The assessment of Mungbean seed yield and quality changes in response to environmental factors in different planting dates

Planting date is one of the important issues in agronomy that could be changing quantitative and qualitative characteristics of crop seeds. Planting date changes seed quality by providing different environmental conditions such as temperature, relative humidity, day length, radiation, and incidence of pathogens. It is also effective on seed yield. It affects duration of growth and development. A suitable planting date leads to optimal utilization of climatic conditions such as temperature, humidity, day length. It is also important for flowering and grain filling period to take place at the same time with favorite environmental conditions. As a result, it can improve seed yield and seed quality. Some factors such as high temperatures, frequent wetting and drying of air, high relative humidity, and continuous rainfalls have a significant negative effects on seed quality. These conditions during seed development reduce seed quality in the field. It is obvious that such conditions can be occurred on both early and late planting dates. Thus, according to an important role of planting date in determination of seed yield and quality, in this study the effects of environmental conditions during seed filling period on seed yield and quality of mungbean (Vigna radiata L.) were evaluated by implementing different planting dates in field conditions. This experiment was conducted as a randomized complete block design with three replications at Agricultural Research Station of Gonbad during 2010, which is located at three kilometers northeast of the city, with latitude and longitude of 37˚16́ N and 55˚12́ E, respectively, and a height of 45 meters above sea level. In this study, the genotype VC-1973A was cultivated in five planting dates. The planting dates included April 21, May 10, June 3, June 20 and July 12. The mungbean developmental stages were divided into three stages includes planting to flowering (PD-R1), flowering to beginning of seed filling (R1-R5), and beginning of seed filling to seed maturity (R5-R7). Afterwards, the relationship between the length of these developmental stages and environmental factors (temperature, relative humidity, rainfall and photoperiod) occurring during these stages were evaluated in terms of yield and seed quality. Seed quality was determined via germination, accelerated aging, Electrical conductivity tests, and field emergence percentage. Data analysis was performed by SAS 9.1.3 (Institute, Inc), and Excel 2007 was used to draw figures. The results showed that seed quality increased as a result of delayed planting date, whereas seed yield decreased. Germination of the aged seeds increased by 0.23% per day of delay in planting. On the other hand, the electrical conductivity per day of delay in planting decreased by 0.43 µs.cm-1.g-1 which reflects the increase in seed vigor. Delaying the planting date led to an increase in field emergence percentage of about 0.23% per day. In fact, in this study the lowest seed quality was observed for planting date April 21 (first planting date). The reason for this occurrence can be related to occurrence of very high temperatures (more than 40°C) and rainfall during seed development (R5-R7). Although the average air temperature during R5-R7 increased in the next planting dates, but the number of days that temperature was more than 40°C decreased, which consequently led to increased seed quality. Also, the length of R5-R7 decreased by delay in planting date, so filling period (R5-R7) was exposed to environmental factors (particularly high temperatures) for a shorter period. In contrast to seed quality, seed yield decreased linearly with delay in planting date. The highest seed yield (1144.4 kg.ha-1) was achieved in the first planting date (April 21). Then, for each day that planting date was delayed, seed yield decreased by 10.84 kg.ha-1, and finally in the last planting date (July 12) reached the lowest amount (299 kg.ha-1). Reduction of the length of developmental stages (especially R5-R7) was the most important cause of seed yield loss. Although, delay in planting date increased seed quality of mungbean, this delay reduced seed yield. Seed yield in the last planting date decreased 845.4 kg.ha-1 compared to the first planting date. In fact, delay in planting date decreased seed quantity, but increased its quality. The optimum planting date is the date that maintains the seed quality at a satisfactory level and produce high seed yield, simultaneously. Therefore, planting of mungbean in mid-June may lead to higher seed yield, as well as production of high-quality seeds