The Application of Paclobutrazol to GA3-Treated Seed Tuber Potato Fields Does Not Shorten the Growth Cycle or Mitigate Tuber Elongation.

Potato seed tubers are a valuable product in potato agriculture. Over the years, studies have been conducted to increase the fraction of mid-size tubers, which are used as a planting material, within the general pool of tuber sizes. Gibberellic acid has been a central component of such studies and has successfully increased the seed-size pool. However, in many cases, misshapen tubers were formed, and the practice has not become widespread. The use of the gibberellic acid inhibitor paclobutrazol has been examined for its ability to increase seed tuber number and tuber growth and to overcome the heat inhibition of tuberization in warm climates. Paclobutrazol has been shown to increase tuber yield and growth rate. In this study, we aimed to test whether the combination of gibberellic acid and paclobutrazol can increase the seed tuber pool, reduce the number of misshapen tubers, and shorten the growth cycle, thus avoiding end-of-season elevated heat conditions and reducing agricultural inputs. Our findings suggest that gibberellic acid on its own can lead to an increase in the number of seed tubers at earlier stages of growth; however, the sequential addition of paclobutrazol was not able to drive even earlier growth or lower the number of misshapen tubers.

Family ties: Root-root communication within Solanaceae.

Root-root communication effects on several physiological and metabolic aspects among Solanaceae relatives were studied. We examined cherry (C) and field (F) tomato (Solanum lycopersicum) and bell pepper (B) (Capsicum annuum), comprising three degrees of relatedness (DOR): high (H-DOR; CC, FF and BB), medium (M-DOR; CF) and low (L-DOR; CB and FB). Plants were grown in pairs of similar or different plants on a paper-based and non-destructive root growth system, namely, rhizoslides. Root growth, including the proliferation of fine roots, and respiration increased as the DOR decreased and were highest in paired L-DOR plants, as was shown for root respiration that increased by 63, 110 and 88 % for C, F, and B when grown with B, B and F, respectively. On the other hand, root exudates of L-DOR plants had significantly lower levels of total organic carbon and protein than those of H-DOR plants, indicating different root-root communication between individuals with different DOR. Our findings indicate, for the first time, that carbon allocation to root growth, exudation and respiration depends on the degree of genetic relatedness, and that the degree of relatedness between individual plants plays a key role in the root-root communication within Solanaceae.

Global evaluation of current and future threats to drylands and their vertebrate biodiversity.

Drylands are often overlooked in broad conservation frameworks and development priorities and face increasing threats from human activities. Here we evaluated the formal degree of protection of global drylands, their land vertebrate biodiversity and current threats, and projected human-induced land-use changes to drylands under different future climate change and socioeconomic scenarios. Overall, drylands have lower protected-area coverage (12%) compared to non-drylands (21%). Consequently, most dryland vertebrates including many endemic and narrow-ranging species are inadequately protected (0-2% range coverage). Dryland vertebrates are threatened by varied anthropogenic factors-including agricultural and infrastructure development (that is, artificial structures, surfaces, roads and industrial sites). Alarmingly, by 2100 drylands are projected to experience some degree of land conversion in 95-100% of their current natural habitat due to urban, agricultural and alternative energy expansion. This loss of undisturbed dryland regions is expected across different socioeconomic pathways, even under optimistic scenarios characterized by progressive climate policies and moderate socioeconomic trends.

Integrated hydroponics systems with anaerobic supernatant and aquaculture effluent in desert regions: Nutrient recovery and benefit analysis.

Hydroponics is a resource-efficient system that increases food production and enhances the overall sustainability of agricultural systems, particularly in arid zones with prevalent water scarcity and limited areas of arable land. This study investigated zero-waste hydroponics systems fed by agricultural waste streams as nutrient sources under desert conditions. Three pilot-scale systems were tested and compared. The first hydroponics system ("HPAP") received its nutrient source internally from an aquaponic system, including supernatant from the anaerobic digestion of fish sludge. The second system ("HPAD") was sourced by the supernatant of plant waste anaerobic digestion, and the third served as a control that was fed by commercial Hoagland solution ("HPHS"). Fresh weight production was similar in all treatments, ranging from 488 to 539 g per shoot, corresponding to 5.7 to 6.0 kg total wet weight per m2. The recovery of N and P from wastes and their subsequent uptake by plants was highly efficient, with rates of 77 % for N and 65 % for P. Plants that were fed using supernatants demonstrated slightly higher plant quality compared with those grown in Hoagland solution. Over the duration of the full study (3 months), water was only used to compensate for evapotranspiration, corresponding to ~10 L per kg of lettuce. The potential health risk for heavy metals was negligible, as assessed using the health-risk index (HRI < 1) and targeted hazardous quotient (THQ < 1). The results of this study demonstrate that careful management can significantly reduce pollution, increase the recovery of nutrients and water, and improve hydroponics production.

Root system response to salt stress in grapevines (Vitis spp.): A link between root structure and salt exclusion.

Accessing freshwater resources for agriculture becomes more complex due to increasing demands and declining water quality. Alternative water sources, such as saline water, require ad hoc solutions. Therefore, understanding roots' response to saline water is crucial for future agriculture. We examined the response of three grapevine rootstocks (Paulsen 1103, Richter 110 and SO4) to salt stress. The rootstocks were subjected to two salinity treatments: 10 mM and 30 mM NaCl (EC = 2 and 4 ds/m, respectively). Root and shoot samples were taken at the end of the experiment for morphologic and ionomic analyses. The specific root area (SRA) increased in response to salinity for all three rootstocks due to root tissue density and average root diameter reductions. Salinity also led to increased root Na+ and Cl- contents and reduced root K+/Na+ ratio, parallel to increased leaf Cl- but not Na+ contents. SO4 showed improved chloride and sodium exclusion, concomitant with its highest SRA, resulting from the increase in its thin roots' contribution to the total root system surface area. We suggest that enhanced SRA combined with decreased root tissue density and diameter may improve grapevines' salt exclusion by less salt uptake from the soil.

Leveraging a graft collection to develop metabolome-based trait prediction for the selection of tomato rootstocks with enhanced salt tolerance.

Grafting has been demonstrated to significantly enhance the salt tolerance of crops. However, breeding efforts to develop enhanced graft combinations are hindered by knowledge-gaps as to how rootstocks mediate scion-response to salt stress. We grafted the scion of cultivated M82 onto rootstocks of 254 tomato accessions and explored the morphological and metabolic responses of grafts under saline conditions (EC = 20 dS m-1) as compared to self-grafted M82 (SG-M82). Correlation analysis and Least Absolute Shrinkage and Selection Operator were performed to address the association between morphological diversification and metabolic perturbation. We demonstrate that grafting the same variety onto different rootstocks resulted in scion phenotypic heterogeneity and emphasized the productivity efficiency of M82 irrespective of the rootstock. Spectrophotometric analysis to test lipid oxidation showed largest variability of malondialdehyde (MDA) equivalents across the population, while the least responsive trait was the ratio of fruit fresh weight to total fresh weight (FFW/TFW). Generally, grafts showed greater values for the traits measured than SG-M82, except for branch number and wild race-originated rootstocks; the latter were associated with smaller scion growth parameters. Highly responsive and correlated metabolites were identified across the graft collection including malate, citrate, and aspartate, and their variance was partly related to rootstock origin. A group of six metabolites that consistently characterized exceptional graft response was observed, consisting of sorbose, galactose, sucrose, fructose, myo-inositol, and proline. The correlation analysis and predictive modelling, integrating phenotype- and leaf metabolite data, suggest a potential predictive relation between a set of leaf metabolites and yield-related traits.

Leaf coordination between petiole vascular development and water demand in response to elevated CO2 in tomato plants.

The rise in atmospheric CO2 has a profound impact on plants physiology and performance. Stomatal gas exchange such as reduction in water loss via transpiration and higher photosynthetic rates are among the key plant physiological traits altered by the increase of CO2. Water acquired in plant roots is transported via the xylem vessels to the shoots. Under conditions of elevated CO2, water flux decreases due to higher water use efficiency and a decline in stomatal conductance. However, the mechanism by which the shoot vascular development is affected under elevated CO2 is still largely unclear in herbaceous crops. In the current study, tomato plants were exposed to either 400 or 800 ppm of CO2 and were analyzed for growth, leaf area, gas exchange rate, and petiole anatomy. Elevated CO2 caused a reduction in metaxylem vessel diameter, which in turn, decreased leaf theatrical conductivity by 400% as compared with plants grown under ambient CO2. This work links anatomical changes in the petioles to the rise in atmospheric CO2 and water use. Plant water demand declined under elevated CO2, while photosynthesis increased. Thus, the decrease in leaf specific conductivity was attributed to lower water consumption in leaf gas exchange and, by extension, to higher leaf water use efficiency. As the global climate changes and water scarcity becomes more common, such anatomical alterations caused by elevated CO2 may affect plant response to water limitation. Further research on petiole anatomical alterations under conditions of combined climate change factors such as drought and heat with elevated CO2 may assist in clarifying the responses expected by future climate scenarios.

Nutrient deficiency effects on root architecture and root-to-shoot ratio in arable crops.

Plant root traits play a crucial role in resource acquisition and crop performance when soil nutrient availability is low. However, the respective trait responses are complex, particularly at the field scale, and poorly understood due to difficulties in root phenotyping monitoring, inaccurate sampling, and environmental conditions. Here, we conducted a systematic review and meta-analysis of 50 field studies to identify the effects of nitrogen (N), phosphorous (P), or potassium (K) deficiencies on the root systems of common crops. Root length and biomass were generally reduced, while root length per shoot biomass was enhanced under N and P deficiency. Root length decreased by 9% under N deficiency and by 14% under P deficiency, while root biomass was reduced by 7% in N-deficient and by 25% in P-deficient soils. Root length per shoot biomass increased by 33% in N deficient and 51% in P deficient soils. The root-to-shoot ratio was often enhanced (44%) under N-poor conditions, but no consistent response of the root-to-shoot ratio to P-deficiency was found. Only a few K-deficiency studies suited our approach and, in those cases, no differences in morphological traits were reported. We encountered the following drawbacks when performing this analysis: limited number of root traits investigated at field scale, differences in the timing and severity of nutrient deficiencies, missing data (e.g., soil nutrient status and time of stress), and the impact of other conditions in the field. Nevertheless, our analysis indicates that, in general, nutrient deficiencies increased the root-length-to-shoot-biomass ratios of crops, with impacts decreasing in the order deficient P > deficient N > deficient K. Our review resolved inconsistencies that were often found in the individual field experiments, and led to a better understanding of the physiological mechanisms underlying root plasticity in fields with low nutrient availability.

A novel approach for long-term spectral monitoring of desert shrubs affected by an oil spill.

Crude oil pollution is a global environmental concern since it persists in the environment longer than most conventional carbon sources. In December 2014, the hyper-arid Evrona Nature Reserve, Israel, experienced large-scale contamination when crude oil spilled. The overarching goal of the study was to investigate the possible changes, caused by an accidental crude oil spill, in the leaf reflectance and biochemical composition of four natural habitat desert shrubs. The specific objectives were (1) to monitor the biochemical properties of dominant shrub species in the polluted and control areas; (2) to study the long-term consequences of the contamination; (3) to provide information that will assist in planning rehabilitation actions; and (4) to explore the feasibility of vegetation indices (VIs), along with the machine learning (ML) technique, for detecting stressed shrubs based on the full spectral range. Four measurement campaigns were conducted in 2018 and 2019. Along with the various stress indicators, field spectral measurements were performed in the range of 350-2500 nm. A regression analysis to examine the relation of leaf reflectance to biochemical contents was carried out, to reveal the relevant wavelengths in which polluted and control plants differ. Vegetation indices applied in previous studies were found to be less sensitive for indirect detection of long-term oil contamination. A novel spectral index, based on indicative spectral bands, named the "normalized blue-green stress index" (NBGSI), was established. The NBGSI distinguished significantly between shrubs located in the polluted and in the control areas. The NBGSI showed a strong linear correlation with pheophytin a. Machine learning classification algorithms obtained high overall prediction accuracy in distinguishing between shrubs located in the oil-polluted and the control sites, indicating internal component differences. The findings of this study demonstrate the efficacy of indirect and non-destructive spectral tools for detecting and monitoring oil pollution stress in shrubs.

Geodiversity impacts plant community structure in a semi-arid region.

Geodiversity refers to the variety of geological and physical elements as well as to geomorphological processes of the earth surface. Heterogeneity of the physical environment has an impact on plant diversity. In recent years, the relations between geodiversity and biodiversity has gained attention in conservation biology, especially in the context of climate change. In this study, we assessed the spatial and temporal change in plant's community structure in a semi-arid region, Sayeret Shaked Long Term Ecosystem Research (LTER) station, Israel. Vegetation surveys were conducted on different hillslopes, either with or without rock covers in order to study the spatial trends of hillslope geodiversity. The surveys were conducted for two consecutive years (2016 and 2017), of which the second year was drier and hotter and therefore permitted to investigate the temporal change of plant's community structure. The results of the spatial trends show that (1) geodiversity increases vegetation biodiversity and promotes perennial plants and those of the temporal change show that (2) the positive effect of geodiversity on plants' community structure and species richness is greater in the drier year than that in a wetter year. The main insight is that in these drylands, hillslopes with higher geodiversity appear to buffer the effect of drier years, and supported a more diverse plant community than lower geodiversity hillslopes.

Dew water-uptake pathways in Negev desert plants: a study using stable isotope tracers.

Dew is an important water resource for plants in most deserts. The mechanism that allows desert plants to use dew water was studied using an isotopic water tracer approach. Most plants use water directly from the soil; the roots transfer the water to the rest of the plant, where it is required for all metabolic functions. However, many plants can also take up water into their leaves and stems. Examining the dew water uptake pathways in desert plants can lend insight on another all water-use pathways examination. We determined where and how dew water enters plants in the water limited Negev desert. Highly depleted isotopic water was sprayed on three different dominant plant species of the Negev desert-Artemesia sieberi, Salsola inermis and Haloxylon scoparium-and its entry into the plant was followed. Water was sprayed onto the soil only, or on the leaves/stems only (with soil covered to prevent water entry via root uptake). Thereafter, the isotopic composition of water in the roots and stems were measured at various time points. The results show that each plant species used the dew water to a different extent, and we obtained evidence of foliar uptake capacity of dew water that varied depending on the microenvironmental conditions. A. sieberi took up the greatest amount of dew water through both stems and roots, S. inermis took up dew water mainly from the roots, and H. scoparium showed the least dew capture overall.

Aeroponic systems: A unique tool for estimating plant water relations and NO3 uptake in response to salinity stress.

The study of transpiration, water, and nutrient uptake during abiotic stress in the root zone is hindered because of the hidden nature of the root zone. In this study, a modified aeroponic system was used to evaluate whole plant transpiration, nitrate and water uptake in the growth and development of tomato plants in response to salinity. Tomato seedlings were exposed to three levels of salinity (1.5, 4.5, and 9 dSm-1) and three levels of nitrate (1, 4, and 8 mM NO3) in a separate experiments conducted concurrently. Whole plant transpiration, water and nitrate uptake were estimated. Our study revealed that ~30 to 35 days after treatment (DAT), water uptake rate per plant increased from a common initial rate of about 0.05 to 1.1, 0.6, and 0.4 kg/day at 1.5, 4.5, and 9 dSm-1 respectively. The NO3 uptake rates in tomatoes grown in 1 and 4 mM NO3 were 5.5 and 22% respectively, of the uptake of tomatoes grown in 8 mM NO3. The estimation of nitrate uptake and lower sensitivity to salinity stress in the aeroponic showed the effectiveness and cost efficiency of the system in the cultivation of vegetables during abiotic stresses. The novelty of the system described is the continuous estimation of root and nutrient uptake by the whole plant at any given time.

Bacterial Community Structure Dynamics in Meloidogyne incognita-Infected Roots and Its Role in Worm-Microbiome Interactions.

Plant parasitic nematodes such as Meloidogyne incognita have a complex life cycle, occurring sequentially in various niches of the root and rhizosphere. They are known to form a range of interactions with bacteria and other microorganisms that can affect their densities and virulence. High-throughput sequencing can reveal these interactions in high temporal and geographic resolutions, although thus far we have only scratched the surface. In this study, we have carried out a longitudinal sampling scheme, repeatedly collecting rhizosphere soil, roots, galls, and second-stage juveniles from 20 plants to provide a high-resolution view of bacterial succession in these niches, using 16S rRNA metabarcoding. Our findings indicate that a structured community develops in the root, in which gall communities diverge from root segments lacking a gall, and that this structure is maintained throughout the crop season. We describe the successional process leading toward this structure, which is driven by interactions with the nematode and later by an increase in bacteria often found in hypoxic and anaerobic environments. We present evidence that this structure may play a role in the nematode's chemotaxis toward uninfected root segments. Finally, we describe the J2 epibiotic microenvironment as ecologically deterministic, in part, due to the active bacterial attraction of second-stage juveniles.IMPORTANCE The study of high-resolution successional processes within tightly linked microniches is rare. Using the power and relatively low cost of metabarcoding, we describe the bacterial succession and community structure in roots infected with root-knot nematodes and in the nematodes themselves. We reveal separate successional processes in galls and adjacent non-gall root sections, which are driven by the nematode's life cycle and the progression of the crop season. With their relatively low genetic diversity, large geographic range, spatially complex life cycle, and the simplified agricultural ecosystems they occupy, root-knot nematodes can serve as a model organism for terrestrial holobiont ecology. This perspective can improve our understanding of the temporal and spatial aspects of biological control efficacy.

Optimizing root yield of cassava under fertigation and the masked effect of atmospheric temperature.

BACKGROUND: Fertigation is a rare and an expensive method of fertilizer application to cassava, and hence there is a need to optimize its efficiency for profitability. This study's objective was to optimize root yield of cassava through fertigation using a logistic model. RESULTS: The field treatments were six fertigation concentrations against three cassava varieties, selected according to their maturity period. The logistic model predicted 52%, 116% and 281% benefit of fertigation for the varieties Mweru, Kampolombo and Nalumino, respectively. Furthermore, only half of the amount of fertilizer applied for Mweru was required to achieve twice the root yield of Kampolombo. During the experiment, an unknown importance of atmospheric temperature to cassava and its relationship to fertigation was observed. An elevation of 3.7 °C in atmospheric temperature led to 226%, 364% and 265% increase in root yield of Mweru, Kampolombo and Nalumino, respectively. Conversely, shoot biomass and root yield declined when the average atmospheric temperatures dropped by 3.6 °C. However, the cold temperatures affected the short-growth-duration (Mweru) and medium-growth-duration (Kampolombo) varieties earlier, 22 days after the drop, than the long-growth-duration variety (Nalumino) - 50 days after the drop. CONCLUSION: Fertigation induced resilience of the shoot biomass production to cold which was most pronounced in the root yield of Mweru in response to the highest fertigation concentration. Thus, while fertigation improved cassava's resilience to cold, it only did so effectively for short-growth-duration variety, Mweru. Also, enhanced performance of cassava under increased atmospheric temperature indicated its importance as a climate-smart crop. © 2020 Society of Chemical Industry.

Wide vessels sustain marginal transpiration flux and do not optimize inefficient gas exchange activity under impaired hydraulic control and salinity.

Plants optimize water use and carbon assimilation via transient regulation of stomata resistance and by limiting hydraulic conductivity in a long-term response of xylem anatomy. We postulated that without effective hydraulic regulation plants would permanently restrain water loss and photosynthetic productivity under salt stress conditions. We compared wild-type tomatoes to a transgenic type (TT) with impaired stomatal control. Gas exchange activity, biomass, starch content, leaf area and root traits, mineral composition and main stems xylem anatomy and hydraulic conductivity were analyzed in plants exposed to salinities of 1 and 4 dS m-1 over 60 days. As the xylem cannot easily readjust to different environmental conditions, shifts in its anatomy and the permanent effect on plant hydraulic conductivity kept transpiration at lower levels under unstressed conditions and maintained it under salt-stress, while sustaining higher but inefficient assimilation rates, leading to starch accumulation and decreased plant biomass, leaf and root area and root length. Narrow conduits in unstressed TT plants were related to permanent restrain of hydraulic conductivity and plant transpiration. Under salinity, TT plants followed the atmospheric water demand, sustained similar transpiration rate from unstressed to salt-stressed conditions and possibly maintained hydraulic integrity, due to likely impaired hydraulic regulation, wider conduits and higher hydraulic conductivity. The accumulation of salts and starch in the TT plants was a strong evidence of salinity tolerance via osmotic regulation, also thought to help to maintain the assimilation rates and transpiration flux under salinity, although it was not translated into higher growth.

Potassium and storage root development: focusing on photosynthesis, metabolites and soluble carbohydrates in cassava.

The linkage between K and the development of storage roots in root crops is partially understood, hence this experiment determined some of the mechanisms involved in cassava. The effects of 10, 40, 70, 100, 150 and 200 mg K l-1 fertigation on photosynthetic attributes, soluble carbohydrates, starch, metabolites, growth and yield were studied in a greenhouse. Storage root yield, number of storage roots, stomatal conductance and net photosynthesis reached maximum at 150 mg K l-1 . However, soluble carbohydrates and starch in the leaves significantly declined with an increasing concentration of K solution, similarly to the trend of glycerol in the leaves. Conversely, malic acid, citric acid and propionic acid gradually increased reaching maximum at 150, 150 and 70 mg K l-1 respectively. Combined, these results suggest that sugars were transported from the leaves to a stronger sink - the bulking storage roots. This and the increase of intermediate metabolites of tricarboxylic acid cycle provided the energy required for the bulking process and the development of the storage roots. Although the measured parameters indirectly link K to storage root development, they nonetheless form a basis for studies on direct interactions.

High Nitrogen Availability Limits Photosynthesis and Compromises Carbohydrate Allocation to Storage in Roots of Manihot esculenta Crantz.

Cassava (M. esculenta Crantz), feeding countless people and attracting markets worldwide, is a model for traditional crops that need physiology-based fertigation (fertilization through irrigation) standards in intensive cultivation. Hence, we studied the effects of 10 to 200 mg L-1 nitrogen (N) fertigation on growth and yields of cassava and targeted alterations in their photosynthetic, transpiration, and carbohydrate management. We found that increasing irrigation N from 10 to 70 mg L-1 increased cassava's photosynthesis and transpiration but supported only the canopy's growth. At 100 mg N L-1 cassava reached a threshold of sugar in leaves (∼47 mg g-1), began to accumulate starch and supported higher yields. Yet, at 200 mg N L-1, the canopy became too demanding and plants had to restrain transpiration, reduce photosynthesis, decrease carbohydrates, and finally lower yields. We concluded that the phases of cassava response to nitrogen are: 1) growth that does not support yields at low N, 2) productive N application, and 3) excessive use of N. Yet traditional leaf mineral analyses fail to exhibit these responses, and therefore we propose a simple and inexpensive carbohydrate measurement to guide a precise use of N.

CO2 and nitrogen interaction alters root anatomy, morphology, nitrogen partitioning and photosynthetic acclimation of tomato plants.

MAIN CONCLUSION: Nitrogen and CO2 supply interactively regulate whole plant nitrogen partitioning and root anatomical and morphological development in tomato plants. Nitrogen (N) and carbon (C) are the key elements in plant growth and constitute the majority of plant dry matter. Growing at CO2 enrichment has the potential to stimulate the growth of C3 plants, however, growth is often limited by N availability. Thus, the interactive effects of CO2 under different N fertilization rates can affect growth, acclimation to elevated CO2, and yield. However, the majority of research in this field has focused on shoot traits, while neglecting plants' hidden half-the roots. We hypothesize that elevated CO2 and low N effects on transpiration will interactively affect root vascular development and plant N partitioning. Here we studied the effects of elevated CO2 and N concentrations on greenhouse-grown tomato plants, a C3 crop. Our main objective was to determine in what manner the N fertilization rate and elevated CO2 affected root development and nitrogen partitioning among plant organs. Our results indicate that N interacting with the CO2 level affects the development of the root system in terms of the length, anatomy, and partitioning of the N concentration between the roots and shoot. Both CO2 and N concentrations were found to affect xylem size in an opposite manner, elevated CO2 found to repressed, whereas ample N stimulated xylem development. We found that under limiting N and eCO2, the N% increase in the root, while it decreased in the shoot. Under eCO2, the root system size increased with a coordinated decrease in root xylem area. We suggest that tomato root response to elevated CO2 depends on N fertilization rates, and that a decrease in xylem size is a possible underlying response that limits nitrogen allocation from the root into the shoot. Additionally, the greater abundance of root amino acids suggests increased root nitrogen metabolism at eCO2 conditions with ample N.

Thermal Benefits From White Variegation of Silybum marianum Leaves.

Leaves of the spiny winter annual Silybum marianum express white patches (variegation) that can cover significant surface areas, the outcome of air spaces formed between the epidermis and the green chlorenchyma. We asked: (1) what characterizes the white patches in S. marianum and what differs them from green patches? (2) Do white patches differ from green patches in photosynthetic efficiency under lower temperatures? We predicted that the air spaces in white patches have physiological benefits, elevating photosynthetic rates under low temperatures. To test our hypotheses we used both a variegated wild type and entirely green mutants. We grew the plants under moderate temperatures (20°C/10°C d/n) and compared them to plants grown under lower temperatures (15°C/5°C d/n). The developed plants were exposed to different temperatures for 1 h and their photosynthetic activity was measured. In addition, we compared in green vs. white patches, the reflectance spectra, patch structure, chlorophyll and dehydrin content, stomatal structure, plant growth, and leaf temperature. White patches were not significantly different from green patches in their biochemistry and photosynthesis. However, under lower temperatures, variegated wild-type leaves were significantly warmer than all-green mutants - possible explanations for that are discussed These findings support our hypothesis, that white variegation of S. marianum leaves has a physiological role, elevating leaf temperature during cold winter days.

The Phosphorus Economy of Mediterranean Oak Saplings Under Global Change.

While a severe decrease in phosphorus (P) availability is already taking place in a large number of ecosystems, drought and nitrogen (N) deposition will likely further decrease the availability of P under global change. Plants have developed physiological strategies to cope with decreasing P resources, but it is unclear how these strategies respond to elevated N deposition and summer droughts. We investigated the influence of N and P availability and soil drought on P uptake (H3 33PO4 feeding experiment) and use efficiencies in young Quercus calliprinos Webb. trees. We hypothesized that (H1) the expected increases in soil N:P ratios will increase the efficiencies of P uptake and use of oak saplings but will decrease the efficiencies of N uptake and use, whereas (H2) drought will affect P uptake efficiency more than N uptake efficiency. In confirmation of (H1) we found that a sharp increase of the soil N:P ratio from 4 to 42 g g-1 significantly increased the instantaneous 33P uptake efficiency (33PUptakeE) by five-fold and long-term P uptake efficiency (PUptakeE) by six-fold, while it decreased N uptake efficiency (NUptakeE) and N use efficiency (NUE). In contradiction to (H1), P use efficiency (PUE) did not respond to the simulated extended gradient of soil N:P ratios but remained relatively constant. (H2) was only partially confirmed as soil drought reduced PUptakeE by up to a fourth at high soil N:P ratios but had no significant effect on NUptakeE. As a consequence, increasing summer droughts may decrease the response of PUptakeE to increasing P limitation, which - in the absence of adjustments of the efficiency of P use - can aggravate growth reductions in this eastern Mediterranean tree species under global change.