Vaccinium corymbosum was selected due to its unique root anatomy and morphology and its reported limited ability to redistribute water. It was hypothesized that root tissue rehydration at night would be delayed, especially in the distal root orders because of their greater hydraulic constraints .The experiment was conducted in November 2005 in a greenhouse at The Pennsylvania State University, University Park, PA, USA. Plant material consisted of 9-year-old V. corymbosum L. ‘Nelson’ plants. At the end of the summer of 2004, individual plants were transplanted to a split-pot system filled with coarse sand . The 40 l pots were separated into two sections using a vertical plastic partition , with the edges sealed with silicone and polyurethane foam. There were five replicate plants. Plants were always watered on the same side of the pot , with the other side not irrigated for the duration of the experiment. Drought conditions were induced gradually over a 5-month period previous to the actual measurements, by exposing plants to decreasing amounts of irrigation that began at 500 ml twice a day, then decreased to 250 ml twice a day, 250 ml once a day, and, finally, to 150 ml once a day. Midday stem water potentials were measured with a pressure chamber on bagged and covered leaves between 12.00 h and 14.00 h and ranged between –2.0 MPa and –2.5 MPa at the lowest level of irrigation. Soil water content was monitored at 09.00 h each day over the drought imposition period and at 20.00 h before the nocturnal patterns of root rehydration measurements by time domain reflectometry .
Moisture probes consisted of three parallel stainless steel rods that were inserted in each side of the pots perpendicular to the soil surface. Typical night-time leaf transpiration for a well-watered blueberry plants, 30 plant pot was about 25-fold lower than that observed during daytime . Greenhouse temperatures ranged between 22C and 30 C during the daytime and between 13 C and 18 C during the nighttime. Supplemental light was provided with three, 400 W halide lamps from 07.00 h to 19.00 h each day.Once shoots exhibited water potentials below –2.0 MPa, patterns of internal hydraulic redistribution in the root system were examined. Measurements were completed on one plant per day. At 18.00 h, one hour after sunset , supplemental lights were turned off and the wet-side was irrigated to field capacity. Nocturnal patterns of root rehydration were determined by sampling the roots at 2, 6, and 11 h after irrigation . Roots were collected by cutting a root section using small scissors, at a distance of 10–15 cm from the centre of the plant, and at a soil depth of 10–15 cm. Once roots were detached from the plant, they were immediately transferred to a humidified chamber to prevent tissue dehydration. After cleaning the roots of sand particles , representative root samples were selected based on three root-order categories. The fine root category consisted of a single root branch that included root orders one to four, where order one consisted of the distal and finest unbranched roots in the root system ; the medium root classification sample consisted of root orders five to seven; and the coarse root classification sample consisted of the distal section of an 8th root order.
Because blueberry has an extremely fine , densely branched root system , it would have been too time-consuming to collect accurate water potential estimates and still precisely separate roots by order in the finest category. As soon as each root sample was collected, it was loaded into a stainless steel chamber with a thermocouple psychrometer . Sampling all root samples for each specific time interval took no more than 30 min. Transferring one root sample to the humidified chamber took less than 5 s, and less than 2 min for cleaning the roots and sealing them into the chambers. As soon as all the psychrometers were loaded, they were transferred to an insulated water bath at 25 C, connected to a CR-7 data logger and measured every 30 min for the next 4 h until they reached temperature and vapour equilibrium. Estimations of root water potentials were based on the average of three measured points .The V. corymbosum root system is characterized as being very efficient in terms of biomass allocation for the production of a large amount of root surface area. Its highly branched root system is composed of very fine roots that usually proliferate in the top 20–30 cm of the soil . Water and nutrient uptake presumably occurs mostly in the first three root orders, which correspond to the mycorrhizal and non-woody, ephemeral section of the root system . These non-woody roots have very few and small-diameter vessels. Such root anatomical characteristics may lead to a high cavitation vulnerability of the distal roots during severe drought, especially when water is available to only a fraction of the root system such as might occur with drip irrigation or when there are only a few deeper roots.
Plants may cope with drought under conditions of heterogeneous soil moisture conditions by internally redistributing water from roots in moist soil to those in dry soil during the night, such as in grape , but to a much more limited extent in blueberry. In the present work, patterns of root water potentials measured throughout the night revealed how water redistributes internally within the root system ofVaccinium. Vaccinium rehydrated the distal first five root orders in dry soil from 60% to 70% in a period of 11 h when well irrigated in another portion of the root system. No other work has previously quantified the rate or magnitude of change of tissue rehydration over the night as water moved through different branch orders in the root system. Root rehydration at night was influenced by several factors, including root hydraulic constraints, duration of the nocturnal period, water availability in the wet-side, and water potential gradients among roots. The overall final rehydration achieved by the distal root orders in dry soil served as an indicator of how efficiently or inefficiently water was hydraulically redistributed through the root system. As suspected, the distal, finest root orders experienced the lowest water potentials at the onset of the nocturnal period, followed by medium and coarse roots. The simulations also demonstrated the importance of the duration of the nocturnal period. Although water was readily available to roots in the wet soil and transpiration was minimal, it took the whole night-time period of 12 h for the distal finest roots under dry soil conditions to reach the same water potentials as fine roots in wet soil. Even though roots in dry soil equilibrated with roots in wet soil, the equilibrium point reached before sunrise was still approximately –1.2 MPa, indicating that the tissues were not fully rehydrated, i.e. not fully in equilibrium with the soil water potential in the wettest portion of the rooted soil. Even with an additional hour and a half of no transpiration, water potentials for all root orders were predicted to range between –0.6 MPa to –0.7 MPa . Therefore, even with the additional time, roots in dry soil would not be predicted to reach values greater than –0.5 MPa, suggesting that the duration of the nocturnal period was not sufficient for roots in dry soil to be fully rehydrated. The main factors influencing water transport were the hydraulic properties of the conductive system. In very fine roots, internal water movement was probably delayed by either very high hydraulic resistances due to small diameter vessels in these roots or by additional resistances caused by the occurrence of xylem embolisms associated with severe water stress conditions . With the single-branch model, grow raspberries in a pot it was possible to estimate the water potential of each of the seven root orders over the night and to identify those orders with the highest hydraulic resistance.
As expected, the magnitude of hydraulic resistance per individual root was highest in 1st-order roots due to these roots having the fewest number of vessels and the smallest vessel diameters. However, it was found that in a root branch composed of seven root orders, 3rd-, 4th-, and 5th-orders , exhibited the highest overall hydraulic resistances within the root branch . Thus, when many resistances are arranged in parallel , the total resistance added to the system was not highest in the 1st- and 2nd-order roots but in the medium root orders, which had fewer individuals within the root branch . In the case of root orders greater than five, although the number of these roots was small, numerous and wider vessels helped to compensate for the limited length and number of roots . Therefore, the high hydraulic resistances exhibited by 3rd-, 4th-, and 5th-order roots may contribute considerably to delayed rehydration of the finest root orders. Interestingly, these roots represented the transition from the more permanent roots with secondary development to the more ephemeral roots without secondary development . The possibility of intermediate-order roots serving as hydraulic controllers has important implications for the function of the whole root system. The observed pattern of hydraulic resistances in Vaccinium roots is consistent with the segmentation concept proposed by Zimmermann for above-ground hydraulic architecture. Similar to the occurrence of embolism within stem junctions, which may cause the sacrifice of minor branches and leaves during severe water stress conditions, hydraulic failure in 5th- or 4th-order roots may lead to the sacrifice of the lower root orders under drought conditions, but the maintenance of higher order roots. In summary, it has been found that under severe water stress conditions the root system of V. corymbosum did not fully redistribute water from roots in wet soil to roots in dry soil. This was mainly attributed to anatomical constraints on water movement and because of the severe degree of water stress of roots in dry soil. Root orders with the highest hydraulic resistances corresponded to the lowest orders of the permanent root system , indicating the possible location of a hydraulic safety control in the root system of this species.Species interactions have received less attention in global change biology than individual species’ responses. In large part, this is because long-term data on species interactions spanning the period of intense anthropogenic environmental change are rare. For example, first flower dates of Japanese cherry blossoms have been recorded in diaries since the ninth century, but we have no equivalent long-term records of cherry tree pollination, leaf microbial communities, or disease incidence. Data describing species interactions are laborious to collect and, in many cases, require technology, such as electron microscopy or DNA sequencing, that was not available until recent years. The lack of long-term data inhibits assessment of how species interactions are impacted by global change and limits our ability to determine how these effects mediate individual species’ distributions, abundances, and ecologies. Variation in species responses to global change has generated concern that interactions which were tightly coupled historically might become decoupled owing to phenological asynchronies. Phenological asynchrony arises when interacting species respond differently to global change—for example, if earlier flowering as a consequence of global warming is not matched by earlier pollinator emergence. Recent meta-analyses have suggested that phenological sensitivity to climate change differs among trophic levels, with lower trophic levels advancing more than higher trophic levels. In one well-documented example, great tit reproduction did not advance in sync with peak food availability for young, leading to potential fitness costs. Similar changes in interactions between trophic levels may happen as a consequence of other differential responses to global change, such as spatial mismatches between species whose ranges expand poleward or upward in elevation to different extents. While predictions for phenological and spatial asynchronies are clear, empirical data are sparse, and there is still no consensus on whether asynchrony is common or rare, or which traits regulate when asynchronies arise. In the absence of long-term observational data, global change biologists increasingly mine museum collections to investigate how species interactions have shifted over time . Diverse types of data are available only in natural history collections, and they could therefore have wide applicability in global change biology. While natural history specimens are not collected systematically—and their use in ecological and evolutionary research can present challenges—they represent time-series data across much of the globe, span the tree of life, and may be able to fill gaps in species interactions data.