Declining soil-root hydraulic conductance drives stomatal closure of tomato under drought

<p>The fundamental question as to what triggers stomatal closure during soil drying remains contentious. Thus, we urgently need to improve our understanding of stomatal response to water deficits in soil and atmosphere.<strong> </strong>Here, we investigated the role of soil-plant hydraulic conductance (K<sub>sp</sub>) on transpiration (E) and stomata regulation. We used a root pressure chamber to measure the relation between E, leaf xylem water potential (ψ<sub>leaf-x</sub>) and soil water potential (ψ<sub>soil</sub>) in tomato. Additional measurements of ψ<sub>leaf-x</sub> were performed with unpressurized plants. A soil-plant hydraulic model was used to simulate E(ψ<sub>leaf-x</sub>) for decreasing ψ<sub>soil</sub>. In wet soils, E(ψ<sub>leaf-x</sub>) had a constant slope while in dry soils the slope decreased, with ψ<sub>leaf-x</sub> rapidly and nonlinearly decreasing for moderate increases in E. The ψ<sub>leaf-x</sub> measured in pressurized and unpressurized plants matched well, which indicates that the shoot hydraulic conductance did not decrease during soil drying and that the decrease in K<sub>sp</sub> is caused by a decrease in soil-root conductance. The decrease of E matched well the onset of hydraulic nonlinearity. Our findings demonstrate that stomatal closure prevents the drop in ψ<sub>leaf-x</sub> caused by a decrease in K<sub>sp</sub> and elucidate a strong correlation between stomatal regulation and belowground hydraulic limitation.</p>

Plant hydraulic conductance adapts to shoot number but limits shoot vigour in grapevines

The rate of shoot growth (vigour) in grapevines tends to decrease as the number of shoots per plant increases. Because the underlying causes of this relationship remain unclear, they were studied by variable pruning of field-grown, deficit-irrigated Merlot grapevines (Vitis vinifera L.). Shoot number ranged from 11 to 124 per vine and was inversely correlated with shoot growth rate, leaf appearance rate, axillary bud outgrowth, internode length, leaf size, shoot leaf area, carbon partitioned to the fruit (Cfruit) per shoot, average daily maximum photosynthesis (Amax), stomatal conductance (gmax), and leaf-specific hydraulic conductance (Kl). Shoot number was positively correlated with canopy leaf area, whole-vine Cfruit, whole-plant hydraulic conductance (Kv), and canopy conductance (Kc). Higher shoot vigour was associated with higher Amax, gmax, predawn leaf water potential (Ψpd), shoot hydraulic conductance (Ks), Kl, and Kv. Vigorous shoots supported both more vegetative growth and more reproductive growth; thus fruit growth did not compete with shoot growth for photosynthates. These results indicate that the hydraulic capacity of grapevines adapts to varying shoot numbers to support leaf physiology, growth, and carbon partitioning, but adaptation may be limited, putting upper bounds on the growth of individual shoots and fruit.

Making the best of the worst of times: traits underlying combined shade and drought tolerance of Ruscus aculeatus and Ruscus microglossum (Asparagaceae)

The genus Ruscus (Asparagaceae) consists of evergreen, woody monocot shrubs with modified photosynthetic stems (phylloclades) that occur in dry, shaded woodland areas of the Mediterranean Basin and southern Europe. The combined drought and shade tolerance of Ruscus species challenges the ‘trade-off model’, which suggests that plants can be either drought or shade adapted, but not both. To clarify the potential mechanisms that enable Ruscus species to survive in shaded environments prone to pronounced soil drought, we studied form–function relations based on a detailed trait survey for Ruscus aculeatus L. and Ruscus microglossum Bertol., focusing on gas exchange, hydraulics, morphology, anatomy, and nutrient and isotope composition. We then compared these trait values with published data for other species. R. aculeatus and R. microglossum exhibited numerous traits conferring drought and shade tolerance via reduced demand for resources in general and an ability to survive on stored water. Specific traits include thick phylloclades with low rates of maximum photosynthetic CO2 assimilation, low stomatal conductance to water vapour (gs), low respiration rate, low light compensation point, low shoot hydraulic conductance, low cuticular conductance, and substantial water storage tissue. Ruscus carbon isotope composition values of –33 ‰ were typical of an understory plant, but given the low gs could be associated with internal CO2 recycling. Ruscus appears to be a model for extreme dual adaptation, both physiologically and morphologically, enabling its occupation of shaded sites within drought prone regions across a wide geographical range, including extremely low resource understory sites.

Light sensitivity of shoot hydraulic conductance in five temperate deciduous tree species

The light sensitivity of the shoot hydraulic conductance in five temperate deciduous tree species was measured using two methods to clarify the role of light sensitivity and the suitability of the methods used to study it. The light sensitivity measured using a method that included an interruption of ≤10 min in shoot light acclimation did not differ from that measured using a method with continuous illumination. The ‘noncontinuous light’ methods are suitable for measuring hydraulic conductance and its light response. Light sensitivity correlated with other leaf water traits as follows: positively with the ion-mediated increase in xylem hydraulic conductance; a relative decrease in the hydraulic conductance of the laminae in response to HgCl2; a relative change in stomatal conductance in response to changes in PAR intensity or atmospheric CO2 concentration, or to a decrease in air humidity or leaf water potential; and with instantaneous water use efficiency. The traits correlated negatively with shoot hydraulic conductance, stomatal conductance and relative increases in stomatal conductance in response to increases in leaf water potential. We suggest that high light sensitivity should be considered as one of the characteristics of conservative water use in trees. Low blue light increased shoot hydraulic conductance to a similar extent to moderate white light and twice as much as moderate red light. Blue light perception is important in the light sensitivity mechanism.

Leaf anatomical characteristics associated with shoot hydraulic conductance, stomatal conductance and stomatal sensitivity to changes of leaf water status in temperate deciduous trees

Some anatomical characteristics in leaves relating to hydraulic conductance and stomatal conductance were examined in six temperate deciduous tree species. The fourth power of the radius of the conducting elements in xylem (r4) and the area of mesophyll and epidermal cells per unit length of leaf cross-section (u) were high in leaves with high hydraulic conductance (L). Stomatal conductance (gs) and stomatal sensitivity to an increase in leaf water potential (si) correlated positively with the length of stomatal pore (l), but negatively with the guard cell width (z) and the length of the dorsal side of the guard cells (ld). Stomatal sensitivity to a decrease in leaf water potential (sd) correlated negatively with l and positively with z and ld. The anatomical characteristics associated with hydraulic conductance (r4 and u) and those associated with stomatal conductance and sensitivity to changes of leaf water potential (l, z and ld) were correlated. We conclude that hydraulic conductance may depend on anatomical characteristics of xylem, mesophyll and epidermis, and stomatal conductance and its sensitivity to changing water potential may depend on anatomical characteristics of stomata. The correlation of shoot hydraulic conductance with stomatal conductance and its sensitivity may be based largely on the correlation between the anatomical characteristics of the water conducting system and stomata in these trees.