Stomatal Resistance of Phosphorus-Deficient Soybean.

Little information exists concerning the stomatal resistance of soybean [Glycine max (L.) Merr.] cultivars grown under low phosphorus conditions.  Therefore, we measured the stomatal resistance of two cultivars of soybean, one a traditional Thai variety (Chiangmai 60 or CM 60) and one an improved cultivar developed at Khon Kaen University in Thailand (KKU 74) for three months under greenhouse conditions in soil (Morrill loam) that was low in phosphorus (6.5 ppm P via a Mehlich-3 soil test) before fertilizer addition.  There were two phosphorus rates:  0 P (no phosphorus added to the soil) and 40 mg P per kg soil added using triple superphosphate   Plants were kept at pot capacity.  Stomatal resistance was measured every two days between 43 and 97 days after planting (28 times) with a steady state diffusion porometer (Decagon Devices, Pullman, WA, Model SC-1).  At harvest, shoot dry weights were determined.  The average stomatal resistances and standard errors of CM 60 without and with P fertilizer were 468+47 and 1050+108 s/m.  The average stomatal resistances and standard errors of KKU 74 without and with P fertilizer were 688+60 and 1333+130 s/m.  Dry weights of CM 60 shoots without and with P fertilizer were 16.29+1.24 and 33.40+4.84 g/plant.  Dry weights of KKU 74 shoots without and with phosphorus fertilizer were 20.96+2.44 and 39.61+5.56 g/plant. Despite higher stomatal resistances of plants grown with phosphorus fertilizer, growth was increased with it.  This finding is contrary to most results that show that stomatal resistance and growth are inversely related.  The effect of phosphorus on stomata needs further elucidation.