Agronomic and physiological responses of soybean and sorghum crops to water deficits. 3. Components of leaf water potential, leaf conductance, C14-dioxide photosynthesis and adaptation to water deficits


Turner, N.C.; Begg, J.E.; Rawson, H.M.; English, S.D.; Hearn, A.B.

Australian Journal of Plant Physiology 5(2): 179-194

1978


Concurrent measurements of leaf water potential, leaf osmotic potential, leaf relative water content, quantum flux density, leaf conductance, 14CO2 photosynthesis, soluble and insoluble sugars, and starch and K concentrations were made diurnally on 6 occasions between flowering and maturity on upper leaves of irrigated and rainfed crops of [Glycine max] (cv. Ruse and Bragg) and a rainfed crop of sorghum [Sorghum bicolor] ('TX 610'). With adequate soil water, sorghum had lower values of leaf conductance than did soybeans at high light, and yet had higher rates of photosynthesis. Stage of plant development had no effect on either leaf conductance or photosynthesis of the youngest fully expanded leaves of sorghum and soybean, but starch accumulation in the leaf over the day was less at grain-filling than at flowering in the soybean. Starch and sugar levels in the leaf had no apparent effect on photosynthesis. The daily minimum leaf water potential decreased in 'Ruse' soybean from -1.5 to -2.7 MPa [megapascal] as soil water was depleted. Late in the drying cycle the daily minimum leaf water potential was higher in 'Bragg' than in 'Ruse'. In both cultivars, stomatal closure and decrease in 14CO2 photosynthesis commenced at leaf water potential below -1.5 MPa. The effect of water deficits on leaf conductance and photosynthesis occurred later in the drying cycle in 'Bragg' than 'Ruse'. As photosynthesis decreased with the depletion of soil water, starch accumulation in leaves of both cultivars of soybean decreased; changes in soluble and insoluble sugars and in K were small. The relationships among leaf water potential, osmotic potential, turgor potential and leaf relative water content did not change with season or soil water depletion. 'Bragg' and 'Ruse' soybeans showed a similar and both approached zero turgor at the same relative water content (82-83%) and the same leaf water potential (-1.5 to 1.7 MPa). No evidence of osmotic adjustment was found in either soybean cultivar.