Morphological Plasticity in Watermelon in Response to Interspecific Competition in a Low-Resource Intercropping System.

Interspecies specific interactions are generally regarded as drivers of plant productivity in multispecies agroecosystems. Positive interactions such as facilitation can dominate over competition under high abiotic stress conditions. Furthermore, complementary use of resource in diverse communities can further enhance community productivity through optimal use of plant-available resources. We studied the effects of a multifunctional species intercropping system consisting of peanut, watermelon, okra, cowpea and pepper plant alone or in various intercropping combinations in a low fertilizer input system in the peak of summer heat in Texas. There were generally no differences detected in gas exchange measurements and no differences in d¹³C composition within individual species between cropping system. Peanut grown in strip intercropping with watermelon, and in the 3 and 5 species systems in 2012 and in the 5 species system in 2011 had the highest midday water potential values as compared to peanut grown in monoculture, indicating peanut may have benefited from facilitative interactions with companion crops in the intercropping systems through a reduction in plant water stress. Differences in specific leaf area (SLA), leaf nitrogen, and carbon to nitrogen (C:N) ratios in watermelon may at least partially explain the lack of changes in gas exchange measurements detected. In 2012, when watermelon was a subordinate crop, specific leaf area and leaf N content were highest in the multispecies systems, particularly W_pwoc. Carbon to nitrogen ratio (C:N) was also lowest in W_pwo and W_pwoc as compared to watermelon grown in monoculture indicating watermelon underwent morphological changes at the leaf-level due to competition for light and allocated less C to leaves when competition was reduced. There were species differences, however, when comparing species within cropping system with peanut and pepper consistently having the lowest midday water potentials as well as the highest photosynthetic nitrogen-use efficiency values. d¹³C values were generally highest for peanut indicating higher water-use efficiency, and lowest for watermelon indicating low water-use efficiency. The results from this study suggest there may be a benefit to a multifunctional intercropping system in the form of enhanced plant resource-use efficiency and reduced water stress for some component species but not all. These findings also demonstrate innate interspecific variation in leaf-level measurements irrespective of intercropping system.