Stem girdling uncouples soybean stomatal conductance from leaf water potential by enhancing leaf xylem ABA concentration

To understand the impact of shoot-to-root ABA transport on water potential of, and xylem ABA concentration in, different plant tissues during soil drying, soybean (Glycine max cv. Siverka) plants were subjected to drought and girdling in a factorial experiment. Girdling was achieved by surgically excising the phloem tissue from just above the cotyledonary node. After girdling and withholding water, ABA concentrations were determined in xylem saps extracted from individual leaves, detached shoots and de-topped roots, after measuring stomatal conductance (gs), tissue water potentials, and root ABA concentrations. Soil drying decreased water potential throughout the plant and approximately doubled xylem ABA concentrations, coinciding with stomatal closure. Girdling slightly enhanced water potential, especially in droughted plants. Girdling diminished the soil-drying induced increase in xylem sap ABA concentration, and completely prevented root tissue ABA accumulation. Furthermore, girdling decreased root ABA concentration and increased leaf xylem ABA concentration of well-watered (WW) plants. Stomatal conductance declined linearly with leaf water potential only in intact plants, while gs declined as leaf xylem ABA concentration increased, independently of girdling. Thus shoot to root ABA transport not only determines (soil-drying induced) root ABA accumulation, but also limits ABA accumulation in the shoot to maintain stomatal opening of WW plants

Soil moisture heterogeneity regulates water use in Populus nigra L. by altering root and xylem sap phytohormone concentrations

Soil moisture heterogeneity in the root-zone is common during both the establishment of tree seedlings and in experiments aiming to impose semi-constant soil moisture deficits, but its effects on regulating plant water use compared to homogenous soil drying are not well known in trees. Pronounced vertical soil moisture heterogeneity was imposed on black poplar (Populus nigra L.) grown in soil columns by altering irrigation frequency, to test whether plant water use, hydraulic responses, root phytohormone concentrations, and root xylem sap chemical composition differed between wet (well-watered, WW), homogeneously (infrequent deficit irrigation, IDI) and heterogeneously dry soil (frequent deficit irrigation, FDI). At the same bulk soil water content, FDI plants had greater water use than IDI plants, probably because root abscisic acid (ABA) concentration was low in the upper wetter layer of FDI plants, which maintained root xylem sap ABA concentration at basal levels in contrast with IDI. Soil drying did not increase root xylem concentration of any other hormone. Nevertheless, plant-to-plant variation in xylem jasmonic acid (JA) concentration was negatively related to leaf stomatal conductance within WW and FDI plants. However, feeding detached leaves with high (1,200 nM) JA concentrations via the transpiration stream decreased transpiration only marginally. Xylem pH and sulphate concentration decreased in FDI plants compared to well-watered plants. Frequent deficit irrigation increased root accumulation of the cytokinin trans-zeatin (tZ), especially in the dry lower layer, and of the ethylene precursor ACC, in the wet upper soil layer. Root hormone accumulation might explain the maintenance of high root hydraulic conductance and water use in FDI plants (similar to well-watered plants) compared to IDI plants. In irrigated tree crops, growers could vary irrigation scheduling to control water use by altering the hormone balance

Effects of nitrogen fertilization and temperature on frost hardiness of Aleppo pine (Pinus halepensis Mill.) seedlings assessed by chlorophyll fluorescence

In a 14-week study, 1-year-old Aleppo pine seedlings were grown in two growth chambers. Seedlings were artificially hardened by decreasing photoperiod and temperature. In each chamber half of the seedlings were fertilized with nitrogen (8.4 mg seedling(-1)). In order to determine the relative importance of the hardening environment versus fertilization, each chamber was programmed to decrease night temperatures down to a low of 8 or 4 degrees C. Chlorophyll fluorescence and frost hardiness was measured five times during the experiment. A sample of seedlings from each treatment was exposed to an artificial frost at -5 degrees C and the freezing effects were assessed by measurements of chlorophyll fluorescence and visual evaluation of needle damage. Seedlings increased their frost hardiness during the experiment in all the treatments but the ratio of variable to maximal chlorophyll fluorescence (F(v)/F(m)) measured before freezing did not vary during the experiment. This indicates that Aleppo pine maintains its photosynthetic ability during hardening in contrast to other coniferous species from colder climates. The effect of nitrogen fertilization on frost hardiness was small in comparison with chamber effect. Nitrogen fertilization slightly delayed the acquisition of hardening in the coldest chamber. Seedlings in the warmest chamber did not become fully resistant to -5 degrees C, but in the coldest chamber, where night temperature reached 4 degrees C, all the seedlings were resistant to the frost. Severe damage caused by frost could be related to a rapid rise of minimal fluorescence (F(0)) but the best index of damage was the drop of F(v)/F(m) after freezing

Effects of nutritional status and seedling size on field performance of Pinus halepensis planted on former arable land in the Mediterranean basin

Seedling size and nutrient concentration of nine Aleppo pine (Pinus halepensis Mill.) seedling sets, which differed in nursery fertilization, were measured before being planted out. Height and survival were monitored for 3 years after planting. Transplant stress indexes (TSI), defined as the relationships between initial height and growth were analysed for five growth periods: December 1998 (planting date) to June 1999, June 1999 to November 1999, November 1999 to November 2000, November 2000 to November 2001, and December 1998 to November 2001. Analyses of covariance were performed to separate set effects on growth from initial height effects, obtaining a TSI adjusted for set effects (TSI(adj)). Analysis of covariance revealed a negative TSI(adj) for the first period. Nitrogen and phosphorus concentrations were related to height growth in this initial period. For subsequent periods, only seedling height influenced growth. Therefore, large seedlings (root-collar diameter >2.3 mm) are recommended for afforestation purposes. In addition, high nutrient content (>20 mg N seedling(-1)) is also desirable to enhance early growth of Aleppo pine after planting in the Mediterranean basin

An empirical model predicting xylem sap ABA concentration from root biomass and soil moisture distribution in plants under partial root-zone drying

Partial root-zone drying (PRD) is predicted to increase ABA concentration in the xylem sap ([X-ABA]) without changing leaf water status, thus partially closing the stomata which may enhance plant water use efficiency. Predicting ([X-ABA]) from soil moisture and root distribution data are essential to optimize the application of this technique. Bean (Phaseolus vulgaris) plants were grown in split pots, designed to allow a wide range of root mass distribution to either compartment, and PRD was applied. [X-ABA] was best predicted with an exponential model using the weighted (by root water uptake fraction - RWUF) average of root ABA concentration ([R-ABA]) in each compartment. This confirmed previous results reporting that a model accounting for RWUF is the best predictor of [X-ABA]. RWUF was equal to root mass fraction (RMF) when soil volumetric water content (θv) in the dry compartment was above 0.18 cm3 cm-3, linearly related to RMF when θv0.7, and null otherwise. [R-ABA] in the dry side had basal values (of 100-150 ng g-1 DW) when θv>0.18 and increased linearly with RMF above that threshold. Therefore, the proportion of roots in dry soil affected [X-ABA] by increasing both root water uptake fraction and ABA accumulation. [R-ABA] in the wet side was also stable when the dry side had θv>0.18 but was linearly related to local θv above that threshold and not to RMF. Simulation modelling showed that irrigating more than 1/3 of the total root mass during PRD had little impact on [X-ABA]

Modeling the environmental response of leaf net photosynthesis in Pinus pinea L. natural regeneration

Environmental factors as incident light, temperature and soil water content mainly determine the dynamics of growth and survival of natural forest regeneration in Mediterranean forests. The complex interactions among these factors highlight the need for physiology-based models which describe seedling and sapling performance under current and changing climatic conditions. These models should be flexible enough to take into account the effect of growth light environment changes on physiological processes and, therefore, to assist in the decision-making process on different silvicultural management alternatives under climatic uncertainty. In the present work the net photosynthesic rate of Pinus pinea L. natural regeneration is modeled as a function of light irradiance using the non-rectangular hyperbola function. The model fit was carried out following a two-step procedure, where the parameters of the original function were expanded over the most influential factors: leaf temperature, soil moisture, global site factor and needle type. The developed model allows defining the optimal niche conditions for the natural regeneration of the species and permits identifying the most limiting conditions which prevent natural regeneration. Regeneration niche was assessed by using the model to simulate net CO2 assimilation of a seedling over twelve contrasting light environments during both a normal and an extremely dry vegetative period. The model predicts that the most favorable carbon balance would be found in mid-shaded expositions, while fully exposed plants at midday during summer would exhibit longer periods of negative assimilation rates, especially on severe dry years

Defining the optimal regeneration niche for Pinus pinea L. through physiology-based models for seedling survival and carbon assimilation

Key message Seedling survival inPinus pineais controlled by both water status and photosynthetic performance. Optimal regeneration niche for the species is found at mid-shaded sites. Abstract Summer survival has been identified as the main bottleneck preventing natural regeneration in Mediterranean forests, although the physiological processes resulting in seedling mortality are not sufficiently known. In the present work, the effect of water status and photosynthetic performance on seedling survival and regeneration niche in a Mediterranean pine (Pinus pinea L.) was analyzed by means of a modeling approach. Midday water potential was modeled as a nonlinear function of environmental factors, and this model was coupled with an existing model for carbon assimilation. A model for seedling survival was then constructed using lifetime analysis techniques, including predicted values of daily and cumulative net assimilation and probability for critical midday water potentials as predictors. The model was applied over a wide range of irradiance environments in order to identify the optimal regeneration niche for the species. Results indicate that midday water potential for P. Pinea seedlings is affected by relative soil water content, leaf temperature, and irradiance, with younger seedlings being more likely to reach critical values. Seedling survival in P. pinea is controlled by both water status and photosynthetic performance, with mortality being triggered by the joint occurrence of low water potentials and negative assimilation rates, although photoassimilates stored during the spring season increase survival in older seedlings. Simulations indicated that seedling survival is optimized in shaded environments, while carbon assimilation reaches maximum values on more open sites, thus the optimal regeneration niche for the species is found on mid-shaded locations

Establishing quercus ilex under Mediterranean dry conditions:sowing recalcitrant acorns versus planting seedlings at different depths and tube shelter light transmissions

Success of Mediterranean dry areas restoration with oaks is a challenging goal. Testing eco-techniques that mimic beneficial effects of natural structures and ameliorate stress contributes to positive solutions to overcoming establishment barriers. We ran a factorial experiment in a dry area, testing two levels of solid wall transmission of tube shelters (60 and 80 %) plus a control mesh, and two depths (shallow and 15 cm depth) of placing either planted seedlings or acorns of Quercus ilex. Microclimate of the planting or sowing spots was characterized by measuring photosynthetically active radiation, temperature and relative humidity. Plant response was evaluated in terms of survival, phenology, acorn emergence and photochemical efficiency (measured through chlorophyll fluorescence). We hypothesize that tube shelters and deep planting improve Q. ilex post-planting and sowing performance because of the combined effects of reducing excessive radiation and improving access to moist soil horizons. Results show that temperature and PAR was reduced, and relative humidity increased, in deep spots. Midsummer photochemical efficiency indicates highest level of stress for oaks in 80 % light transmission shelter. Optimum acorn emergence in spring was registered within solid wall tree shelters, and maximum summer survival of germinants and of planted seedlings occurred when acorns or seedlings were placed at 15 cm depth irrespectively of light transmission of shelter. Survival of germinants was similar to that of planted seedlings. The importance of techniques to keep high levels of viability after sowing recalcitrant seeds in the field is emphasized in the study

Elevated atmospheric CO2 does not modify osmotic adjustment to light and drought in the Mediterranean oak Quercus suber L

The current ongoing increase in the atmospheric CO2 concentration is an unquestionable fact. Thus, plants are bound to live in a more enriched CO2 world in a not far-off future. In this new framework, regeneration of forest tree species may be modified as a consequence of the change in the current patterns of seedling response to other environmental resources, such as water or light. We studied the impact of an elevated CO2 concentration on the interaction of drought and light upon the water relations of cork oak (Quercus suber L.) seedlings. In a complete factorial design of contrasting light (HL vs LL), water (WW vs S) and CO2 levels (800 ppm vs 370 ppm), we analysed the influence of each factor and its interaction in the modification of different leaf water parameters in potted seedlings after a 6 months experimental period. These parameters were derived from the construction, with leaf materials, of the P-V curves: osmotic potential at full turgor (Ψπ100), osmotic potential at zero turgor (Ψπ0), modulus of elasticity at full turgor (ε max), and the ratio dry/turgor weight (DW/TW). Doubling of the CO2 levels over the current concentration (380 ppm) did not change any of the studied leaf water parameters, while light and water availabilities had a significant influence. This result does not exclude changes in other basic physiological parameters that could modify the pattern of cork oak regeneration responding to a CO2 enriched atmosphere in the future, and under climatic conditions different to the current ones