Lima bean growth, leaf stomatal and nonstomatal limitations to photosynthesis, and 13C discrimination in response to saline irrigation

Authors: Liu, Xuan; Suarez, Donald

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2020
Publication Date: 12/18/2021
Citation: Liu, X., Suarez, D.L. 2021. Lima bean growth, leaf stomatal and nonstomatal limitations to photosynthesis, and 13C discrimination in response to saline irrigation. Journal of the American Society for Horticultural Science. 146(2):132-144. https://doi.org/10.21273/JASHS04996-20.
DOI: https://doi.org/10.21273/JASHS04996-20

Interpretive Summary: Soil salinity and limited fresh water supplies are of increasing concern for agricultural production in arid and semiarid regions. Little information is available on lima bean leaf photosynthetic performance under salt stress. In this study we examined, leaf gas exchange and growth of lima bean in response to irrigation with 6 waters of varying salinity. We determined that the reduction in net carbon fixation accounted for most of the growth reduction at the vegetative growth stage, pod growth stage as well as bean yield loss. Further we determined that the reduction in carbon fixation was mainly due to stomatal closure rather than photosynthetic biochemical limitations. The effect of lima bean leaf photosynthesis by salt stress may be partially alleviated by an elevated ambient CO2 concentration thus lima bean may be more salt tolerant under increased atmospheric CO2 . This research is of interest to extension specialists and plant scientists examining the interaction of various plant stresses and climatic conditions.

Technical Abstract: Soil salinization is a widespread problem severely impacting crop production. Understanding how salt stress affects growth-controlling photosynthetic performance is essential for improving crop salt tolerance and alleviating the salt impact. Lima bean (Phaseolus lunatus) is an important crop, but little information is available on its growth and leaf gas exchange in relation to a wide range of salinity. In this study, the responses of leaf gas exchange and whole plant growth of lima bean (cv. Fordhook 242) to six salinities with electrical conductivity (EC) of 2.9 (control), 5.7, 7.8, 10.0, 13.0, and 15.5 dS·m-1 in irrigation waters were assessed. Significant linear reduction by increasing salinity was observed on plant biomass, bean yield, and leaf net carbon assimilation rate (A). As EC increased from the control to 15.5 dS·m-1, plant biomass and A decreased by 87% and 69%, respectively, at the vegetative growth stage, and by 96% and 83%, respectively, at the pod growth stage, and bean yield decreased by 98%. Judged by the linear relations, the reduction in A accounted for a large portion of the growth reduction and bean yield loss. Salinity also had a significantly negative and linear effect on leaf stomatal conductance (gS). Leaf intercellular CO2 concentration (Ci) and leaf C13 isotope discrimination ('13) declined in parallel significantly with increasing salinity. The A-Ci curve analysis revealed that stomatal limitation [Lg (percent)] to A increased significantly and linearly, from 18% to 78% and from 22% to 87% at the vegetative and pod-filling stages, respectively, as EC increased from the control to the highest level. Thus, relatively nonstomatal or biochemical limitation [Lm (percent), Lm = 100 - Lg] to A responded negatively to increasing salinity. This result is coincident with the observed '13 salt-response trend. Furthermore, leaf carboxylation efficiency and CO2-saturated photosynthetic capacity [maximum A (Amax)] were unaffected by increasing salinity. Our results strongly indicate that the reduction in lima bean A by salt stress was mainly due to stomatal limitation and biochemical properties for photosynthesis might not be impaired. Because stomatal limitation reduces A exactly from lowering CO2 availability to leaves, increasing CO2 supply with an elevated CO2 concentration may raise A of the salt-stressed lima bean leaves and alleviate the salt impact. This is supported by our finding that the external CO2 concentration for 50% of Amax increased significantly and linearly with increasing salinity at the both growth stages. Leaf water use efficiency showed an increasing trend and no evident decline in leaf chlorophyll soil plant analysis development (SPAD) readings was observed as salinity increased.