Abstract
High relative humidity (RH) perturbs plant growth, stomatal functioning and abscisic acid (ABA) homeostasis, but the role of ABA in this physiological regulation is equivocal. To determine the role(s) of ABA in plant responses to high RH, wild-type (WT) tomato and barley plants and their respective ABA-deficient mutants flacca and Az34 (which are mutated in the same locus of the ABA biosynthesis pathway) were grown in contrasting RHs (60% and 90%) to measure biomass partitioning, stomatal traits and water relations. Surprisingly, growth RH did not affect foliar ABA levels in either species. While Az34 showed similar stomatal size and density as WT plants, flacca had larger and more abundant stomata. High RH increased stomatal size in tomato, but decreased it in barley, and decreased stomatal density in tomato, but not in barley. Altered stomatal responses in ABA-deficient plants to high RH had little effect on tomato photosynthesis, but Az34 barley showed lower photosynthesis. ABA deficiency decreased relative shoot growth rate (RGRSHOOT ) in both species, yet this was counteracted by high RH increasing leaf water status in tomato, but not in barley. High RH increased RGRSHOOT in flacca, but not in WT tomatoes, while having no effect on RGRSHOOT in barley, but affecting barley net assimilation rate, leaf area ratio (LAR) and specific leaf area in an ABA-dependent manner. ABA-RH interaction affected leaf development in tomato only. Thus, different crop species show variable responses to both high RH and ABA deficiency, making it difficult to generalise on the role of ABA in growth regulation at contrasting RHs.
Highlights
Plant responses to low air relative humidity (RH, corresponding to high vapour pressure deficit, VPD, provided no change in temperature) are important to prevent excessive water loss, yet responses to high RH (> 85%) (Torre et al, 2003) are arguably as important
The barley mutant, Az34, was initially characterised as a nitrate reductase (NR)-deficient mutant, but the nar2 locus codes for the same molybdenum cofactor of the molybdoenzyme aldehyde oxidase (AO) (Walker-Simmons et al, 1989), indicating that both flacca and Az34 are deficient in the enzyme which catalyses the conversion of abscisic aldehyde to abscisic acid (ABA) in the final step of ABA biosynthesis (Bauer et al, 2013; McAdam et al, 2015; Sagi et al, 2002)
These results show that tomato and barley WT and ABA-deficient mutants respond to high RH
Summary
Plant responses to low air relative humidity (RH, corresponding to high vapour pressure deficit, VPD, provided no change in temperature) are important to prevent excessive water loss, yet responses to high RH (> 85%) (Torre et al, 2003) are arguably as important. To compensate for the high rates of water loss in the mutants, the ABA-deficient and the WT plants can be grown at different RHs to ensure the effects of ABA deficiency are compared between leaves of the same RWC and/or leaf water potential (Mäkelä et al, 2003; Okamoto et al, 2009; Sharp et al, 2000; Yaaran et al, 2019). Since growth in high RH affects plant morphology and water relations (Fanourakis et al, 2016; Innes et al, 2018; Innes et al, 2019; Torre et al, 2003), and high RH decreases ABA concentration (Aliniaeifard et al, 2014; Arve et al, 2013; Fanourakis et al, 2011; Okamoto et al, 2009), separating the effects of these two main factors is important but has not been previously investigated. The plants were watered daily to drip point and were kept in the greenhouse for 14 days
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