Study site: The experimental site was located in Mao county eastern Qinghai-Tibet Plateau (31° 41′ N, 103° 53′ E, elevation 1686 m). According to meteorological monitoring data from the Mao County Ecological Station of Chinese Academy of Science, the mean annual precipitation in the area is 920 mm, mean annual temperature is 8.9 ℃ and extreme minimum and maximum temperatures recorded are -11.6 ℃, and 32.2 ℃, respectively. The total precipitation in August is approximately 90 mm, and it is considered as the month with the most rainfall. The soils are classified as Udic Luvisols. Experimental design: A batch of uniform, two-year-old seedlings of Z. bungeanum were planted in April 2013. Six experimental treatments were set up as a randomized design with three replicates, with 18 plots of 2.6 m × 2.6 m spaced at least 1 m apart from each other. The three planting systems were as follows: (1) Z. bungeanum + Glycine max (Z-G); (2) Z. bungeanum + Z. bungeanum + Capsicum annum (Z-C); (3) Z. bungeanum monoculture (Z). G. max and C. annuum were planted in April 2015. One Z. bungeanum was grown in the center of each plot, while species of G. max and C. annuum were planted at the same density (0.27 m²/individual) in all plots. No additional fertilization was applied after the experiment commenced, and the weeds in each plot were completely removed by hand each week. Z. bungeanum and intercrops were grown under natural rainfall before simulating extreme precipitation. In August 2015, we exposed our plots, to the precipitation treatment at random, in which triplicate plots per system received either normal (control) or extreme rainfall. To avoid external rainfall effects, all plots were kept under rainout shelters during the experimental period (from 1″t August to 30th September 2015) to control soil moisture. To minimize greenhouse effects, the rainout shelters for each plot were situated 2 m aboveground. Tap water was used to mimic extreme rainfall events, and a watering pot was used to compensate for rain. Rainfall regimes were designated, based on the average rainfall in the area during August of 3 mm/day (based on the average rainfall data during 1983-2013 from the Mao County Ecological Station of Chinese Academy of Science). This was designed as the control rain regime, while extreme rainfall was designated according to the abnormally high rainfall in August of 9.5 mm/day. Each planting system was first divided into two groups of different treatments: (1) Extreme rainfall (9.5 mm/day) and (2) Mean rainfall (Control, 3.0 mm/day). During the two-month-long experimental period, all the plots were watered in the morning (7-9 am) and evening (6-8 pm). After one month of extreme rainfall and control treatments, the systems were subsequently subjected to one month of recovery with rainfall of 3.0 mm/day. Around all plots, thick PVC panels were inserted to a depth of 0.5 m into the soil to prevent the lateral water movement between the plots and prevent interactions with roots from neighboring plots. Plant leaf collection: At the end of each stage, the youngest fully expanded Z. bungeanum, G. max, and C. annum leaves at the same developmental stage among plots were collected and placed in a liquid nitrogen container.

The results indicated that, extreme rainfall had significantly negative impacts on Z. bungeanum in three intercropping systems. However, intercropping with G. max improved the transpiration rate (Tr) and stomatal conductance (Gs), raised leaf relative water content (LRWC), increased chlorophyll a (Chl a) and carotenoid (Car) content, and enhanced the superoxide dismutase activity (SOD) of Z. bungeanum. After recovery, the Z. bungeanum + G. max mixed culture significantly increased soil NO3 -N, improved the intercellular carbon dioxide concentration (Ci) and Tr, upregulated soluble sugar and proline, and enhanced hydrogen peroxidase activity (CAT). Moreover, the higher root biomass of G. max provided much more nitrogen for Z. bungeanum via the return of organic matter. However, intercropping with C. annum significantly increased active oxygen (ROS). Compared with neighboring species, in intercropping systems, G. max could have improved the tolerance of the focal species Z. bungeanum in response to extreme rainfall and its recovery after extreme rainfall.