An investigation of abscisic acid as a chemical signal of root-restriction

Abstract

Physically restricting the rooting volume (RV) available to a plant will eventually reduce plant growth. Changes within the soil environment of a root-restricted plant are thought to be perceived by the root-system, resulting in de novo changes within the root system. These changes within the functioning of the root-system are then either directly or non-directly communicated to the rest of the plant via hydraulic (water) or non-hydraulic (chemical) signals. This transduction process results in a well-characterised dwarfing response, which bears a number of similarities to the dwarfing response shown by a plant acclimatising to drought.

The intention of this research was to ascertain if abscisic acid (ABA) formed the chemical basis for a transduction mechanism, whereby changes in the soil associated with physically restricting the RV result in changes in communication processes between the root and shoot. This was achieved by investigating the response of tomato plants (Lycopersicon esculentum Mill. cv. Red Dwarf) and changes within the soil environment (50:50 v/v vermiculite/perlite) during root volume restriction (RVR) at three rooting volumes (200 cm³, 400 cm³ or 800 cm³).

Significant variations in the matric potential (Ψm) and temperature of the soil within different pots of the same size and also between pots of different sizes were observed. The root-systems of tomato plants grown in 200 cm³ sized pots experienced a lower mean soil Ψm of -14 kPa (compared with -7 kPa in the 800 cm³ sized pots), greater rates of decrease in soil Ψm and lower overall soil Ψm magnitudes of -30 kPa (compared with -12 kPa in the 800 cm³ sized pots). All pots were also subject to large diurnal soil temperature fluctuations (range = 12°C). The root-systems of plants grown in the 200 cm3 sized pots were consistently 2°C higher (mean 200 cm³ temperature = 25.4°C, mean 800 cm³ temperature = 22.3°C) at all times of the 24 hour cycle. Enclosing the pots within a water-based heat-exchange system and incorporation of a very frequent watering regime (96 events per 24 hour cycle) eliminated differences in soil Ψm and reduced differences in soil temperature between pots of different sizes.

The 2-cis, 4-trans enantiomer of ABA occurs naturally within plants and has been widely implicated as a general "stress hormone". During periods of stress, the affects of this hormone enable the plant to mediate short-term and long-term environmental pressures with metabolic demands, especially regulation of water relations during periods of water stress. To this end, an accurate, precise and economical ABA assaying capability was developed. The suitability of a small C₁₈ column (35 mm in length, 3 µm diameter packing matrix) for the quantitation of ABA was assessed. Although there were a number of theoretical advantages in using the particular type of analytical column, this was the first time this type of column had been used for hormonal assaying. The column had a retention time (tᵣ) of 105 sec and a minimum mass sensitivity (m) of 500 pg compared with a standard C₁₈ column where tᵣ=1920 sec and m=2500 pg, respectively. The tᵣ and m for the small C₁₈ column appeared to be the lowest reported figures currently available for the quantitation of ABA using a C₁₈ column, and offered considerable advantages in terms of solvent consumption, reduced analysis time and increased mass sensitivity. This type of column is ideally suited for the quantitation of ABA where the resolution requirements do not exceed about 5000 plates.

Resistance to root growth, measured as mechanical impedance (MI) with a soil penetrometer, was lowest (0.41 MPa) in the largest RV, increasing to 0.66 MPa in the smallest RV. Final dry weight of plants showed substantial growth reductions from 132g (RV=800 cm³) to 80.5 g (RV=200 cm³), with a concomitant decrease in stomatal conductance from 350 mmolm⁻²s⁻¹ to 260 mmolm⁻²s⁻¹ respectively, even though leaf Ψw was unaffected by RVR and all leaves were similarly well-hydrated at about -0.5 MPa. The concentration of ABA within the root system ([ABA]rt) and xylem sap ([ABA ]xy) varied inversely with RVR, decreasing from a high of 90 ngABAgfwt⁻¹ and 26 ngABAml⁻¹, down to a plateau of about 30 ngABAgfwt⁻¹ and 10 ngABAml⁻³ respectively.

Conflicting results and ideas from other researchers relating to ABA as a transducer of MI and to the role of ABA during RVR are discussed. It is suggested that the root tips may perceive an increase in MI within the soil environment of the restricted root system, resulting in the initiation of a cascade of drought acclimatising mechanisms. The initiation of these mechanisms may be regulated by a transduction mechanism that is [ABA]xy mediated, thereby supporting the proposed role of ABA during RVR.