High-Night Temperature-Induced Changes in Chlorophyll Fluorescence, Gas Exchange, and Leaf Anatomy Determine Grain Yield in Rice Varieties

Night temperature plays a key role in the growth and development of rice plants, specifically the flowering and grain filling stages. An increase in night temperature more than the normal range is regarded as high night temperature (HNT). The HNT results in reduced yield; largely the action mechanism of HNT induced rice grain yield loss remains unknown. The present study provides a comprehensive and holistic analysis of HNT induced impact on several morphological, physiological and anatomical factors which individually or in combination may be linked with yield loss. Normal night temperature (AMB) and + 2° C and + 5° C increase (referred to as HNTs) from ambient temperature was imposed on seven rice varieties viz. Ronga Ahu, Haringa, Inglonkiri, Dishang, Maibee, and Banglami over 2 years in bioreactor facilities. The Nagina-22 (N-22), a drought tolerant variety was used as a reference line. Based on grain yield and grain filling percentage, Maibee was highly sensitive; while Bangalami was the most tolerant to HNT. Key parameters of reproductive growth were reduced by HNT among all the varieties, with Bangalami and Maibee proving tolerant and sensitive to HNT. Among the physiological attributes viz. Fv/Fm, NPQ, qP, Pn, gs, iCO2, ETR, and Φ yield showed deviation from normal values under HNT, which led to a reduction in carboxylation efficiency and carbohydrate synthesis in all the varieties. Leaf anatomical features negatively impacted by HNT included the size of the stomatal pore, xylem, and phloem profiles. The SPAD showed a linear correlation with a reduction in leaf chlorophyll, and leaf and grain nitrogen content under HNT. Structured equation modeling (SEM) estimated the contribution of all the morphological and physiological variables toward grain yield loss under HNT. Among the 27 variables, mainly carboxylation efficiency (CO_S), quantum yield (QY_S), dark respiration (RES_S), Fv/Fm (FVFM_S), and non-photochemical and photochemical quenching (NPQ/PQ_S), chlorophyll stability (CS_S) were able to explain variance in grain yield loss under HNT in Bangalami, Maibee, and N-22. The findings of the present study provided a novel insight into the HNT action mechanism responsible for a reduction in growth, development, and yield in rice. Furthermore, SEM indicates important attributes which were crucial for HNT induced yield loss in rice varieties under study.