Low Si combined with drought causes reduced transpiration in sorghum Lsi1 mutant

Background and aims: High and stable plant productivity is a major aim in agricultural research. Silicon fertilization improves yields of various crops under stress. Nonetheless, broad application of silicon is inhibited by the lack of a mechanism explaining this effect. Experimental System: To study the role of silicon in soil-grown plants under drought, we utilized a sorghum (Sorghum bicolor) mutant plant lacking the key silicon root channel – Low silicon 1 (SbLsi1). The sblsi1 mutant plants absorb 1/15 of the silicon absorbed by wild type plants, making them a suitable tool to examine silicon physiology in soil and under field conditions. Results: In mutant plants grown in pots, significant reductions in momentary and accumulated whole plant transpiration, photosynthesis rate, and stomatal conductance were found only under water stress. Root structure, root hydraulic conductance, and stomatal density were similar between wild type and sblsi1 plants. Similar leaf water contents between the genotypes suggested that the water uptake was balanced with transpiration. Conclusions: The similarity between the genotypes under benign conditions are in accordance with minor to no effects of silicon fertilization in non-stressed plants, and support the minor pleiotropic effects of the mutation. Early stomatal closure in the mutant plants under drought stress caused the reduced transpiration. Thise early response suggests that silicon may delay the onset of drought physiology by either reduced stress signaling or reaction.