Water Stress in Dwarfing Cherry Rootstocks: Increased Carbon Partitioning to Roots Facilitates Improved Tolerance of Drought

Cherry orchards are transitioning to high-density plantings and dwarfing rootstocks to maximize production, but the response of these rootstocks to drought stress is poorly characterized. We used a 16-container, automated lysimeter system to apply repeated water stress to ungrafted Krymsk® 5 and 6 rootstocks during two growing cycles. Drought stress was imposed by withholding irrigation until the daily transpiration rate of each tree was 25% and 30% of the unstressed rate during the first trial and second trial, respectively. After this point was reached, the root-zone water status was restored to field capacity. Whole-tree transpiration measurements were supplemented with leaf-level gas-exchange measurements. Krymsk® 6 had a higher rate of photosynthesis, more vigorous vegetative growth and less conservative stomatal regulation during incipient drought than Krymsk® 5. At harvest, carbon partitioning to roots was greater in Krymsk® 6 than Krymsk® 5. The conservative rate of water use in Krymsk® 5 could be a function of greater stomatal control or reduced carbon partitioning to roots, which thereby limited transpiration rates. Further studies are needed to confirm that these results are applicable to trees grown using a common grafted scion under field conditions.

Transpiration Rate of White Clover (Trifolium repens L.) Cultivars in Drying Soil

Determining the performance of white clover cultivars under drought conditions is critical in dry climates. However, comparing the differences in cultivar performance requires equivalent soil water content for all plants, to reduce the water deficit threshold eliciting stomatal closure. In this study, the objective was to compare the rate of stomatal closure in eighty white clover cultivars in response to soil drying. Two glasshouse experiments were conducted, and the daily transpiration rate was measured by weighing each pot. The transpiration rate of the drought-stressed plants were normalized against the control plants to minimize effects from transpiration fluctuations and was recorded as the normalized transpiration rate (NTR). The daily soil water content was expressed as the fraction of transpirable soil water (FTSW). The FTSW threshold (FTSWc) was estimated after which the NTR decreases linearly. The FTSWc marks the critical point where the stomata start to close, and transpiration decreases linearly. The significant difference (p < 0.05) between the 10 cultivars with the highest and lowest FTSWc demonstrates the cultivars would perform better in short- or long-term droughts.

Modeling the Physiological Responses of a Desert Shrub to Rainfall Pulses in an Arid Environment

<p>A physically-based model for soil-plant-atmosphere continuum (SPAC) is parameterized and evaluated against field-measured physiological responses of a desert shrub, Haloxylon Ammodendron (HA), to rainfall pulses in a desert environment in northwestern China. Despite its simplicity, the model was successfully employed to assess the complexity and uncertainty involved in the physiological responses of HA following pulsed rainfall events. Through modelling efforts, we report a systematic evaluation of the non-linear relationship between the physiological responses of HA and pulse magnitude or antecedent moisture. The results show that following the rainfall pulses, the modeled daily transpiration and assimilation rates either stayed the same or decreased monotonically with water stress. However, the stomatal conductance (g<sub>s</sub>) and photosynthetic rate (A<sub>n</sub>) responses were relatively weaker when compared to the increase in water potential. We found that rainfall events with <5 mm cannot induce any substantial response of A<sub>n</sub> (Δ<4μmol m<sup>-2</sup>s<sup>-1</sup>), and at least 13 mm of rain is required to increase A<sub>n</sub> by 10 μmol m<sup>-2</sup>s<sup>-1</sup>. Significant responses of water use efficiency (WUE) were not even discernible from viewing the simulation. Our analysis reproduced the judgements with a certain uncertainty that HA is basically a kind of drought resistant species, it tends to have a more conservative water-use strategy and thus a safer photosynthetic behavior. The inverse–texture hypothesis is much more clearly supported by the modeling experiments, suggesting that soil texture drives differences in the effects of pulses on the magnitude and sensitivity of the physiological responses of plants, and the interaction between rainfall and soil texture may lead to the preferred acquisition and use of pulsed precipitation by HA. The modelling work and findings in this study is likely to shed light on the quantitative understanding of the physiological behavior of other plants in water-limited environments.</p>

A Hyperspectral-Physiological Phenomics System: Measuring Diurnal Transpiration Rates and Diurnal Reflectance

A novel hyperspectral-physiological system that monitors plants dynamic response to abiotic alterations was developed. The system is a sensor-to-plant platform which can determine the optimal time of day during which physiological traits can be successfully identified via spectral means. The directly measured traits include momentary and daily transpiration rates throughout the daytime and daily and periodical plant weight loss and gain. The system monitored and evaluated pepper plants response to varying levels of potassium fertilization. Significant momentary transpiration rates differences were found between the treatments during 07:00–10:00 and 14:00–17:00. The simultaneous frequently measured high-resolution spectral data provided the means to correlate the two measured data sets. Significant correlation coefficients between the spectra and momentary transpiration rates resulted with a selection of three bands (ρ523, ρ697 and ρ818nm) that were used to capture transpiration rate differences using a normalized difference formula during the morning, noon and the afternoon. These differences also indicated that the best results are not always obtained when spectral (remote or proximal) measurements are typically preformed around noon (when solar illumination is the highest). Valuable information can be obtained when the spectral measurements are timed according to the plants’ dynamic physiological status throughout the day, which may vary among plant species and should be considered when planning remote sensing data acquisition.

Differences in Drought Tolerance among Gisela® Cherry Rootstocks Determined Using Automated Weighing Lysimeters

The Gisela® series of dwarfing rootstock are widely used because they enable high-density production, but they may be sensitive to drought. Drought tolerance may be associated with root-zone distribution and depth or with physiological adaptation to low water potential. Here we describe a novel technique for determining physiological tolerance to drought when root distribution is held constant. In two matching studies, we continuously measured transpiration of two groups of eight trees using a 16-container automated weighing lysimeter system in a greenhouse. With this system, Gisela® 3, 5, and 12 (G.3, G.5, and G.12) rootstocks were subjected to multiple, controlled drought cycles based on reductions in whole-tree transpiration. To provide an equivalent amount of stress for each tree, water was withheld until the daily transpiration rate had decreased to less than 250 g of water transpired per tree per day. Each tree was then drip-irrigated to bring the root-zone back to about field capacity. G.3 and G.12 rootstocks more rapidly recovered to maximum transpiration rates compared with G.5 (an indication of ability to resume normal growth after a drought). At harvest, G.3 and G.12 rootstocks also had greater leaf area and trunk diameter. Both transpiration data and harvest data indicate physiological differences among rootstocks. Because root-zone volume was constant, these differences are not associated with changes in root distribution or depth. These data indicate that G.5 is less adapted for regulated deficit irrigation strategies that include long irrigation intervals.

Estimation of beech tree transpiration in relation to their social status in forest stand

The results of sap flow continuous measurements by a tree-trunk heat balance method (THB) on beech model trees are analysed in this paper. Experimental research works were carried out in a mature mixed fir-spruce-beech stand in the research area Poľana – Hukavský grúň (j= 48o39´, l = 19o29´, H = 850 m a.s.l.) in UNESCO Biosphere Reserve on two co-dominant and one sub-dominant beech trees. A mathematical model of daily transpiration dynamics was proposed for <br />a quantitative analysis of the daily course of sap flow intensity. The model works on a one-tree level and enables to consider the influence of the tree social position in the stand on the sap flow intensity of model beech trees and to express the dependence of sap flow intensity on the tree height and crown projection.

Transpiration and Drought Stress Recovery of Three Zinnia Cultivars1

Transpiration and drought stress recovery were investigated in three container-grown zinnia cultivars [Zinnia elegans Jacq. (‘Lilliput' and ‘Thumbelina') and Z. haageana Reger (‘Persian Carpet')] by measuring daily changes in the normalized transpiration ratio (NTR) of well-watered (control), water-stressed, and water-stressed/re-watered plants. Transpiration of plants grown in gradually drying substrate did not decline until the fraction of transpirable substrate water (FTSW) reached 0.16 to 0.12. Symptoms of plant-water stress (i.e. foliar wilt) were first observed on the leaves of ‘Persian Carpet', which was also the cultivar with the highest average daily transpiration rate. By comparison, the remaining two cultivars (‘Lilliput' and ‘Thumbelina') exhibited lower average daily transpiration rates and took significantly longer to reach the same dry-down endpoint (NTR ≤0.15). Drought stress recovery was assessed by comparing xylem water potential and root and shoot dry weight in well-watered and in drought-stressed plants following a 7-day stress amelioration period. Xylem water potential of all three drought-stressed cultivars increased (i.e. became less negative) one week after re-watering. Root biomass and root:shoot ratio were both significantly greater in water-stressed plants than in well-watered plants of the same cultivar, a finding that suggests the likelihood of osmotic adjustment in response to drought. Index words: normalized transpiration ratio, fraction of transpirable substrate water, foliar wilt, containerized horticultural crops. Species used in this study: ‘Lilliput' and ‘Thumbelina' zinnia (Zinnia elegans Jacq.), ‘Persian Carpet' zinnia (Zinnia haageana Reger).

Luxury transpiration of winter wheat and its responses to deficit irrigation in North China Plain

Reducing crop luxury transpiration is an important step in improving water productivity; water shortage regions are potential hotspots for studying physiological water conservation. This study investigated the amount of luxury transpiration in winter wheat and its responses to different irrigation treatments in North China Plain. The results showed that luxury transpiration existed and increased with growth of winter wheat and after rainfall. In each sampling day, the amount of luxury transpiration under full irrigation was significantly higher than that under deficit irrigation. The average amount of luxury transpiration was 258.87 g/m² under full irrigation, and 125.18 g/m² under deficit irrigation during the experimental period. Although the amount of luxury transpiration was 2.09-fold higher under full irrigation than that in deficit irrigation, the water loss ratio due to luxury transpiration in deficit irrigation (8.13%) was significantly higher than that in full irrigation (6.75%). Furthermore, the ratio between luxury transpiration amount and crop daily transpiration was revealed in all sampling dates. Therefore, deficit irrigation should be generalized in the water shortage area, because it can save irrigation water and reduce the amount of luxury transpiration. Full irrigation should be carried out in the water abundant region mainly for higher production.