Paclobutrazol improves leaf carbon-use efficiency by increasing mesophyll conductance rate, while abscisic acid antagonizes this increased rate

Our experiment was conducted to understand the effects of foliar sprays of paclobutrazol (PBZ), its mixture with abscisic acid (ABA), or water (control) on leaf carbon-use efficiency (CUEl), photosynthesis, and respiration of Euonymus japonicus seedlings. Plants were pot-grown in a greenhouse. The study revealed that elevated stomatal conductance, mesophyll conductance, and maximum rate of Rubisco carboxylation (Vcmax) in plants treated with PBZ could strongly improve net photosynthetic rate (PN). ABA could antagonize the increase of mesophyll conductance and stomatal conductance by PBZ, thereby inhibiting the improvement of photosynthesis by PBZ. The combination of PBZ and ABA significantly reduced day respiration (Rd) and dark respiration (Rdk), resulting in the increased CUEl, PN/Rd, and PN/Rdk. The increased PN in the PBZ treatment led to slightly increased PN/Rd and CUEl. Thus, the CUEl could be improved by PBZ and its mixtures with ABA by regulating leaf photosynthesis and respiration. Additional key words: gas exchange; leaf respiration; photosynthetic respiration ratio; plant growth inhibitors. Received 13 September 2019, accepted 19 March 2020. +Corresponding author; e-mail: cyxu@bjfu.edu. cn Abbreviations: ABA ‒ abscisic acid; Cc ‒ CO2 concentration in the chloroplasts; Ci ‒ intercellular CO2 concentration; CUEl ‒ leaf carbon-use efficiency; Fv/Fm ‒ maximal quantum yield of PSII photochemistry; gm – mesophyll conductance; gs – stomatal conductance; Jmax – maximum electron transport rate; PBZ – paclobutrazol; PGI – plant growth inhibitor; PN ‒ net photosynthetic rate; Rd – day respiration; Rdk – dark respiration; Rp – photorespiration rate; Vcmax – maximum rate of Rubisco carboxylation. Acknowledgements: This study was funded by the Scientific Research and Graduate Training Co-construction Project of Beijing Education Commission and Major Project of Forestry Public Welfare Industry of China (20140430102). The authors would like to thank Prof. Dayong Fan as well as Prof. Richard J. Hauer for valuable suggestions. Tari 2003, Jaleel et al. 2007). However, the changes in biochemical components [including maximum electron transport rate (Jmax) and maximum rate of Rubisco carboxylation (Vcmax)] and mesophyll conductance (gm), which are important photosynthesis-limiting factors, have been overlooked. Researchers often use Vcmax and Jmax to illustrate the photosynthetic capacity (Sago 1994). Mesophyll conductance is considered as another limiting factor of photosynthetic efficiency which could be equally important as stomatal conductance (gs) (Flexas et al. 2012). In addition, photorespiration (Rp) is also considered to be closely related to photosynthesis, and the reduction of photorespiration is conducive to improving photosynthetic efficiency (Zelitch 1989). However, the effects of PBZ on Rp have not been studied. The effect of PBZ on photosynthesis has been investigated extensively, whereas the research of respiratory responses in leaves is relatively scarce. Respiration which mainly includes day respiration (Rd) and dark respiration (Rdk) is a way of releasing carbon (Gonzàlez-Meler et al. 2009). There are only a few studies on the effects of PBZ on Rdk (Zeng et al. 1992), and the studies on Rd are missing. Rd is directly related to CUEl. Abscisic acid (ABA), an antagonist of auxin synthesis, Introduction In order to maintain the landscape of some ornamental plants for urban greening, frequent pruning is needed to control the growth of ornamental plants, and as such growth can be regulated by applying chemical inhibitors (Papafotiou and Chronopoulos 2015). Paclobutrazol (PBZ) commonly referred as a plant growth regulator is effective and universal in regulating plant size of urban ornamental plants (Fletcher et al. 2000). In addition to being effective in controlling plant growth, PBZ could affect leaf carbon-use efficiency (CUEl) which depends on the dynamic balance between photosynthesis and respiration. The CUEl is an important indicator of carbon balance in leaves which could determine the carbon budget at individual and ecosystem level (Gifford 2003). A PBZ-mediated improvement of CUEl may result from enhanced photosynthesis (Dwivedi et al. 2017) and decreased plant respiration (Zeng et al. 1992). There have been many studies on the effect of PBZ on photosynthesis. Previous studies found that the enhanced photosynthesis may result from an increase of chlorophyll (Chl) content and synthesis and stomatal opening and density (see e.g.,