies among species and has been correlated with the hydraulic conductance of leaves (K_leaf), but the hydraulic conductance of other tissues has also been implicated in plant responses to changing D. Among the 19 grass species, we found that K_leaf was correlated with the hydraulic conductance of large longitudinal veins (K_lv, r² = 0.81), but was not related to K_root (r² = 0.01). Stomatal sensitivity to D was correlated with K_leaf relative to total leaf area (r² = 0.50), and did not differ between C₃ and C₄ species. Transpiration (E) increased in response to D, but 8 of the 19 plants showed a decline in E at high D, indicative of an ‘apparent feedforward’ response. For these individuals, E began to decline at lower values of D in plants with low K_root (r² = 0.72). These results show the significance of both leaf and root hydraulic conductance as drivers of plant responses to evaporative demand.     

Limitation of transpiration by hydraulic conductance and xylem cavitation in Betula occidentalis

The extent to which stomatal conductance (g_s) was capable of responding to reduced hydraulic conductance (k)and preventing cavitation-inducing xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air-injection technique. From 1 to 18 d after shoot injection we measured midday transpiration rate (E), g_s, and xylem pressure (Ψ_p-xylem) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2–3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and g_s. The k measurement was expressed per unit leaf area (k_l, leaf-specific conductance). Leaves measured within 2 d of shoot injection showed reduced g_s and E relative to non-injected controls, and both parameters were strongly correlated with k_l At this time, there was no difference in leaf Ψ_p-xylem between injected shoots and controls, and leaf Ψ_p-xylem was not significantly different from the highest cavitation-inducing pressure (Ψ_p-cav) in the branch xylem (-1.43 ± 0.029 MPa, n=8). Leaves measured 7–18 d after shoots were injected exhibited a partial return of g_s and E values to the control range. This was associated with a decrease in leaf Ψ_p-xylem below Ψ_p-cav and loss of foliage. The results suggest the stomata were incapable of long-term regulation of E below control values and that reversion to higher E caused dieback via cavitation.     

Beyond soil water potential: An expanded view on isohydricity including land–atmosphere interactions and phenology

Over the past decade, the concept of isohydry or anisohydry, which describes the link between soil water potential (Ψ_S), leaf water potential (Ψ_L), and stomatal conductance (g_s), has soared in popularity. However, its utility has recently been questioned, and a surprising lack of coordination between the dynamics of Ψ_L and g_s across biomes has been reported. Here, we offer a more expanded view of the isohydricity concept that considers effects of vapour pressure deficit (VPD) and leaf area index (A_L) on the apparent sensitivities of Ψ_L and g_s to drought. After validating the model with tree‐ and ecosystem‐scale data, we find that within a site, isohydricity is a strong predictor of limitations to stomatal function, though variation in VPD and leaf area, among other factors, can challenge its diagnosis. Across sites, the theory predicts that the degree of isohydricity is a good predictor of the sensitivity of g_s to declining soil water in the absence of confounding effects from other drivers. However, if VPD effects are significant, they alone are sufficient to decouple the dynamics of Ψ_L and g_s entirely. We conclude with a set of practical recommendations for future applications of the isohydricity framework within and across sites.