Effects of Time, Number and Type of Neighboring Plant on Shade Avoidance Syndromes on Red Bean

The light received by plants is composed of different types of wavelength, ranged from ultraviolet to infrared. The range between 400 nm and 700 nm is used by plants to drive photosynthesis and is typically referred as to Photosynthetically Active Radiation (PAR). Plants are very good filters of light. They absorb and use most of the R light available to them for photosynthesis, but reflect or transmit most of the FR light, since FR light is not useful in photosynthesis. As a result, the ratio of R: FR will be decreased. Red to far red ratio (R: FR) is considered as an indicator of the quality of light. Recent physiological studies indicate that plants can perceive the quality of light reflected from neighbors as an accurate predictor of future competition, and respond morphologically even before they are directly shaded. Understanding the loss of quality of light, and taking away red wavelengths, creates a series of reactions in the plant such as stem elongation, reduction in stem diameter, and a reduction in shoot and root biomass and so forth that are known as shade avoidance syndrome (SAS). In this study, we established a pot experiment to evaluate the effects of time, number and type of neighboring plants on shade avoidance mechanisms in red bean.

In order to investigate the effects of the neighboring plants consist of Phaseolus vulgaris, Zea maize, Amaranthus retroflexus and Chenopodium album with Phaseolus vulgaris (as target plant), a pot experiment was conducted in Ferdowsi University of Mashhad during spring and summer in 2016. The experiment was designed as a factorial with three factors and four replicates based on a randomized complete block design. The first, second, and third factors were, respectively, the duration of the neighboring period of the plants with red bean (15, 30 and 45 days), the number of plants neighbors with red bean (1, 2 and 3 plants), and the type of neighboring species with red bean, including Phaseolus vulgaris, Zea maize, Amaranthus retroflexus and Chenopodium album. The plants were planted, watered, fertilized and treated carefully. After completing the neighboring time for each treatment, all plants neighbored with target plant (central) were removed and the target plant remained to grow. This work was performed 15, 30 and 45 days after planting. At third stage (45 days of neighboring), after removing the third groups of neighboring plants, all target plants were harvested and some morphological traits such as plant height, number of branches, leaf dry mater, shoot dry mater were measured. Finally, all statistical analyses were performed using SAS 9.1 ((SAS Institute, Cary, NC)) with a type 1 error rate set at P ≤ 0.05.

The results of this experiment showed that among all the investigated factors, the duration of neighboring plants had a significant effect on most traits. Only leaf dry matter reacted to the number of neighboring species and also none of the traits was affected by the type of species. The comparison of means test related to the neighboring duration factor showed that among time of neighboring with central plant (15, 30 and 45 days), third treatment i.e. 45 days neighboring had the significant impact on most of the traits. The amount of plant height for time of 15, 30 and 45 of neighboring was 32.1, 35.2 and 41.2, respectively, demonstrating that time of neighboring is a vital factor in shade avoidance mechanism, regardless of type and number. In this experiment, neighboring did not affect lateral shoot. However, further assessment showed that the lateral shoot is a gene-based trait and the number of lateral shoot is constant. But neighboring time influenced the rate of lateral shoot number per height. The amount of that for the trait of 15, 30 and 45 days neighboring was 0.49, 0.46 and 0.38, respectively. In fact, the continuity of presence of neighboring plants besides the central plants is considered as an effective factor in the appearance of shade avoidance mechanisms. In this experiment, the presence of neighboring species type was one of the evaluated factors assumed to have different influences on shade avoidance mechanisms in beans. Considering that in shade avoidance mechanisms, the biomass volume produced by a plant is considered as reflection of farther red light with higher impact on neighboring plant, but the species treated has no effect on shade avoidance mechanism on red bean. Finally, the quality of light is more strongly affected, with increasing the number of neighboring species. This should be sensitive to the occurrence of shade avoidance mechanisms from the central plant, but number of species did not significantly impact trait.