The regulatory role of silicon on water relations, photosynthetic gas exchange, and carboxylation activities of wheat leaves in field drought conditions

The effects of silicon on water relations, photosynthetic gas exchange, and carboxylation activities of wheat (Triticum aestivum L.) leaves were investigated in field drought conditions. Silicon application improved the leaf relative water content and water potential under drought. The leaf net photosynthetic rate and stomatal conductance were significantly decreased between 7:30 and 17:30 under drought, whereas silicon application increased the leaf net photosynthetic rate between 7:30 and 15:30 with an exception at 9:30. Silicon application also increased the leaf stomatal conductance at 13:30 and 17:30 under drought. The leaf transpiration rate was decreased by drought but it was increased by silicon from 13:30 to 17:30. The intercellular CO2 concentration was increased at 7:30 under drought, while it was decreased most of the time from midday to the afternoon. The leaf stomatal limitation was increased under drought from 11:30 to 17:30, whereas it was intermediate in silicon treated plants. The instantaneous water use efficiency was significantly increased by silicon application at 7:30 under drought. Silicon application slightly decreased the activity of ribulose-1, 5-bisphosphate carboxylase, but it increased the activity of phosphoenolpyruvate carboxylase and the concentration of inorganic phosphorus under drought. These results suggest that silicon could improve the photosynthetic ability of wheat in field drought conditions, and both stomatal and non-stomatal factors were involved in the regulation. In the early morning (at 7:30), the non-stomatal factor was the main contributor; 9:30 was a turning point, after which the stomatal factor was the main contributor.