Exposing the hidden half: root research at the forefront of science

Plant and Soil - Soil organic matter (SOM) supports multiple soil ecosystem functions, underpinned by processes such as C sequestration, N mineralization, aggregation, promotion of plant health and...     

Above and below ground development of Acacia saligna shrubs grown under different irrigation frequencies in an arid environment

Many arid and semiarid areas of the world remain barren due to the lack of water even though the use of ephemeral local water sources, such as runoff water, could change the biomass production patterns. The objective of the present study was to determine the effects of levels of water application and modes of application on the biomass below and above ground development of mature Acacia saligna (Labill.) H.L. Wendl. shrubs. The application treatments were: flooded once a year, low and high frequency irrigation with and without an additional annual flooding, and well-watered. An analysis of the standing biomass data after 5 years of growth indicated that the yearly runoff floods contributed significantly to increase the total above ground biomass. The data collected during the 1999 season, indicated however that during this season flooding had no significant effect on any of the measured growth parameters, while application frequency had a significant effect on the growth rates, water use efficiency and leaf and stem production of shrubs. Below ground, two periods of root growth were observed for the higher irrigation frequency treatments: an initial moderate increase followed by a rapid reduction. The period of rapid reduction in root biomass matched well with the period of rapid increments in above ground biomass production. Furthermore, increased irrigation frequencies resulted in bigger root systems but for lower irrigation frequencies rooting depth increased.     

Phenotypic plasticity and water flux rates of Citrus root orders under salinity

Knowledge about the root system structure and the uptake efficiency of root orders is critical to understand the adaptive plasticity of plants towards salt stress. Thus, this study describes the phenological and physiological plasticity of Citrus volkameriana rootstocks under severe NaCl stress on the level of root orders. Phenotypic root traits known to influence uptake processes, for example frequency of root orders, specific root area, cortical thickness, and xylem traits, did not change homogeneously throughout the root system, but changes after 6 months under 90 mM NaCl stress were root order specific. Chloride accumulation significantly increased with decreasing root order, and the Cl(-) concentration in lower root orders exceeded those in leaves. Water flux densities of first-order roots decreased to <20% under salinity and did not recover after stress release. The water flux densities of higher root orders changed marginally under salinity and increased 2- to 6-fold in second and third root orders after short-term stress release. Changes in root order frequency, morphology, and anatomy indicate rapid and major modification of C. volkameriana root systems under salt stress. Reduced water uptake under salinity was related to changes of water flux densities among root orders and to reduced root surface areas. The importance of root orders for water uptake changed under salinity from root tips towards higher root orders. The root order-specific changes reflect differences in vulnerability (indicated by the salt accumulation) and ontogenetic status, and point to functional differences among root orders under high salinity.     

Influence of saline drip-irrigation on fine root and sap-flow densities of two mature olive varieties

Salt stress is known to influence water use and carbon allocation in trees; however, information about the effects of salt exposure on water uptake and below-ground carbon investment is scant, especially for adult trees. Consequently, this study examined these variables in two mature olive varieties (Olea europaea L.) that differ in their NaCl tolerance: Barnea (tolerant) and Proline (sensitive). Trees were irrigated using water with electrical conductivities of 1.2, 4.2 (both varieties) and 7.5 dS m⁻¹ (Barnea only) for 11 years. At each treatment level, we measured soil properties, root morphology, root biomass:necromass ratio, root xylem sap osmolality, and root sap-flow as well as leaf conductance and morphology. Both varieties exhibited reduced fine root biomass under salinity which was only partially compensated for by higher specific root areas under moderate salinity. Proline variety exhibited a smaller fine root system under moderate salinity than Barnea trees, likely causing the lower sap-flow density in coarse roots of Proline compared to Barnea. The higher biomass:necromass ratio of the Barnea root system under moderate salinity is indicative of lower root turnover rates and thus a more efficient carbon use than in Proline trees. Besides differences in ion exclusion capacities, the ability of the fine root system to resist the deleterious effects of salinity seems to affect the salt resistance of mature olive varieties by influencing water uptake and carbon allocation.     

Calibration of minirhizotron readings against root length density data obtained from soil cores

The minirhiozotron (MR) root observation method was studied versus root length density (RLD) obtained from soil cores. Two plant species, acacia (Acacia saligna) and wheat (Triticum aestivum L.) were grown in a 1-m³ container on Silt Loam (Typic Torrifluvent) and on fine dune sand (Typic Torripsamment), respectively. Roots of both plants were measured periodically by the two methods. The MR observation tubes (MROT) were inserted, either vertically or at 45°. The correlation between the number of roots obtained by the MR and RLD was significant for the entire profile. However, an appreciable error in root estimation by the MR root observation method at the upper 10-cm soil might occur. No significant difference was obtained from MROT oriented vertically or at 45°. The differences between the correlation coefficients of the two methods were not significant, for both plants and soils, indicating that this correlation expresses the geometry of the two measurement systems, not affected by plant or soil types. We concluded that the MR method may be used as an in situ, non-destructive root measuring method with reasonable confidence. This revised version was published online in June 2006 with corrections to the Cover Date.     

Lower leaf gas‐exchange and higher photorespiration of treated wastewater irrigated Citrus trees is modulated by soil type and climate

Water quality, soil and climate can interact to limit photosynthesis and to increase photooxidative damage in sensitive plants. This research compared diffusive and non‐diffusive limitations to photosynthesis as well as photorespiration of leaves of grapefruit trees in heavy clay and sandy soils having a previous history of treated wastewater (TWW) irrigation for >10 years, with different water qualities [fresh water (FW) vs TWW and sodium amended treated wastewater (TWW + Na)] in two arid climates (summer vs winter) and in orchard and lysimeter experiments. TWW irrigation increased salts (Na⁺ and Cl⁻), membrane leakage, proline and soluble sugar content, and decreased osmotic potentials in leaves of all experiments. Reduced leaf growth and higher stomatal and non‐stomatal (i.e. mesophyll) limitations were found in summer and on clay soil for TWW and TWW + Na treatments in comparison to winter, sandy soil and FW irrigation, respectively. Stomatal closure, lower chlorophyll content and altered Rubisco activity are probable causes of higher limitations. On the other hand, non‐photochemical quenching, an alternative energy dissipation pathway, was only influenced by water quality, independent of soil type and season. Furthermore, light and CO₂ response curves were investigated for other possible causes of higher non‐stomatal limitation. A higher proportion of non‐cyclic electrons were directed to the O₂ dependent pathway, and a higher proportion of electrons were diverted to photorespiration in summer than in winter. In conclusion, both diffusive and non‐diffusive limitations contribute to the lower photosynthetic performance of leaves following TWW irrigation, and the response depends on soil type and environmental factors.     

Correction to: Exposing the hidden half: root research at the forefront of science

Plant and Soil - In the published version of this editorial paper, the sentence in the first paragraph should be corrected as shown below.     

A model for nutrient and water flow and their uptake by plants grown in a soilless culture

The objective of this study was to develop a sensitive means of control to optimize nutrient concentrations in the root zone of a soilless system, considering plant water and nutrient uptake, and solution circulation rates. A model is proposed to simulate ornamental plants growth in a channel with a non-interacting soilless substrate, irrigated by point sources with constant discharge rates, spaced uniformly along the channel. The model accounts for compensation for transpiration water losses and consequent salinity buildup, and its interactions with plant growth and nutrient uptake. The added water may contain given concentrations of nutrients and/or toxic (saline) compounds, which would cause salinity buildup. Uptake of each solute is specific, according to a Michaelis–Menten kinetics mechanism, but passive uptake by the transpiration stream is also accounted for. Plant growth is affected by time/age and ionic balance in the solution. The model was calibrated with lettuce (Lactuca sativa L.) plants grown in volcanic ash. Simulation of potassium concentration change as a result of discharge rate and emitter spacing revealed that the two parameters could compensate one for the other, once a target lower limit is set. Potassium appeared to be most sensitive to sodium accumulation in the growth medium; this accumulation changed ionic concentration balance, which affected pH and bicarbonate concentration. Passive uptake of calcium by the transpiration stream is highly affected by the root fraction involved, but its calculated contribution is below published values is highly affected by the root fraction involved, but its calculated contribution is below published values.     

A new method for in-situ monitoring of the underground development of Orobanche cumana in sunflower (Helianthus annuus) with a mini-rhizotron

AIMS: To develop an in-situ, non-destructive method for observation and monitoring of the underground developmental stages of the root parasite Orobanche cumana. SCOPE: The parasitic weed Orobanche causes severe damage to vegetables and field crops. Most of the damage caused to the crops occurs during the underground, unobservable parasitism stage. Sunflower (Helianthus annuus 'Adi') plants were planted in soil that was artificially inoculated with O. cumana seeds. Clear Plexiglas mini-rhizotron plastic observation tubes were inserted into the soil. Seed germination, early stage of penetration, and formation of tubercles and spikes were observed non-destructively and were monitored throughout the growing season by mean of a mini-rhizotron camera. Use of this technology enabled the complete individual parasite life cycle from the very early development (including germination) to Orobanche shoot to be monitored. In addition, the effect of the systemic herbicide Cadre (imazapic) on the development of O. cumana was inspected and quantified. CONCLUSIONS: This novel methodology facilitates the in-situ study of major aspects of the host-parasite interaction and of parasite suppression, such as parasitism dynamics, parasite growth rate, and the effect of chemical treatments on the parasite.     

Phelipanche aegyptiaca parasitism impairs salinity tolerance in young leaves of tomato

The parasite Phelipanche aegyptiaca infests tomato, a crop plant that is commonly cultivated in semi‐arid environments, where tomato may be subject to salt stress. Since the relationship between the two stresses —salinity and parasitism – has been poorly investigated in tomato, the effects of P. aegyptiaca parasitism on tomato growing under moderate salinity were examined. Tomatoes were grown with regular or saline water irrigation (3 and 45 mM Cl^?, respectively) in soils infested with P. aegyptiaca . The infested plants accumulated higher levels of sodium and chloride ions in the roots, shoots and leaves (old and young) under both salinity levels vs. non‐infected plants. There was a positive linear correlation between P. aegyptiaca biomass and salt accumulation in young tomato leaves, and a negative linear correlation between parasite biomass and the osmotic potential of young tomato leaves. Concentrations of the osmoprotectants proline, myoinositol and sucrose were reduced in infected tomato plants, which impaired the host's osmotic adjustment ability. The sensitivity of P. aegyptiaca to salt stress was manifested as a decrease in biomass. In conclusion, P. aegyptiaca parasitism reduced the salt tolerance of tomato plants by promoting the accumulation of salts from the rhizosphere and impairing the host's osmotic adjustment ability.