Evaluation of Radiation use Efficiency of Maize (Zea mays L.) and Styrian Oil Pumpkin (Cucurbita pepo var. styriaca) under Different Intercropping Ratios

Mixed cropping is one of the oldest and most commonly used agricultural activities in low-yield systems in many parts of the world. One of the factors contributing to the superiority of the mixed cropping to sole cropping is the correct selection of its components. When plants with a distinct root system and different canopy structures are placed next to each other, they occupy different ecological niches and have different spatial and temporal uses of environmental resources in a more efficient way than a combination of resources. Light is one of the main components of growth and production of dry matter of plants. In non-stressed conditions, dry matter production is a function of the time and amount of photosynthesis active radiation received, the fraction of radiation absorbed by the plant, and the efficiency of absorbed radiation converted to dry matter. Among these components, it is possible to improve the absorbance of photosynthetic active radiation by increasing leaf area index and canopy leaf arrangement. Due to the fact that maize leaves are placed in a more vertical arrangement, while absorbing better light, allows the light to penetrate into the lower canopy portions. On the other hand, Cucurbitaceae family plants because of the rapid covering of the surface of the land through crawling growth and the ability to grow in the shade, are most of the plants that are cultivated mixed with maize. Styrian oil Pumpkin (Cucurbita pepo L.) is an annual herbaceous plant which, due to its numerous medicinal properties, is one of the most important medicinal plants in the Cucurbitaceae family. Therefore, in maize and Styrian oil pumpkin mix cropping, most of the light is absorbed by the maize canopy surface due to its higher height and a different morphological structure, than the Styrian oil pumpkin and the rest of the light that passes through the upper layers is absorbed by the Styrian oil pumpkin canopy and hence, the efficiency of light consumption will be maximized. The aim of this study was to investigate the radiation use efficiency as an important aspect in crop growth and yield improvement.

In order to evaluate radiation use efficiency of maize and Styrian oil Pumpkin under different intercropping ratios, an experiment was conducted in a randomized completely block design with three replications in a field located in 10 km west of Shirvan, Iran in 2015. The experimental treatments were included intercropping replacement ratio of 75% maize: 25% Styrian oil Pumpkin, 50% maize: 50% Styrian oil Pumpkin, 25% maize: 75% Styrian oil Pumpkin and pure culture of both plants. A linear PAR Ceptometer (Sun Scan) was used to measure the radiation at top and under the canopy to calculate the amount of absorbed radiation by the canopy.

The results showed that there was a linear relationship between dry matter accumulation of both plants with cumulative photosynthetically active radiation. The correlation coefficient between them was greater than 0.9 in different intercropping treatments. Radiation use efficiency of maize was varied from 4.35 g MJ-1 in monoculture (the maximum amount) to 0.92 g MJ-1 (the minimum amount) in 25% maize: 75% Styrian oil Pumpkin treatment. The maximum and minimum amount of radiation use efficiency of Styrian oil Pumpkin (3.7 and 0.87 g MJ-1) were observed in monoculture and 75% maize: 25% Styrian oil Pumpkin treatments. The highest amount of total absorbed radiation among different intercropping ratios, was obtained in 50% maize: 50% Styrian oil Pumpkin and 25% maize: 75% Styrian oil Pumpkin. The maximum amount of total radiation absorbed by the canopy obtained70-110 days after sowing date. The results showed that canopy in mixed cropping treatment absorbed all received radiation in 70-110 days after sowing date. The maximum harvest index of maize (59.5) and Styrian oil pumpkin (24.1) was obtained from 25% maize: 75% Styrian oil pumpkin and 50% maize: 50% Styrian oil pumpkin, respectively.

The results indicated that there was a linear relationship between accumulation of dry matter of both plants and cumulative active photosynthesis radiation in different cropping treatments with correlation coefficient greater than 0.9. The slope of this line, which shows the efficiency of light consumption, varied from 4.35 g MJ-1 to pure maize, up to 0.92 g MJ-1 in the 25% maize ratio, and the highest and lowest amount of light consumption efficiency of Styrian oil pumpkin (3.7 and 0.87 g MJ-1) were observed in pure crop and 25% ratio of Styrian oil pumpkin, respectively.