Steeve Pepin

Variable mesophyll conductance revisited: theoretical background and experimental implications

The CO(2) concentration at the site of carboxylation inside the chloroplast stroma depends not only on the stomatal conductance, but also on the conductance of CO(2) between substomatal cavities and the site of CO(2) fixation. This conductance, commonly termed mesophyll conductance (g(m) ), significantly constrains the rate of photosynthesis. Here we show that estimates of g(m) are influenced by the amount of respiratory and photorespiratory CO(2) from the mitochondria diffusing towards the chloroplasts. This results in an apparent CO(2) and oxygen sensitivity of g(m) that does not imply a change in intrinsic diffusion properties of the mesophyll, but depends on the ratio of mitochondrial CO(2) release to chloroplast CO(2) uptake. We show that this effect (1) can bias the estimation of the CO(2) photocompensation point and non-photorespiratory respiration in the light; (2) can affect the estimates of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) kinetic constants in vivo; and (3) results in an apparent obligatory correlation between stomatal conductance and g(m) . We further show that the amount of photo(respiratory) CO(2) that is refixed by Rubisco can be directly estimated through measurements of g(m) .

Threshold response of mesophyll CO2 conductance to leaf hydraulics in highly transpiring hybrid poplar clones exposed to soil drying

Mesophyll conductance (g m) has been shown to impose significant limitations to net CO2 assimilation (A) in various species during water stress. Net CO2 assimilation is also limited by stomatal conductance to water (g sw), both having been shown to co-vary with leaf hydraulic conductance (K leaf). Lately, several studies have suggested a close functional link between K leaf, g sw, and g m. However, such relationships could only be circumstantial since a recent study has shown that the response of g m to drought could merely be an artefactual consequence of a reduced intercellular CO2 mole fraction (C i). Experiments were conducted on 8-week-old hybrid poplar cuttings to determine the relationship between K leaf, g sw, and g m in clones of contrasting drought tolerance. It was hypothesized that changes in g sw and K leaf in response to drought would not impact on g m over most of its range. The results show that K leaf decreased in concert with g sw as drought proceeded, whereas g m measured at a normalized C i remained relatively constant up to a g sw threshold of ~0.15mol m–2 s–1. This delayed g m response prevented a substantial decline in A at the early stage of the drought, thereby enhancing water use efficiency. Reducing the stomatal limitation of droughted plants by diminishing the ambient CO2 concentration of the air did not modify g m or K leaf. The relationship between gas exchange and leaf hydraulics was similar in both drought-tolerant and drought-sensitive clones despite their contrasting vulnerability to stem cavitation and stomatal response to soil drying. The results support the hypothesis of a partial hydraulic isolation of the mesophyll from the main transpiration pathway.

Greater efficiency of water use in poplar clones having a delayed response of mesophyll conductance to drought

Improvement of water use efficiency is a key objective to improve the sustainability of cultivated plants, especially fast growing species with high water consumption like poplar. It is well known that water use efficiency (WUE) varies considerably among poplar genotypes, and it was recently suggested that the use of the mesophyll-to-stomatal conductance ratio (gm/gs) would be an appropriate trait to improve WUE. The responses of 7-week-old cuttings of four hybrid poplar clones and one native Balsam poplar (Populus balsamifera L.) to a water stress-recovery cycle were examined to evaluate the relation between the gm/gs ratio and transpiration efficiency (TE), a leaf-level component of WUE. A contrasting gs response to water stress was observed among the five clones, from stomatal closure early on during soil drying up to limited closure in Balsam poplar. However in the hybrids, the decline in gm was consistently delayed by a few days compared with gs. Moreover, in the most water use-efficient hybrids, the recovery following rehydration occurred faster for gm than for gs. Thus, the delay in the response of gm to drought and its faster recovery upon rewatering increased the gm/gs of the hybrids and this ratio scaled positively with TE. Our results support the use of the gm/gs ratio to select genotypes with improved WUE, and the notion that breeding strategies focusing mainly on stomatal responses to soil drying should also look for a strong curvilinearity between net carbon assimilation rate and gs, the indication of a significant increase in gm/gs in the earlier stages of stomatal closure.

Determination of irrigation set points for cranberries from soil- and plant-based measurements

Abstract: Cranberry production requires accurate irrigation management to optimize crop yield and reduce water use. However, irrigation guidelines for that crop are scarce and empirical. The objective of this study was to identify appropriate soil matric potential (ψ) irrigation set points for cranberry production. A three-step process was used to evaluate the set points. Crop water requirements were first evaluated in the field and, second, combined to soil physical properties with a hydrological model to estimate irrigation set points. Third, experimental measurements were carried out in a growth cabinet and in the field to validate the set point estimates from independent observations. Irrigation set point estimates obtained from yield response curves, photosynthesis and transpiration measurements, and soil physical properties were all consistent and suggest that soil matric potential be maintained between -4.0 and -7.0 kPa to ensure an adequate water supply to the crop and optimal fruit yield. Yield responses suggest that cranberries are highly sensitive to small changes in soil matric potential, showing differences of about 20 000 kg ha-1 when outside of the -4.0 to -7.0 range, with a maximum yield between 35 000 and 40 000 kg ha-1, depending on the site.

Vulnerability of low temperature induced needle retention in balsam fir (Abies balsamea L.) to vapor pressure deficits

Low temperature (LT) exposure has been shown to delay postharvest needle abscission and senescence (NAS) in balsam fir and changes in vapor pressure deficit (VPD) are expected to alter these processes. Two and half year-old seedlings were exposed to a LT of 5°C for 15 days while the control (CT) group was maintained at 22°C. Seedlings were then exposed to four different VPD (0.22, 0.87, 1.3 and 1.86 kPa) and observed for relative water content (RWC), xylem pressure potential (XPP), membrane injury index (MII), stem capacitance (SC) and NAS. An interactive effect of LT and VPD was observed in RWC, XPP, MII and NAS. Low VPD (0.22 and 0.87 kPa) resulted in less negative XPP, lower electrolyte leakage, higher SC and ultimately higher NAS than those at high VPD. Maximum NAS was recorded at 0.22 kPa. At 1.86 kPa, LT had 5× lower RWC (13%), 3× more negative XPP (−1.1 MPa), 1.8× higher membrane damage and 35% lower NAS (47 days) than CT. The SC declined with an increase in VPD with no effect of LT. The XPP and RWC of LT seedlings showed a positive relationship with NAS with R2 values of 0.54 and 0.59, respectively. LT offered no benefits to NAS at high VPD environments.

Thermal acclimation of photosynthesis and respiration of southern and northern white spruce seed sources tested along a regional climatic gradient indicates limited potential to cope with temperature warming

Abstract Background and Aims Knowledge of thermal acclimation of physiological processes of boreal tree species is necessary to determine their ability to adapt to predicted global warming and reduce the uncertainty around the anticipated feedbacks of forest ecosystems and global carbon cycle to climate change. The objective of this work was to examine the extent of thermal acclimation of net photosynthesis (An) and dark respiration (Rd) of two distant white spruce (Picea glauca) seed sources (from south and north of the commerial forest zone in Québec) in response to latitudinal and seasonal variations in growing conditions. Methods The temperature responses of An, its biochemical and biophysical limitations, and Rd were measured in 1-year-old needles of seedlings from the seed sources growing in eight forest plantations along a regional thermal gradient of 5.5 °C in Québec, Canada. Key Results The average optimum temperature (Topt) for An was 19 ± 1.2 °C and was similar among seed sources and plantation sites along the thermal gradient. Net photosynthesis at Topt (Aopt) varied significantly among plantation sites and was quadratically related to the mean July temperature (MJT) of plantation sites. Topt for mesophyll conductance, maximum electron transport rate and maximum rate of carboxylation were 28, 22 and 30 °C, respectively. Basal respiration rate (Rd at 10 °C) was linearly and negatively associated with MJT. Q10 of Rd (the rate of change in Rd with a 10 °C increase in temperature) did not show any significant relationship with MJT and averaged 1.5 ± 0.1. The two seed sources were similar in their thermal responses to latitudinal and seasonal variations in growing conditions. Conclusions The results showed moderate thermal acclimation of respiration and no evidence for thermal acclimation of photosynthesis or local genetic adaptation for traits related to thermal acclimation. Therefore, growth of local white spruces may decline in future climates.

Impact of potassium sulfate salinity on growth and development of cranberry plants subjected to overhead and subirrigation1

Abstract: New recommendations in cranberry production suggest reducing overhead irrigation and the use of subirrigation as an alternative irrigation method, two strategies suspected to increase the risk of salt buildup in soil. Because very little is known about cranberry tolerance to salinity, this study was conducted to determine if deficit irrigation and subirrigation could cause salinity issues and affect plant yield. In a greenhouse, cranberry plants were submitted to eight different treatments combination from two irrigation methods (overhead irrigation and subirrigation) and four salinity levels created by increasing amounts of applied K2SO4 (125 (control), 2500, 5000, and 7500 kg K2O ha-1). Irrigation methods showed no significant difference in measured electrical conductivity of soil solution (ECss). Meanwhile, growth and yield parameters decreased significantly with soil salinity in both irrigation treatments, and an average ECss of 3.2 dS m-1 during flowering caused a 22% drop in relative photosynthetic rate and a 56% decrease in yield when compared with the control. Cranberry seems to be salt sensitive, and further work should investigate ECss levels under different field and irrigation practices to make sure that it does not reach critical levels.

Persistent negative temperature response of mesophyll conductance in red raspberry (Rubus idaeus L.) leaves under both high and low vapour pressure deficits: A role for abscisic acid?: Persistent negative temperature response of mesophyll conductance in red raspberry (Rubus idaeus L.) leaves under both high and low vapour pressure de

The temperature dependence of mesophyll conductance (gm) was measured in well-watered red raspberry (Rubus idaeus L.) plants acclimated to leaf-to-air vapour pressure deficit (VPDL) daytime differentials of contrasting amplitude, keeping a fixed diurnal leaf temperature (Tleaf) rise from 20°C to 35°C. Contrary to the great majority of gm temperature responses published to date, we found a pronounced reduction of gm with increasing Tleaf irrespective of leaf chamber O2 level and diurnal VPDL regime. Leaf hydraulic conductance was greatly enhanced during the warmer afternoon periods under both low (0.75 to 1.5 kPa) and high (0.75 to 3.5 kPa) diurnal VPDL regimes, unlike stomatal conductance (gs), which decreased in the afternoon. Consequently the leaf water status remained largely isohydric throughout the day, and therefore cannot be evoked to explain the diurnal decrease of gm. However, the concerted diurnal reductions of gm and gs were well correlated with increases in leaf abscisic acid (ABA) content, thus suggesting that ABA can induce a significant depression of gm under favourable leaf water status. Our results challenge the view that the temperature dependence of gm can be explained solely from dynamic leaf anatomical adjustments and/or from the known thermodynamic properties of aqueous solutions and lipid membranes.

Effects of CO2 enrichment, LED inter-lighting, and high plant density on growth of Nicotiana benthamiana used as a host to express influenza virus hemagglutinin H1

Plants are being recognized as promising hosts for molecular farming and several molecular tools have been developed over the last two decades to optimize recombinant protein yields. However, the effects of basic growth factors on protein yield have been much less studied. Here, we investigated the effects of supplemental light emitting diode (LED) inter-lighting, CO2 enrichment, and plant density on growth and recombinant protein yield of Nicotiana benthamiana used as a host to express the vaccine antigen influenza virus hemagglutinin H1. LED inter-lighting improved plant growth and recombinant protein yield on a per-plant basis. CO2 enrichment also enhanced plant growth, but its effect on recombinant protein yield was not significant. By comparison, high plant density decreased recombinant protein production per plant, mainly because of its negative impact on protein accumulation on a per-plant basis. On a whole-crop area basis, supplemental lighting, CO2 enrichment, and high plant density improved plant growth, while only LED inter-lighting and high plant density positively impacted recombinant protein yield. We suggest that LED inter-lighting and an elevated plant density should be used to maximize H1 antigen yield in large-scale protein production systems using N. benthamiana.

Precision and accuracy of time-domain reflectometry and capacitive probes to determine soil electrical conductivity in cranberry production — Technical note1

Abstract: A recent study suggests a sensitivity of cranberry to saline stress. Consequently, monitoring of soil electrical conductivity may help growers to identify areas where plants could be under stress due to salt deposits. We used two different types of probes, a time-domain reflectometry (TDR; model CS645 probe) and a capacitive approach (model GS3 probe) to estimate electrical conductivity (EC) or conductance (G). The estimates were compared with measurements of EC in soil pore water using suction lysimeters in a sandy soil exposed to two different irrigation methods and a wide range of salt concentrations in a greenhouse. Linear regression analysis of TDR conductance versus measured EC in pore water gave coefficients of determination (R 2) between 0.24 and 0.98 and required specific calibration to accurately reproduce the suction lysimeter EC values. The GS3 probes had higher R 2 values, between 0.54 and 0.98, and were generally easier to work, gave a better accuracy, and had a regression slope not significantly different from 1, result better than with the TDR probes. For both probes, data averaging increased the accuracy in estimates of soil solution EC, as did specific calibration of the probes for the EC values value within the range of 0–5 dS m-1.