Claudia Keitel

Okra is a vegetable crop adapted to summer temperatures, but heat stress has been shown to reduce its growth and productivity. We measured physiological traits of 104 okra genotypes in response to high temperature, augmented by the molecular characterization of selected genotypes to identify parents for crossing. Genotypes were exposed to a short heat shock (45 °C, 4 h) in a controlled environment, followed by the assessment of chlorophyll fluorescence (Fv/Fm, Fv′/Fm′) and stomatal conductance (gs). DNA was isolated from all genotypes using a modified CTAB method with additional PVP and RNase, and the amplification of 8 polymorphic SSR markers was used to generate a dendrogram. This preliminary screening identified 33 polymorphic genotypes with less than 50% genetic similarity and contrasting Fv′/Fm′ and gs responses. More detailed physiological measurements (Fv/FmFv′/Fm′, gs, photosynthesis (A), efficiency of the open reaction centre (ΦPSII), and electrolyte leakage (EL)) were conducted after exposure to 45 °C for 6 h and compared to the control (30 °C). EL did not significantly increase in the heat treatment; in contrast, there were significant genotype and treatment effects observed for fluorescence (Fv/Fm, Fv′/Fm′) and photosynthetic parameters (A, ΦPSII, gs). In conclusion, cell membranes in okra remained unaffected after short periods of heat stress, whereas the ranking of differences of measured physiological traits (∆) between control and heat-treated plants (∆Fv′/Fm′, ∆Fv′/Fm′, ∆A, ∆ΦPSII, ∆gs) was indicative of genotype sensitivity to heat.

Summary We compiled hydrogen and oxygen stable isotope compositions (δ 2 H and δ 18 O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ 2 H was more closely correlated with δ 2 H of xylem water or atmospheric vapour, whereas leaf water δ 18 O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ 2 H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ 18 O. We next expressed leaf water as isotopic enrichment above xylem water (Δ 2 H and Δ 18 O) to remove the impact of xylem water isotopic variation. For Δ 2 H, leaf water still correlated with atmospheric vapour, whereas Δ 18 O showed no such correlation. This was explained by covariance between air relative humidity and the Δ 18 O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2 H and 18 O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.