Effect of drought stress caused by partial root zone irrigation on water use efficiency and grain yield of maize (Zea mays L.) and mung bean (Vigna radiata L.) in different intercropping planting patterns

Introduction Intercropping, defined as planting two or more crops simoultanously in a same field during a specific growing season, is an agronomical operation which has important advantages such as improving water use efficiency, increasing land use efficiency and yield compared with sole crop systems. Because of lowering available water for agronomical operations, water conservation methods are in high importance in agronomical systems. Partial root zone irrigation has been considered during recent years. In this method, half of root system is irrigated and other half is subjected to dry soil. In wheat maize intercropping it has been reported that partial root zone irrigation improved water use efficiency (Yanh et al., 2011). The purpose of this experiment was evaluating the effect of water deficit induced by partial root zone irrigation in growth and grain yield of maize and mung bean in intercropping and sole cropping systems to assess the possibility of improving water use efficiency.  Materials and Methods The experiment was conducted as a field experiment during 2016-17 growing season in Lorestan province. A tw-factors factorial experiment based on randomized complete block design with three replications was employed to compare the treatments. The fist factor was irrigation (including conventional irrigation (I1) and partial root zone irrigation (I2)) which was applied after seedling establishment. The second factor was planting pattern including sole maize, sole mung bean, within-row intercropping and between-row intercropping. Water consumption and leaf area index were measured during growing period. At harvest time, grain yield and yield components were determined. Water use efficiency was measured by dividing grain yield on water consumption. Land equivalent ratio was used to evaluate the advantage of intercropping.  Results and Discussion The highest grain yield of maize was recorded in sole cropping system under conventional irrigation. The lowest grain yield of maize was achieved in between-row intercropping under partial root zone irrigation. There was a reduction in grain yield with lowering water availability. Leaf area index of maize in partial root zone irrigation system was lower than that of conventional irrigation. Partial irrigation reduced all yield component of maize including grain number and weight. However, the effect of water deficit on grain weight was more than grain number. Lower plant number per unit area can be considered as a cuase of the lower grain yield of maize in between-row intercropping. Furtheremore, reducing leaf area index under partial root zone irrigation showed a reduction in leaf size and number, resulting in lower solar radiation absorption and, finally, reduction of photo assimilates synthesis.      The highest yiel of mung bean was achieved under conventional irrigation and sole cropping system which was not significanlt difference with within-row intercropping. Mung bean produced the lowest grain yield in between-row intercropping in partial root zone irrigation. Generally, partial root zone irrigation reduced mung bean grain yield in all treatments with the amount of 29%, 22% and 21% for soel cropping, between-row intercropping and within-row intercropping, respectively.    Although the highest grain yield was achived from maize sole cropping under conventional irrigation, the highest waer use efficiency of this treatment was lower than that of sole maize and within-row maize-mung bean intercropping. In all treatment, water use efficiency was improved by applying the partial root zone irrigation system which its highest value was recorded for maize-mung eban intercropping (43%) and mung bean sole cropping (39%). Land equivalent ratio of both interropping systems under partial and conventional irrigation was more than one, indicating the advantage of intercropping. The advantage of within-row intercropping under both irrigation systems was two times more than that of sole cropping.  Conclusion Yield and yield components of maize and mung bean were reduced under partial root zone irrigation due to the reduction of grain weight and number. Reduction of water consumption through partial root zone irrigation improved water use efficiency. The results of this experiment revealed that maize mung bean intercropping under partial root zone irrigation can produce optimum grain yield while improving water use efficiency.