Blackgram [Vigna mungo (L.) Hepper] also known as urdbean, is one of the important pulse crops in India. It is a protein-rich food which contains about 26% protein, almost three times that of cereals. Sowing time, a non-monetary input, is the single most important factor to obtain optimum yield. So determination of optimum sowing time for blackgram is inevitable. Optimum time of sowing of blackgram may vary from genotype to genotype. Therefore, there must be a specific sowing period during spring season for different genotypes to obtain maximum yield. The rate of plant development for any genotype is directly related to temperature, so the length of time between the different stages will vary as the temperature varies, both between and within growing seasons. Changes in seasonal temperature affect the productivity through the changes in phenological development process of the crop. All the crops are vulnerable to different temperature stresses during the crop season and differential response of temperature change to various crops has been noticed under different production environments (Kalra et al. 2008). The occurrence of different phenological events during growing season of any crop and the effect of temperature on plant growth can be inferred using accumulated heat units or growing degree days. The present investigation was, therefore, undertaken to identify the suitable genotypes and sowing time of spring blackgram. MATERIALS AND METHODS A field experiment was conducted during spring season of 2008, 2009 and 2010 at the Research Farm of Punjab Agricultural University, Ludhiana (30° 56’N, 75° 52'E, altitude 247 m), India. The soil of the experimental site was loamy sand in texture. The experiment comprising four dates of sowing (10 March, 15 March, 20 March and 25 March) and six genotypes (Sekhar 2, Pant U 19, Pant U 31, WBU 109, Mash 364 and Mash 1008) was conducted in a split plot design by keeping dates of sowing in main plot and genotypes in sub-plot. The treatments were replicated four times in all 1 Senior Agronomist (Pulses) (e mail: singhguriqbal@ rediffmail.com), 2 Assistant Agronomist (Pulses) (e mail: hkmand@rediffmail.com), 3 Assistant Agronomist (Pulses) (e mail: navneetpulsespau@yahoo.com), 4 Wheat Agronomist (e mail: hr_saharan@yahoo.com), Department of Plant Breeding and Genetics; 5 Agrometeorologist (e mail: kgill2002@gmail.com), Department of Agricultural Meteorology; 6 Senior Microbiologist (e mail: veenadk@rediffmail.com), Department of Plant Breeding and Genetics 57 Indian Journal of Agricultural Sciences 83 (9): 953–8, September 2013/Article Symbiotic efficiency, thermal requirement and yield of blackgram (Vigna mungo) genotypes as influenced by sowing time GURIQBAL SINGH 1 , HARPREET KAUR 2 , NAVNEET AGGARWAL 3 , HARI RAM 4 , K K GILL 5 and VEENA KHANNA 6 Punjab Agricultural University, Ludhiana, Punjab 141 004 Received: 28 June 2011; Revised accepted: 15 April 2013 ABSTRACT A field investigation was carried out during summer 2008, 2009 and 2010 at Research Farm of Punjab Agricultural University, Ludhiana, to study the performance of spring blackgram [Vigna mungo (L.) Hepper] genotypes (Sekhar 2, Pant U 19, Pant U 31, WBU 109, Mash 364 and Mash 1008) on different dates of sowing (10 March, 15 March, 20 March and 25 March). The grain yield recorded in 20 March sowing was significantly higher than the other sowing dates. Blackgram sown on 20 March registered 17.2, 3.2 and 4.5% higher grain yield over 10, 15 and 25 March sowing dates, respectively. Number of nodules was not significantly influenced by sowing dates. Nodule dry weight was significantly higher in 15, 20 and 25 March sowings than 10 March sowing. Early sown crop took more number of days to 50% flowering and maturity than late sown crop in 2008 but these were increased with delay in sowing in 2009 and 2010. Early sown crop required lower agroclimatic indices, i.e. growing degree days, heliothermal units, photothermal units than the late sown crop. Among genotypes, Mash 364 yielded the highest (1 136 kg/ha), closely followed by Mash 1008 (1 125 kg/ ha), both being significantly superior to all other genotypes. The optimum time of sowing for spring blackgram was found to be 15-25 March and Mash 364 and Mash 1008 were the promising genotypes. Key words: Blackgram, Genotypes, Sowing dates, Urdbean