Received: 5 June, 2010. Accepted: 18 September, 2010. Original Research Paper International Journal of Plant Breeding ©2011 Global Science Books Biomass, Harvest Index and Yield in Relation to Changes in Photo-thermal Regimes in Soybean (Glycine max L. Merrill.) Genotypes Gaurav Khosla 1 • B. S. Gill, T. P. Singh 2 • Rahul Kapoor 2* 1 Krishi Vigyan Kendra, Gurdaspur, Regional station, Punjab Agricultural University, Ludhiana, Punjab, India 2 Department of Plant Breeding and Genetics, Punjab Agricultural University, Ludhiana-141 004, Punjab, India Corresponding author: * rahulkapoor@pau.edu, rahul200189@gmail.com ABSTRACT Fifteen photo- and thermo-insensitive early maturing genotypes and five promising main season normal maturing genotypes of soybean which were determinate in growth and photo-thermo sensitivity were evaluated for their agronomic performance under a wide range of photoperiod and temperature conditions manipulated through five sowing dates (February 23, March 20, April 14, May 9 and June 3). Genotypes, sowing dates and their interactions with environments were highly significant for the traits studied. The significant GE interactions for biomass production, harvest index (HI) and grain yield (GY) indicated that the tested genotypes ranked differently across diverse environments for these characters. In early maturing genotypes, longer day-length and higher temperature produced bold seeds and high HI but fewer yields. Main season genotypes produced higher biomass and GY than that of early maturing genotypes in all the sowings. Early maturing genotypes viz. E7 and E19 had higher GY whereas SL 295 and Pb. No. 1 recorded higher GY among main season genotypes over all the sowing dates. Biomass and HI were important determinants of GY as evident from their significant positive regression coefficients with GY. It is concluded that for main season genotypes (May and June sowing) GY can be increased by increasing HI and enhancement of biomass production in early maturing-photo and thermo insensitive in February-March sowing could lead in yield improvement. Study also demonstrated that early maturing photo-thermo insensitive genotypes of soybean could be grown successfully during spring/summer (February to June). _____________________________________________________________________________________________________________ Keywords: Grain yield components, genotype × environment interaction, photo-insensitiveness, sowing dates INTRODUCTION Soybean (Glycine max L. Merrill.), believed to have origi- nated in China, is a nitrogen fixing legume crop that may be a good component of a general plan to improve cropping system efficiency. For this purpose, crop suitability to spe- cific environments must be established, as the economic yield of the crop will depend largely on its adaptability to various environments. So, the understanding of the growth dynamics and of the growth realized in the grain component is important to improve the crop management. Soybean is a short-day plant, and photoperiod and tem- perature control the duration of both pre- and post-flower- ing phases (Kantolic and Slafer 2007). Its growth depends on the ability of different genotypes to capture light and the efficiency of conversion of intercepted light into biomass (Confalone et al. 2010). Photo-thermal sensitivity in soy- bean influences to a considerable extent the area of its adap- tation and the time of maturity of cultivars (Hartwig and Kihl 1979). Temperature along with photoperiod affects the flower- ing time but temperature along with humidity affects initia- tion and normal development of pods (Singh and Khehra 1988). Roberts et al. (1996) had also emphasized the impor- tance of photoperiod-insensitivity in the improvement of soybean crop after characterizing soybean genotypes in conjunction with an analysis of the world-wide range of photo-thermal environments in which soybean crops are grown. Most of the Indian soybean cultivars (> 95%) were found to be highly sensitive to photoperiod that limits their cultivation in only localized area (Bhatia et al. 2003). In soybean three loci E 1 /e 1 , E 2 /e 2 and E 3 /e 3 affect the photoperiod sensitivity of the duration from the sowing to first flowering (Upadhyay et al. 1994a, 1994b), and from first flowering to the end of flowering (Asumadu et al.1998; Summerfield et al. 1998). The genotypes differing in one or more alleles show negligible differences in vegetative and reproductive durations in short days but substantial dif- ferences in long days (Upadhyay et al. 1994a, 1994b; Asu- madu et al. 1998; Summerfield et al. 1998). Not surpri- singly, the greater the reproductive duration of a genotype in long days the more biomass was accumulated by matu- rity (Asumadu et al. 1998). However, the variation in dura- tion accounted for only about 70% of the variation in the biomass accumulated at maturity (Hunt 1982). Different soybean varieties behave differently under dif- ferent photoperiod and temperature conditions. Photoperiod affects vegetative growth, flowering and seed maturity while temperature influences time of flowering, rate of growth, normal pod development and pod retention (Piper et al. 1996). Both of these parameters i.e. photoperiod and temperature can be manipulated by varying sowing dates under field conditions. The seed yield of grain legumes is the result of two interconnected processes: plant growth and development of sink strength which will determine the proportion of growth partitioned to grain, and the harvest index (HI). Compo- nents of yield, seed numbers and seed weight provide some clues as to the environmental effects on yield-forming pro- cesses (Ayaz et al. 2004). Duration of vegetative and repro- ductive development is reported to vary in response to dif- ferent planting dates. Vegetative growth of soybean has been reported to be associated with seed yield. Therefore, sufficient vegetative dry matter is required to support and ®