Citation: Hussain, M.E.; Sharma, S.; Joel, A.J.; Kilian, B. Photoperiod Insensitivity in Pigeonpea Introgression Lines Derived from Wild Cajanus Species. Agronomy 2022, 12, 1370. https://doi.org/10.3390/ agronomy12061370 Academic Editors: Yuan-Ming Zhang and Fengjie Sun Received: 18 March 2022 Accepted: 28 May 2022 Published: 6 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). agronomy Article Photoperiod Insensitivity in Pigeonpea Introgression Lines Derived from Wild Cajanus Species Mohammad Ekram Hussain 1,2 , Shivali Sharma 1,3, * , A. John Joel 4 and Benjamin Kilian 3 1 International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, Hyderabad 502324, India; ekram.hussain12@gmail.com 2 Department of Genetics and Plant Breeding, Centre for Plant Breeding and Genetics, Tamil Nadu Agricultural University, Coimbatore 641003, India 3 Global Crop Diversity Trust, Platz der Vereinten Nationen 7, D-53113 Bonn, Germany; benjamin.kilian@croptrust.org 4 Department of Plant Biotechnology, Centre for Plant Molecular Biology and Biotechnology, Tamil Nadu Agricultural University, Coimbatore 641003, India; jnjoel@gmail.com * Correspondence: shivalipbg@gmail.com; Tel.: +91-9440397294 Abstract: Pigeonpea is a photoperiod-sensitive crop; therefore, the introgression of photoperiod in- sensitivity could increase its adaptability to new environments. We determined the effect of extended daylength (ED; 16 h light) on the phenotypical traits of extra-early, early, and mid-early maturing pigeonpea introgression lines (ILs) derived from wild Cajanus species belonging to secondary and tertiary gene pools. Plants were grown under natural daylength and extended daylength in a green- house. Comparisons of the time of floral bud initiation, days to flowering, plant height, number of branches, and number of leaf nodes on the main stem at flowering revealed photoperiod-insensitive lines. All traits varied widely among the ILs. Analyses of flowering traits revealed large genetic components with low genotype × treatment interactions and high broad-sense heritability. The photoperiod most strongly affected the number of primary branches, followed by plant height. The extended day advanced flowering by approximately four days in extra-early ILs, confirming that these ILs are quantitative, short-day plants. The photoperiod insensitivity index varied from 0.88 in ICPP 171541 (moderately photoperiod sensitive) to 0.99 in ICPP 171546 and ICPP 171561 (photope- riod insensitive). These photoperiod-insensitive extra-early flowering ILs can be used to enrich the genetic diversity of pigeonpea and to develop photoperiod-insensitive cultivars for cultivation in new environments. Keywords: photoperiod; photoperiod insensitivity index; introgression lines (ILs); facultative; quantitative; wild Cajanus species; pigeonpea 1. Introduction Pigeonpea (Cajanus cajan (L.) Millspaugh), a protein-rich food legume crop with multiple benefits, is adapted to tropical and subtropical regions. Because of its multiple uses, viz., food, fodder, and soil improvement through nitrogen fixation, pigeonpea plays an important role in subsistence agriculture. Globally, it is grown across approximately 6.9 million hectares (ha) with an annual production of 5.9 million tonnes and an average yield of 852.4 kg/ha [1]. India, Myanmar, Malawi, Tanzania, Haiti, Kenya, the Dominican Republic, and Nepal are the primary pigeonpea-producing countries. In India, pigeonpea is cultivated mainly as a rain-fed crop across approximately 5.5 million ha, which accounts for approximately 80% of the global pigeonpea cultivation area. Despite substantial breeding efforts for pigeonpea improvement in India, its productivity has remained stagnant at approximately 0.8 t ha −1 . The low productivity is because of the lack of improved varieties suited to cultivation in varied agroclimatic conditions. Furthermore, because pigeonpea is cultivated in marginal lands with limited inputs, it is exposed to various biotic and Agronomy 2022, 12, 1370. https://doi.org/10.3390/agronomy12061370 https://www.mdpi.com/journal/agronomy