Stomatal and photosynthetic responses of soybean with two types of mulch under different soil water conditions

Abstract

Soybean was planted in clay pots with living mulch of white clover (CL) or shredded paper mulch (SP). Soil moisture in the pots was individually controlled at five levels from around 20% of volumetric soil water content (SWC) to near-saturated, by monitoring SWC every morning and irrigating intermittently. This study focussed on the growth period of the soybean when plant height and LAI reached around maximum. The lower evapotranspiration (ET) and lower stomatal conductance (gs) of the soybean in lower SWC with both SP and CL were caused by water stress. The significantly higher ET but lower gs for CL compared with SP at the same SWC levels revealed that the soybean with CL competed with the clover for water and that the clover extracted water even at the lower SWC. The lowest electron transport rate (ETR) and non-photochemical quenching (NPQ) at the lowest SWC level with both SP and CL were regarded as being caused by water stress, under which the thermal dissipation processes could not be activated under strong light with photoinhibition. The lower ETR and the higher NPQ under the low PPFD condition for CL compared with SP could be interpreted as the soybean with CL having both water and nutrient stresses, even under the low PPFD condition, but the soybean with CL activated NPQ to protect PhSII by reaching the maximum NPQ under lower PPFD compared with the soybean with SP. The lower photosynthetic rate (P) of the soybean at the lower SWC level for both SP and CL resulted from the lower gs and ETR caused by water stress. The lower P for CL compared with SP at the same SWC level resulted from the lower gs and ETR and severer photoinhibition despite the activated function of NPQ caused by the water stress and by the nutrient stress.