Citation: Santanoo, S.; Lontom, W.; Dongsansuk, A.; Vongcharoen, K.; Theerakulpisut, P. Photosynthesis Performance at Different Growth Stages, Growth, and Yield of Rice in Saline Fields. Plants 2023, 12, 1903. https://doi.org/10.3390/ plants12091903 Academic Editor: Dayong Zhang Received: 15 March 2023 Revised: 22 April 2023 Accepted: 4 May 2023 Published: 7 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). plants Article Photosynthesis Performance at Different Growth Stages, Growth, and Yield of Rice in Saline Fields Supranee Santanoo 1 , Watanachai Lontom 1 , Anoma Dongsansuk 2 , Kochaphan Vongcharoen 3 and Piyada Theerakulpisut 1, * 1 Department of Biology, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand; supranee4705@hotmail.com (S.S.); watalo@kku.ac.th (W.L.) 2 Department of Agronomy, Faculty of Agriculture, Khon Kaen University, Khon Kaen 40002, Thailand; danoma@kku.ac.th 3 Faculty of Science and Health Technology, Kalasin University, Kalasin 46000, Thailand; ko-cha_9@hotmail.com * Correspondence: piythe@kku.ac.th; Tel.: +66-43-202-531; Fax: +66-43-202-530 Abstract: Photosynthetic performance and biomass at different growth stages of the salt-sensitive KDML105 rice cultivar, three improved lines (RD73, CSSL8-94, and TSKC1-144), and the salt-tolerant standard genotype (Pokkali) were investigated under non-saline, semi-saline, and the heavy-saline field conditions in the northeast of Thailand. In the non-saline field, net photosynthesis rates (P n ) of all genotypes remained high from the early vegetative stage to the milky stage and then dramatically reduced at maturity. In contrast, in both saline fields, P n was the highest at the early vegetative stage and continuously declining until maturity. Leaf chlorophyll content remained high from the early vegetative to milky stage then reduced at maturity for all three field conditions. During the reproductive phase, P n of KDML105 and the improved lines were reduced by 4–17% in the heavy- saline field, while that of Pokkali was increased (11–19% increase over that of the non-saline). Pokkali also showed a prominent increase in water use efficiency (WUE) under salinity. Nevertheless, rice leaves under saline conditions maintained the PSII integrity, as indicated by the pre-dawn values of maximum quantum yield of PSII photochemistry (F v /F m ) of higher than 0.8. Pokkali under the semi-saline and the heavy-saline conditions exhibited 51% and 27% increases in final biomass, and 64% and 42% increases in filled grain weight plant −1 , respectively. In the semi-saline condition, RD73, TSKC1-144, CSSL8-94, and KDML105 showed moderate salt tolerance by displaying 24%, 18.6%, 15%, and 11.3% increases in final biomass, and 24%, 4%, 13%, and 6% increases in filled grain weight plant −1 , respectively. In contrast, in the heavy-saline field, final biomass of RD73, KDML105, CSSL8-94, and TSKC1-144 showed 48%, 45%, 38%, and 36% reductions from that in the non-saline field, while the filled grain weight plant −1 were reduced by 45%, 58%, 35%, and 32%, respectively. This indicated that the improved lines carrying drought- and/or salt-tolerance genes achieved an increased salt tolerance level than the parental elite cultivar, KDML105. Keywords: biomass; chlorophyll fluorescence; KDML105; net photosynthesis rate; Oryza sativa L.; Pokkali; saline soils 1. Introduction Rice (Oryza sativa L.) has the third largest planting area in the world with a 10-year average, from 2011 to 2020, of 16.25 million ha, while yield production per area was 4545 kg ha −1 [1]. Rice is one of the main sources of daily food for more than 50% of the world’s population, and is cultivated in more than 100 countries with 90% of the total global production from Asia [2]. For Thailand, rice is the most important economic crop with the planting area of 10.82 million ha (yield production per area 3032 kg ha −1 ) and more than half of rice production for domestic consumption and export comes from the northeast (NE) region [3]. Paradoxically, while the NE accounts for the largest rice-growing area (e.g., Plants 2023, 12, 1903. https://doi.org/10.3390/plants12091903 https://www.mdpi.com/journal/plants