A relook into plant wilting: observational evidence based on unsaturated soil-plant-photosynthesis interaction

Abstract

Permanent wilting point (PWP) is generally used to ascertain plant resistance against abiotic drought stress and designated as the soil water content (θ) corresponding to soil suction (ψ) at 1500 kPa obtained from the soil water retention curve. Determination of PWP based on only pre-assumed ψ may not represent true wilting condition for soils with contrasting water retention abilities. In addition to ψ, there is a need to explore significance of additional plant parameters (i.e., stomatal conductance and photosynthetic status) in determining PWP. This study introduces a new framework for determining PWP by integrating plant leaf response and ψ during drought. Axonopus compressus were grown in two distinct textured soils (clayey loam and silty sand), after which drought was initiated till wilting. Thereafter, ψ and θ within the root zone were measured along with corresponding leaf stomatal conductance and photosynthetic status. It was found that coarse textured silty sand causes wilting at much lower ψ (≈ 300 kPa) than clayey loam (≈ 1600 kPa). Plant response to drought was dependent on the relative porosity and mineralogy of the soil, which governs the ease at which roots can grow, assimilate soil O₂, and uptake water. For clay loam, the held water within the soil matrix does not facilitate easy root water uptake by relatively coarse root morphology. Contrastingly, fine root hair formation in silty sand facilitated higher plant water uptake and doubled the plant survival time.

Figure 1

Schematic diagram depicting (a) stomatal functioning, initiated by transport of root water uptake and nutrient to plant leaf. The steps in sequence represents transpiration mechanism, indicated by loss of water via opening and closure of stomata across the leaf section; (b) light dependent photosynthesis process. Photosystems (groups of photosynthetic pigments (including chlorophyll pigment molecules) embedded within the thylakoid membrane) absorbs light energy to energize the delocalized electrons. The energized electrons are transferred to carrier molecules within the thylakoid membrane (electron transfer chain) for further ATP production (phosphorylation) and reduction of NADP+ .

Figure 2

Pictorial presentation of (a) Vegetated columns with instrumentation to monitor soil hydrological parameters, stomatal conductance and in-house temperature; (b) sensors used in the study to measure matric water potential (ψ) and volumetric water content (θ) of the soil medium; (c) photosynthesis measurement setup using DLC-8 dark leaf clip and MINI-PAM-II; (d) ensuring the optimal light intensity for plant growth using lux meter.

Figure 3

Typical photosynthetic parameters derived from (a) light curve and (b) induction curve. The curves were obtained directly from MINI-PAM-II window settings and are individually analyzed to derive the parameters.

Figure 4

(a) Particle size gradation of the selected soils showing higher number of retained particles ranging between 0.001 and 0.01 mm for clay loam, and 0.1–1 mm for silty sand; (b) soil water characteristic curve of vegetated soil columns depicting higher retention at clay loam than silty sand. Also, the matric suction of 1500 kPa determined by Veihmeyer and Hendrickson (1928) and Feddes (1982) is marked to represent the traditionally considered permanent wilting point.

Figure 5

Drought response of vegetated soil columns presented by (a) stomatal conductance with the wilting point determined at both clay loam and clay loam (i.e. 1600 kPa and 263 kPa respectively). Also, the stomata images at different drought progressions are shown; (b) pictorial images of leaf under continued drought stage at different suction stresses.

Figure 6

Pictorial description of excavated roots of the grass species depicting root length and fine “root hairs”.

Figure 7

Photosynthetic parameters (a) photosynthetic efficiency; (b) maximum CO2 fixation rate per unit cell; (c) maximum yield; (d) effective yield vs suction plots under drought progression. The permanent wilting points determined at all the parameters are observed as 323 kPa and 1600 kPa for silty sand and clay loam, respectively.

Figure 8

(a) Dry root and shoot characteristics of the vegetation in the tested soils. (b) Root architecture of the vegetation presented in the form of normalized root area index (i.e. Column area/Root area). (c) Plant available water content and time till wilting for A. compressus.