Increased Ratio of Electron Transport to Net Assimilation Rate Supports Elevated Isoprenoid Emission Rate in Eucalypts under Drought    

Abstract

Plants undergoing heat and low-CO 2 stresses emit large amounts of volatile isoprenoids compared with those in stress-free conditions. One hypothesis posits that the balance between reducing power availability and its use in carbon assimilation determines constitutive isoprenoid emission rates in plants and potentially even their maximum emission capacity under brief periods of stress. To test this, we used abiotic stresses to manipulate the availability of reducing power. Specifically, we examined the effects of mild to severe drought on photosynthetic electron transport rate (ETR) and net carbon assimilation rate (NAR) and the relationship between estimated energy pools and constitutive volatile isoprenoid emission rates in two species of eucalypts: Eucalyptus occidentalis (drought tolerant) and Eucalyptus camaldulensis (drought sensitive). Isoprenoid emission rates were insensitive to mild drought, and the rates increased when the decline in NAR reached a certain species-specific threshold. ETR was sustained under drought and the ETR-NAR ratio increased, driving constitutive isoprenoid emission until severe drought caused carbon limitation of the methylerythritol phosphate pathway. The estimated residual reducing power unused for carbon assimilation, based on the energetic status model, significantly correlated with constitutive isoprenoid emission rates across gradients of drought (r 2 . 0.8) and photorespiratory stress (r 2 . 0.9). Carbon availability could critically limit emission rates under severe drought and photorespiratory stresses. Under most instances of moderate abiotic stress levels, increased isoprenoid emission rates compete with photorespiration for the residual reducing power not invested in carbon assimilation. A similar mechanism also explains the individual positive effects of low-CO 2 , heat, and drought stresses on isoprenoid emission.

Methanol and isoprene emissions from the fast growing tropical pioneer species <i>Vismia guianensis</i> (Aubl.) Pers. (Hypericaceae) in the central Amazon forest

“…Additional studies revealed possible connections between volatile isoprenoid emissions and increased photorespiration during high leaf temperatures and drought stress (Dani et al, 2014). Consistent with these potentially important functional roles for pioneer species, a leaf and branch survey at four neo-tropical sites (Harley et al, 2004) suggested that Vismia guianensis is an Is producer with a single leaf emission value of 48 µg C g −1 h −1 reported (or 11 nmol m −2 s −1 assuming a specific leaf area of 20 m 2 kg −1 ) (Dias-Filho, 1995).…”

Species-specific photorespiratory rate, drought tolerance and isoprene emission rate in plants

“…The emission rates observed in all of our experiments involving eucalypts, including both multiple species and the pairwise comparison cited above, 16 were low (0 to 10 nmol/m 2 /s) compared with those from more widely studied poplars and oaks (20 to 100 nmol/m 2 /s). 6 Many native Australian plants have abundant Rubisco and eucalypts in particular are notable for their extremely high photosynthetic capacities among trees, operating as they generally do in high-light environments.…”

“…Improving global emission algorithms requires a broad consensus of species' responses to drought, which so far has proven a challenge. In this addendum to Keywords: drought, electron transport rate, isoprene emission, MEP pathway, photorespiration the paper by Dani et al, 16 we highlight issues that are yet to be addressed.…”

“…A claim is made that residual reducing power unused by carbon assimilation drives increased isoprene emission under various abiotic stresses 16,17 ( Fig. 1).…”

“…1). Exploiting the contrasting drought tolerance of 2 Eucalyptus species, we provided 16 an exhaustive experimental data-set showing that an increased ETRto-NAR ratio sustained increased isoprene emission rates under drought, which appears to follow a species-specific threshold for drought tolerance (Fig. 1).…”