The effect of vapour pressure deficit on stomatal conductance, sap pH and leaf-specific hydraulic conductance in Eucalyptus globulus clones grown under two watering regimes

Seasonal variation in the xylem sap composition of six Australian trees and shrubs

In recent years, xylem sap composition has been shown to affect xylem hydraulics. However, information on how much xylem sap composition can vary across seasons and specifically under drought stress is still limited. We measured xylem sap chemical composition ([Ca2+], [K+], [Na+], electrical conductivity EC and pH) and surface tension (γ) of six Australian angiosperm trees and shrubs over 1 year, which comprised of exceptional dry and wet periods. Percentage losses of hydraulic conductivity and predawn leaf water potential were also monitored. In all species, measured parameters changed considerably over the annual time course. Ions and pH tended to decrease during winter months whereas γ showed a slight increase. No clear correlation was found between sap and hydraulic parameters, except for pH that was higher when plants suffered higher drought stress levels. Results indicate xylem sap composition to be complex and dynamic, where most variation in its composition seems to be dictated by season, even under severe dry conditions. However, pH might play a role as signals of drought stress.

Effects of Metamitron under Different Relative Humidity Conditions on the Fruit Abscission of Malus domestica Borkh. Cultivars

Metamitron is an apple (Malus domestica Borkh.) fruit thinner that acts by reducing the photosynthetic capacity of trees. Relative humidity may influence thinning efficacy; however, the broadness of this effect is not yet fully understood. Trials were set in Sint-Truiden (Belgium) in 2018 and Lleida and Girona (Spain) in 2019, using 4-year-old cvs. Braeburn and Elstar trees in Belgium, and 16-year-old cv. Golden Reinders trees in Spain. Four treatments were implemented at the stage of 12–14 mm fruit diameter: (a) CTR—control, trees under natural environmental conditions; (b) HH—high humidity, trees submitted to artificially increased air relative humidity applied for 3 h prior to the beginning of the experiment; (c) MET—trees sprayed with 247.5 mg/L metamitron; (d) MET + HH—trees submitted to the combination of increased humidity (HH) and metamitron (MET) application. In Belgium, metamitron absorption by leaves was greater than in Spain. This might be related to morphological characteristics of the leaves which developed under greater natural relative humidity levels in Belgium than in Spain. Compared to MET alone, ‘Braeburn’ and ‘Elstar’ demonstrated significantly greater metamitron absorption, 59% and 84%, respectively, under MET + HH, accompanied by declines in leaf net photosynthesis (10% and 32%, respectively) and sucrose (31% and 26%, respectively). At harvest, MET + HH treatment reduced yield by 24% and 32% in ‘Braeburn’ and ‘Elstar’, respectively, when compared with MET alone. A large reduction (considered over thinning) in the yield of ‘Elstar’ occurred. In contrast, metamitron absorption by ‘Golden Reinders’ using MET alone was similar to MET + HH; however, there was a slight foliar sugar reduction in the latter treatment. In addition, both treatments enhanced shoot growth and increased fruit abscission with similar improvements in fruit weight and size. In this study, high relative humidity enhanced fruit thinning efficacy under certain circumstances, such as age or genetic predisposal, which left the tree more susceptible to a negative carbohydrate balance. For instance, ‘Braeburn’ and ‘Elstar’ were easier to thin when compared to ‘Golden Reinders’. In addition, this study raises a question that requires further research regarding the impact of HH before and after spraying as well as its effect in combination with higher temperatures.

Growth promotion and protection from drought in Eucalyptus grandis seedlings inoculated with beneficial bacteria embedded in a superabsorbent polymer

Eucalyptus grandis is a globally important tree crop. Greenhouse-grown tree seedlings often face water deficit after outplanting to the field, which can affect their survival and establishment severely. This can be alleviated by the application of superabsorbent hydrophilic polymers (SAPs). Growth promoting bacteria can also improve crop abiotic stress tolerance; however, their use in trees is limited, partly due to difficulties in the application and viability loss. In this work, we evaluated the improvement of drought tolerance of E. grandis seedlings by inoculating with two Pseudomonas strains (named M25 and N33), carried by an acrylic-hydrocellulosic SAP. We observed significant bacterial survival in the seedling rhizosphere 50 days after inoculation. Under gradual water deficit conditions, we observed a considerable increase in the water content and wall elasticity of M25-inoculated plants and a trend towards growth promotion with both bacteria. Under rapid water deficit conditions, which caused partial defoliation, both strains significantly enhanced the formation of new leaves, while inoculation with M25 reduced the transpiration rate. Co-inoculation with M25 and N33 substantially increased growth and photosynthetic capacity. We conclude that the selected bacteria can benefit E. grandis early growth and can be easily inoculated at transplant by using an acrylic-hydrocellulosic SAP.

Variability and Plasticity in Cuticular Transpiration and Leaf Permeability Allow Differentiation of Eucalyptus Clones at an Early Age

Background and Objectives. Water stress is a major constraining factor of Eucalyptus plantations’ growth. Within a genetic improvement program, the selection of genotypes that improve drought resistance would help to improve productivity and to expand plantations. Leaf characteristics, among others, are important factors to consider when evaluating drought resistance evaluation, as well as the clone’s ability to modify leaf properties (e.g., stomatal density (d) and size, relative water content at the time of stomatal closure (RWCc), cuticular transpiration (Ec), specific leaf area (SLA)) according to growing conditions. Therefore, this study aimed at analyzing these properties in nursery plants of nine high-productivity Eucalyptus clones. Material and Methods: Five Eucalyptus globulus Labill. clones and four hybrids clones (Eucalyptus urophylla S.T. Blake × Eucalyptus grandis W. Hill ex Maiden, 12€; Eucalyptus urograndis × E. globulus, HE; Eucalyptus dunnii Maiden–E. grandis × E. globulus, HG; Eucalyptus saligna Sm. × Eucalyptus maidenii F. Muell., HI) were studied. Several parameters relating to the aforementioned leaf traits were evaluated for 2.5 years. Results: Significant differences in stomatal d and size, RWCc, Ec, and SLA among clones (p < 0.001) and according to the dates (p < 0.001) were obtained. Each clone varied seasonally the characteristics of its new developing leaves to acclimatize to the growth conditions. The pore opening surface potential (i.e., the stomatal d × size) did not affect transpiration rates with full open stomata, so the water transpired under these conditions might depend on other leaf factors. The clones HE, HG, and 12€ were the ones that differed the most from the drought resistant E. globulus control clone (C14). Those three clones showed lower leaf epidermis impermeability (HE, HG, 12€), higher SLA (12€, HG), and lower stomatal control under moderate water stress (HE, HG) not being, therefore, good candidates to be selected for drought resistance, at least for these measured traits. Conclusions: These parameters can be incorporated into genetic selection and breeding programs, especially Ec, SLA, RWCc, and stomatal control under moderate water stress.

Synthesis and evaluation of a superabsorbent-fertilizer composite for maximizing the nutrient and water use efficiency in forestry plantations

Reducing fertilizer use is a priority in the quest for sustainable forestry systems. In short rotation Eucalyptus plantations, NPK pellets are routinely added to the seedling's top soil layer at planting, potentially leading to increased seedling mortality, nutrient loss and environmental degradation. To address this triple challenge, the development of efficient fertilization practices is essential. In the present work, we synthesized a crosslinked acrylic-cellulosic superabsorbent composite (SAPH-BAL) containing small amounts of specific nutrients integrated in the polymer matrix. We analyzed the composite's chemical and rheological properties, and assessed the viability of Eucalyptus plantations supplied with it at planting. Physiological measurements confirmed the suitability of SAPH-BAL in greenhouse-grown potted seedlings subjected to different growth conditions, showing that it efficiently delivers nutrients while protecting seedlings from drought stress. Field experiments carried out at ten South American locations covering an ample range of environmental conditions confirmed the beneficial effect of SAPH-BAL on growth and survival in comparison to the conventional fertilization scheme (superabsorbent + 75 g NPK). Furthermore, it was found that plants treated with SAPH-BAL were less affected by the differences in rainfall regimes during the experiments compared to those fertilized conventionally. To the best of our knowledge this is the first report describing the successful use of superabsorbents for root targeted delivery of fertilizers in forestry operations.

Genetic adaptation and phenotypic plasticity contribute to greater leaf hydraulic tolerance in response to drought in warmer climates

The ability of plants to maintain an intact water transport system in leaves under drought conditions is intimately linked to survival and can been be seen as adaptive in shaping species climatic limits. Large differences in leaf hydraulic vulnerability to drought are known among species from contrasting climates, yet whether this trait varies among populations within a single species and, furthermore, whether it is altered by changes in growth conditions, remain unclear. We examined intraspecific variation in both leaf water transport capacity (Kleaf) and leaf hydraulic vulnerability to drought (P50leaf) among eight populations of Corymbia calophylla (R. Br.) K.D. Hill & L.A.S. Johnson (Myrtaceae) from both cool and warm climatic regions grown reciprocally under two temperature treatments representing the cool and warm edge of the species distribution. Kleaf did not vary between cool and warm-climate populations, nor was it affected by variable growth temperature. In contrast, population origin and growth temperature independently altered P50leaf. Using data pooled across growth temperatures, cool-climate populations showed significantly higher leaf hydraulic vulnerability (P50leaf = -3.55 ± 0.18 MPa) than warm-climate populations (P50leaf = -3.78 ± 0.08 MPa). Across populations, P50leaf decreased as population home-climate temperature increased, but was unrelated to rainfall and aridity. For populations from both cool and warm climatic regions, P50leaf was lower under the warmer growth conditions. These results provide evidence of trait plasticity in leaf hydraulic vulnerability to drought in response to variable growth temperature. Furthermore, they suggest that climate, and in particular temperature, may be a strong selective force in shaping intraspecific variation in leaf hydraulic vulnerability to drought.