The holoparasitic endophyte Bdallophyton americanum affects root water conductivity of the tree Bursera simaruba

It has been reported that parasitic vascular plants (hemiparasites and holoparasites) may affect host fitness, but the effects produced by root endophyte holoparasitic species on its host have not been documented. Here the effect of the holoparasitic endophyte Bdallophyton americanum (R. Br.) Harms on the root conductivity of Bursera simaruba (L.) Sarg. was studied. Parasitized and non-parasitized root segments were sampled in the rainy and dry seasons in a dry coastal forest in central Veracruz, Mexico. Root diameter, hydraulic (K _h) and specific conductivity (K _s = K _h/root transverse area), percent loss of conductivity and reproductive specific conductivity (K _h /inflorescence or infructescense dry weight) data were obtained. The diameter and number of conductive and non-conductive vessels were recorded in parasitized and non-parasitized root segments in the dry season. Root diameters were not different between root types and seasons, but root specific conductivity was different both between seasons and root types. Specific conductivity on parasitized roots was 61% (wet season) and 85% (dry season) lower than that recorded for non-parasitized roots in the wet season. Root hydraulic conductivity was positively related with the biomass of reproductive structures of B. americanum in the wet season. The parasite appears to alter the xylem morphogenesis of the host, reducing vessel number by 40%, but not plugging or otherwise harming the conductive vessels, and resulted in no change in vessel diameter. Contrary to what has been reported to occur in some plant stems infected with hemiparasitic mistletoes, B. americanum decreases but does not eliminate conductivity to the distal plant parts.     

Drought, abscisic acid and transpiration rate effects on the regulation of PIP aquaporin gene expression and abundance in Phaseolus vulgaris plants

BACKGROUND AND AIMS: Drought causes a decline of root hydraulic conductance, which aside from embolisms, is governed ultimately by aquaporins. Multiple factors probably regulate aquaporin expression, abundance and activity in leaf and root tissues during drought; among these are the leaf transpiration rate, leaf water status, abscisic acid (ABA) and soil water content. Here a study is made of how these factors could influence the response of aquaporin to drought. METHODS: Three plasma membrane intrinsic proteins (PIPs) or aquaporins were cloned from Phaseolus vulgaris plants and their expression was analysed after 4 d of water deprivation and also 1 d after re-watering. The effects of ABA and of methotrexate (MTX), an inhibitor of stomatal opening, on gene expression and protein abundance were also analysed. Protein abundance was examined using antibodies against PIP1 and PIP2 aquaporins. At the same time, root hydraulic conductance (L), transpiration rate, leaf water status and ABA tissue concentration were measured. KEY RESULTS: None of the treatments (drought, ABA or MTX) changed the leaf water status or tissue ABA concentration. The three treatments caused a decline in the transpiration rate and raised PVPIP2;1 gene expression and PIP1 protein abundance in the leaves. In the roots, only the drought treatment raised the expression of the three PIP genes examined, while at the same time diminishing PIP2 protein abundance and L. On the other hand, ABA raised both root PIP1 protein abundance and L. CONCLUSIONS: The rise of PvPIP2;1 gene expression and PIP1 protein abundance in the leaves of P. vulgaris plants subjected to drought was correlated with a decline in the transpiration rate. At the same time, the increase in the expression of the three PIP genes examined caused by drought and the decline of PIP2 protein abundance in the root tissues were not correlated with any of the parameters measured.     

Changes in axial hydraulic conductivity along elongating leaf blades in relation to xylem maturation in tall fescue

Xylem maturation in elongating leaf blades of tall fescue ( Festuca arundinacea ) was studied using staining and microcasting. Three distinctive regions were identified in the blade: (1) a basal region, in which elongation was occurring and protoxylem (PX) vessels were functioning throughout; (2) a maturation region, in which elongation had stopped and narrow (NMX) and large (LMX) metaxylem vessels were beginning to function; (3) a distal, mature region in which most of the longitudinal water movements occurred in the LMX. The axial hydraulic conductivity ( K _h) was measured in leaf sections from all these regions and compared with the theoretical axial hydraulic conductivity ( K _t) computed from the diameter of individual inner vessels. K _t was proportional to K _h throughout the leaf, but K _t was about three times K _h. The changes in K _h and K _t along the leaf reflected the different stages of xylem maturation. In the basal 60 mm region, K _h was about 0.30±0.07 mmol s⁻¹ mm MPa⁻¹. Beyond that region, K _h rapidly increased with metaxylem element maturation to a maximum value of 5.0±0.3 mmol s⁻¹ mm MPa⁻¹, 105 mm from the leaf base. It then decreased to 3.5±0.2 mmol s⁻¹ mm MPa⁻¹ near the leaf tip. The basal expanding region was observed to restrict longitudinal water movement. There was a close relationship between the water deposition rate in the elongation zone and the sum of the perimeters of PX vessels. The implications of this longitudinal vasculature on the partitioning of water between growth and transpiration is discussed.     

Zinc Extraction potential of two common crop plants,Nicotiana tabacum and Zea mays

White spruce [Picea glauca (Moench) Voss] seedlings were inoculated with Hebeloma crustuliniforme and treated with 25 mM NaCl to examine the effects of salinized soil and mycorrhizae on root hydraulic conductance and growth. Mycorrhizal seedlings had significantly greater shoot and root dry weights, number of lateral branches and chlorophyll content than non-mycorrhizal seedlings. Salt treatment reduced seedling growth in both non-mycorrhizal and mycorrhizal seedlings. However, needles of salt-treated mycorrhizal seedlings had several-fold higher needle chlorophyll content than that in non-mycorrhizal seedlings treated with salt. Mycorrhizae increased N and P concentrations in seedlings. Na levels in shoots and roots of salt-treated mycorrhizal seedlings were significantly lower and root hydraulic conductance was several-fold higher than in non-mycorrhizal seedlings. A reduction of about 50% in root hydraulic conductance of mycorrhizal seedlings was observed after removal of the fungal hyphal sheath. Transpiration and root respiration rates were reduced by salt treatments in both groups of seedlings compared with the controls, however, both transpiration and respiration rates of salt-treated mycorrhizal seedlings were as high as those in the non-mycorrhizal seedlings that had not been subjected to salt treatment. The reduction of shoot Na uptake while increasing N and P absorption and maintaining high transpiration rates and root hydraulic conductance may be important resistance mechanisms in ectomycorrhizal plants growing in salinized soil.     

Hydraulics play an important role in causing low growth rate and dieback of aging Pinus sylvestris var. mongolica trees in plantations of Northeast China

The frequently observed forest decline in water‐limited regions may be associated with impaired tree hydraulics, but the precise physiological mechanisms remain poorly understood. We compared hydraulic architecture of Mongolian pine (Pinus sylvestris var. mongolica) trees of different size classes from a plantation and a natural forest site to test whether greater hydraulic limitation with increasing size plays an important role in tree decline observed in the more water‐limited plantation site. We found that trees from plantations overall showed significantly lower stem hydraulic efficiency. More importantly, plantation‐grown trees showed significant declines in stem hydraulic conductivity and hydraulic safety margins as well as syndromes of stronger drought stress with increasing size, whereas no such trends were observed at the natural forest site. Most notably, the leaf to sapwood area ratio (LA/SA) showed a strong linear decline with increasing tree size at the plantation site. Although compensatory adjustments in LA/SA may mitigate the effect of increased water stress in larger trees, they may result in greater risk of carbon imbalance, eventually limiting tree growth at the plantation site. Our results provide a potential mechanistic explanation for the widespread decline of Mongolian pine trees in plantations of Northern China.     

Leaf hydraulic traits and their trade-offs for nine Chinese temperate tree species with different wood properties

Aims Trees with different wood properties display variations in xylem anatomy and leaf vein structure, which may influence tree water transport efficiency and water-use strategy, and consequently constrain tree survival, growth and distribution. However, the effects of wood properties on leaf hydraulic conductance and vulnerability and their potential trade-offs at leaf level are not well understood. Our aims were to examine variations in leaf hydraulic traits of trees with different wood properties and explore potential trade-offs between leaf hydraulic efficiency and safety.
Methods Nine tree species with different wood properties were selected for measuring the leaf hydraulic traits, including three diffuse-porous species (Populus davidiana, Tilia amurensis, Betula platyphylla), three ring-porous species (Quercus mongolica, Fraxinus mandshurica, Juglans mandshurica), and three non-porous species (Picea koraiensis, Pinus sylvestris var. mongolica, Pinus koraiensis). Four dominant and healthy trees per species were randomly selected. The hydraulic traits measured included leaf hydraulic conductance on leaf area (K_area) and dry mass (K_mass) basis, leaf hydraulic vulnerability (P₅₀), and leaf water potential at turgor loss point (TLP), while the leaf structural traits were leaf dry mass content (LDMC), leaf density (LD) and leaf mass per unit area (LMA).
Important findings The K_area, K_mass, and P₅₀ differed significantly among the tree species with different woody properties (p < 0.05). Both K_area and K_mass were the lowest for the non-porous trees, and did not differ significantly between the diffuse-porous and ring-porous trees. The ring-porous trees had the highest P₅₀ values, while the diffuse-porous and non-porous trees showed no significant differences in P₅₀. Both K_area and K_mass were negatively correlated with P₅₀ (p < 0.05) for all the trees, and the relationships for the diffuse-porous, ring-porous, and non-porous trees were fitted into linear, power, exponential functions, respectively. This indicates that significant trade-offs exist between leaf hydraulic efficiency and safety. The K_mass was correlated (p < 0.01) with TLP in a negative linear function for the diffuse- and ring-porous trees and in a negative exponential function for the non-porous trees. The P₅₀ increased with increasing TLP. These results suggest that apoplastic and symplastic drought resistance are strictly coordinated in order to protect living cells from approaching their critical water status under water stresses. The K_mass was negatively correlated (p < 0.01) with LDMC, LD, or LMA, while the P₅₀ was positively correlated with LDMC and LD; this suggests that variations in K_mass and P₅₀ are driven by similar changes in structural traits regardless of wood traits. We conclude that the tree tolerance to hydraulic dysfunction increases with increasing carbon investment in the leaf hydraulic system.     

Hydraulic architecture of leaf venation in Laurus nobilis L.

Veins are the main irrigation system of the leaf lamina and an understanding of the hydraulic architecture of the vein networks is essential for understanding leaf function. However, determination of leaf hydraulic parameters is challenging, because for most leaves the vein system is reticulate, contains a hierarchy of different vein sizes, and consists of leaky conduits. We present a new approach that allows for measurements of pressure differences between the petiole and any vein within the leaf. Measurements of Laurus nobilis leaves indicate that first‐ and second‐order veins have high axial conductance and relatively small radial permeability, thus allowing water to reach distal areas of the leaf with only a small loss of water potential. Higher order veins tend to be more hydraulically resistant and permit greater radial leakage. This design allows for a relatively equitable distribution of water potential and thus reflects the capacity of the venation to provide a relatively homogeneous water supply across the leaf lamina, with only the leaf margins being hydraulically disadvantaged relative to the rest of the leaf.     

Stomatal conductance and photosynthesis vary linearly with plant hydraulic conductance in ponderosa pine

Recent work has shown that stomatal conductance (g_s) and assimilation (A) are responsive to changes in the hydraulic conductance of the soil to leaf pathway (K_L), but no study has quantitatively described this relationship under controlled conditions where steady‐state flow is promoted. Under steady‐state conditions, the relationship between g_s, water potential (Ψ) and K_L can be assumed to follow the Ohm's law analogy for fluid flow. When boundary layer conductance is large relative to g_s, the Ohm's law analogy leads to g_s = K_L (Ψ_soil?Ψ_leaf)/D, where D is the vapour pressure deficit. Consequently, if stomata regulate Ψ_leaf and limit A, a reduction in K_L will cause g_s and A to decline. We evaluated the regulation of Ψ_leaf and A in response to changes in K_L in well‐watered ponderosa pine seedlings (Pinus ponderosa). To vary K_L, we systematically reduced stem hydraulic conductivity (k) using an air injection technique to induce cavitation while simultaneously measuring Ψ_leaf and canopy gas exchange in the laboratory under constant light and D. Short‐statured seedlings (< 1 m tall) and hour‐long equilibration times promoted steady‐state flow conditions. We found that Ψ_leaf remained constant near ? 1·5 MPa except at the extreme 99% reduction of k when Ψ_leaf fell to ? 2·1 MPa. Transpiration, g_s, A and K_L all declined with decreasing k (P < 0·001). As a result of the near homeostasis in bulk Ψ_leaf, g_s and A were directly proportional to K_L (R² > 0·90), indicating that changes in K_L may affect plant carbon gain.     

陕北水蚀风蚀交错带柠条适应不同生境的形态和生理可塑性

以陕北水蚀风蚀交错带4个不同坡位与土壤质地生境[沟底+坝淤绵沙土(A)、梁坡+红黄土(B)、坡顶+绵沙土(C)、坡顶+风沙土盖绵沙土(D)]中生长的柠条为研究对象,研究了其生长、光合和水力性状的可塑性变化,以揭示柠条对不同生境的适应机制。结果表明:(1)生境D和生境A样地1~3m和3m土层的平均含水量明显高于生境B和C;(2)与生境B和C相比,生境A和D中生长的柠条冠幅、株高和新枝长显著增加,叶厚度减小,正午叶水势、净光合速率、气孔导度和蒸腾速率亦显著增加,但不同生境中柠条枝比导水率、Huber值和栓塞程度无显著差异;(3)柠条各测定指标中,新枝长、净光合速率、气孔导度和蒸腾速率表现出较大的可塑性。研究认为,柠条可能主要通过新枝长或光合生理特征的改变来适应不同的水分生境。     

Are functional traits a good predictor of global change impacts on tree species abundance dynamics in a subtropical forest?

Significant changes in the composition of tree species have been observed in various forests worldwide. We hypothesised that these changes might result from variable sensitivities of species to global change, and species sensitivities might be quantified, using functional traits. Employing long‐term (1978–2010) species abundance data of 48 tree species from a permanent subtropical forest plot, where multiple global change factors have been observed, including soil drying, we examined the relationships between temporal trends in abundance and suits of functional traits. We found that species with high photosynthesis rates, leaf phosphorus and nitrogen concentrations, specific leaf area, hydraulic conductivity, turgor loss point and predawn leaf water potential had increased in abundance, while species with opposite trait patterns had decreased. Our results demonstrate that functional traits underlie tree species abundance dynamics in response to drought stress, thus linking traits to compositional shifts in this subtropical forest under global changes.