Phytohormonal effects on the regulation of stem elongation of pea (Pisum sativum) subjected to drying soil

Mohd Zan, Noorliana and Dodd, Ian and McAinsh, Martin (2019) Phytohormonal effects on the regulation of stem elongation of pea (Pisum sativum) subjected to drying soil. PhD thesis, Lancaster University.

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Abstract

To understand whether phytohormones regulate stem elongation of plants grown in drying soil, endogenous phytohormone concentrations in growing internodes were measured along with stem elongation, soil water content (θ) and stem water potential (Ψstem) in tall pea (Pisum sativum cv. Alderman) plants that were well watered or exposed to drying soil. The hormones quantified were abscisic acid (ABA), gibberellins (GA1, GA3, GA4), auxin (IAA), cytokinins (iP, tZ), ethylene (actually its precursor ACC), jasmonic acid (JA) and salicylic acid (SA). After withholding water, stem phytohormone concentrations (ABA, iP, GA3) changed earlier (Days 3, 4) than changes in stem elongation (Day 5) and Ψstem (Day 7). Furthermore, ABA, tZ, iP and ACC concentrations were negatively related to stem elongation, while IAA, GA1 and GA3 concentrations were positively related to stem elongation. Since ABA was amongst the first hormone to respond to drying soil, and accumulated to the highest levels with soil drying, its role was further assessed by measuring responses of wild-type (WT) and wilty (ABA-deficient) peas to soil drying in a factorial experiment with moderate (50%) and high (95%) relative humidity. Since ABA-deficient plants have higher stomatal conductance, growth at high humidity aimed to attenuate any decline in plant water status. High humidity slowed the soil-drying induced decrease in Ψstem, θ, stem elongation and evapotranspiration rate of both genotypes. With soil drying, wilty had a similar leaf expansion rate as WT plants at high humidity, but stem elongation was 28% less. WT plants accumulated more ABA in growing leaves and stems than wilty plants, especially at moderate humidity. Although stem and leaf tissue ABA levels increased with soil drying and were correlated with stem elongation and leaf respectively, there was no unifying relationship across genotypes and relative humidities. Lower stem elongation of wilty indicates that ABA maintains stem elongation and leaf expansion, irrespective of soil drying. Thus, further experiments evaluated the role of endogenous gibberellins and auxin (IAA) concentrations in limiting growth, by independently applying GA3 (modified to the active form GA1 in pea) and IAA as foliar sprays to plants grown in drying soil. Although soil drying decreased θ, Ψstem and stomatal conductance (gs), exogenous GA3 and IAA accelerated the declined in θ, while Ψstem decreased (after the 2nd spray) and increased (after the 3rd spray), respectively. In addition, GA3 and IAA enhanced gs and stem elongation of plants grown in drying soil but did not alter leaf expansion. For plants grown in drying soil, exogenous GA3 (0.03-0.1 mM) increased stem elongation (although had no effect in well-watered plants), while exogenous IAA (0.05-0.1 mM) increased gs, with both GA3 and IAA increasing gs of well-watered plants. This research suggests that different phytohormones have quantitatively different effects on stem elongation, gs and leaf expansion, but play an important role in regulating stem elongation responses to soil drying.