Relating Whole-plant Photosynthesis to Physiological Acclimations at Leaf and Cellular Scales under Drought Stress in Bedding Plants

in Journal of the American Society for Horticultural Science

Bedding plants are at increased risk for exposure to drought stress during production because they are grown in small containers. Physiological mechanisms of bedding plants at leaf and cellular scales that regulate whole-plant photosynthesis under drought conditions are not well understood. This information can be useful for screening bedding plant cultivars with improved drought-tolerance and generate guidelines to mitigate drought stress during production. We subjected drought-sensitive salvia (Salvia splendens ‘Bonfire Red’) and drought-tolerant vinca (Catharanthus roseus ‘Cooler Peppermint’) to gradual drought stress inside whole-plant gas exchange chambers. Substrate water content (Θ), whole-plant net photosynthesis (Pn_avg), whole-plant respiration (Rd_avg), and daily carbon gain (DCG) were measured continuously, whereas stomatal conductance (gS) to water, leaf water (ΨL), osmotic (ΨS), and turgor (ΨP) potentials were measured at the start and end of the drought phase. In addition, ΨS was measured before exposure to stress and after thoroughly rehydrating plants. Dark-adapted quantum efficiency (dark-adapted ΦPSII) was measured after rehydrating plants. The results indicated that, at whole-plant scale, vinca continued to uptake water at lower Θ levels than the Θ level that resulted in wilting of salvia. There were no differences in Rd_avg; however, Pn_avg and DCG of salvia decreased at a higher Θ level than that of vinca. This indicated that salvia experienced drought stress at a higher Θ level than did vinca. At the leaf scale, there were no differences in ΨL; however, a more negative ΨS (P = 0.06) and significantly higher ΨP were observed in vinca (compared to salvia) under drought conditions. In addition, ΨS was not different between species before exposure to drought, whereas ΨS of rehydrated leaves after exposure to drought in vinca was significantly lower than that in salvia. Moreover, ΨS of rehydrated leaves after exposure to drought was significantly lower than that observed before exposure to drought in vinca. This indicated osmotic adjustment (OA) in vinca under drought conditions. Dark-adapted ΦPSII was lower in salvia than in vinca after exposure to drought, indicating damage to photosynthetic mechanisms. Our results suggested that increased OA likely helped to maintain higher ΨP under drought conditions and continuation of water uptake at lower Θ in vinca compared to salvia. In addition, healthier photosynthetic mechanisms of vinca (compared to salvia) under drought conditions likely resulted in its higher Pn_avg and DCG at lower Θ. Screening for OA and dark-adapted ΦPSII may be useful for developing drought-tolerant bedding plant cultivars.

Contributor Notes

Corresponding author. E-mail: knemali@purdue.edu.

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    (A) Substrate volumetric water content (Θ) measured using dielectric moisture sensors on different days after withholding water during the second drought cycle for salvia and vinca. Error bars represent sem (n = 5–6). The substrate water content was not different between species, but the average Θ significantly differed with time. An exponential equation, Θ = 0.06 + 0.245 e(−0.387·days) (R2 = 0.72), was fitted using data from all replicates and both species to describe the change in Θ with time. (B) Relationship between daily decrease in substrate water content (Θ change) and Θ in salvia (open circles) and vinca (closed circles). Error bars represent sem (n = 5–6). The interactive effect of species × Θ on Θchange was significant. Linear equations Θchange = −0.03 + 0.57·Θ (r2 = 0.67; P < 0.0001) and Θchange = −0.01 + 0.24·Θ (r2 = 0.65 ; P < 0.0001) were fitted for salvia and vinca, respectively.

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    Net CO2 exchange rates (solid line) and substrate water content (Θ) (dashed line) of vinca during the past 168 h (7 d) of the drought cycle. Similar data were collected for salvia. Plants were exposed to cycles of 4 h of light (positive CO2 exchange rate or net photosynthesis rate) and 2 h of dark (negative CO2 exchange rate or dark respiration rate) periods. Note the gradual decrease in the net CO2 exchange rate with decreasing Θ.

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    Relationship between normalized (percent maximum) whole-plant photosynthesis (Pn_avg) (A and B)], whole-plant respiration (Rd_avg) (C and D), and daily carbon gain (DCG) (E and F)] compared to the substrate water content (Θ) in salvia and vinca during the second drought cycle. Data were normalized to account for differences in canopy size between replications and species. Each data point represents a daily average value. Data from all replications (n = 5–6) are shown. The Gompertz function was fitted to the data: f=100· expexp[(θθo)b ] . Arrows indicate the substrate water content at which measured responses decreased to 90% of the maximum observed value.

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    Average ΨS of fully rehydrated leaves of salvia and vinca before exposing plants to drought and after exposure to two drought cycles. The osmotic potential of three to four leaves within each replication was measured (n = 6) at both times. Error bars indicate sem. Means separated by a different letter indicate statistical significance at P ≤ 0.05.

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