Photosynthetic performance of soybean plants to water deficit under high and low light intensity

• The effect of water deficit under high or low intensity on the photosynthetic performance of soybean plants.
• Experimental light treatments were used to simulate the field conditions: sole cropping and relay strip intercropping.
• The photosynthetic performance was severely reduced in drought conditions, shading alleviated the drought impact.

The two major challenges to relay strip intercropping soybean production in Southwest China are drought and low light intensity. This study tests whether the impact of drought on the photosynthetic performance of soybean plants is different between low and high light intensity conditions. To investigate this, soybean plants were grown in pots in a factorial experiment at two irrigation regimes (75 ± 2% and 45 ± 2% of soil field capacity) and two light intensity treatments (100% and 65% light intensity) in 2011. In 2012, soybean plants were grown in two irrigation regimes (75 ± 2% of soil field capacity vs. progressive soil drying) and two light intensity treatments (sole cropping soybean and relay strip intercropping soybean). Photosynthetic performance was assessed by measuring parameters such as net photosynthetic rate (Pn), stomatal conductance (Gs), water use efficiency (WUE), which were decreased significantly in drought stressed plants. We also observed differences in the photosynthetic responses of soybean plants to drought depending on the light intensity treatment the plants were subjected to. Shaded soybean plants in response to drought conditions had increased chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll (Chl), carotenoid (Car), ratio of Car/Chl, leaf relative water content (RLWC), leaf area per plant, specific leaf area (SLA), Pn, Gs, intercellular CO2 concentration (Ci), transpiration rate (Tr), photochemical quenching (qP) and electron transport rate (ETR). The above-mentioned photosynthetic changes may play an important role in determining how shaded soybean plants adjust their photosynthetic rate when experiencing drought conditions.