Significant genetic variation in leaf photosynthetic rate has been reported in grain sorghum [Sorghum biocolor (L.) Moench]. The relationships between leaf photosynthetic rates and total biomass production and grain yield remain to be established and formed the purpose of this experiment. Twenty two grain sorghum parent lines were tested in the field during the 1988 growing season under well-watered and water-limited conditions. Net carbon assimilation rates were measured at mid-day during the 30 day period from panicle initiation to head exertion on upper-most fully expanded leaves using a portable photosynthesis system (LI-6200). Total biomass and grain production were determined at physiological maturity. The lines exhibited significant genetic variation in leaf photosynthetic rate, total biomass production and grain yield. Significant positive correlations existed between leaf photosynthesis and total biomass and grain production under both well-watered and water-limited conditions. The results suggest that leaf photosynthetic rate measured prior to flowering is a good indicator of productivity in grain sorghum.

Osmotic adjustment, defined as a lowering of osmotic potential (*In) due to net solute accumulation in response to water stress, has been considered to be a beneficial drought tolerance mechanism in some crop species. The objective of this experiment was to determine the relative contribution of passive versus active mechanisms involved in diurnal 'II changes Loss of water from turgid leaf tissue in response to transpiration results in not only a significant decline in 'I',' but also a decline in 'In, to a lesser extent. As the leaf tissue 'I,, declines even more in response to soil water deficits, 'I' declines accordingly as reported in a large number of crop species (2,(13)(14)(15)(16)19). A decline in measured 'I' can result from a simple passive concentration of solutes due to dehydration or due to net solute accumulation. The lowering of 'n by net solute accumulation is termed OA. OA has been considered a beneficial drought tolerance mechanism in some 'Abbreviations: I', water potential (MPa); 'n, osmotic potential (MPa); Ip, turgor potential (MPa); In°"°, 'n at full hydration (MPa); RWC, relative water content (%); OA, osmotic adjustment (MPa); E, modulus of elasticity (MPa); TW, turgid weight (g); DW, dry weight (g); WC, water content (g).field crop species (6,9,20,22). Processes such as cell expansion are dependent on the influx of water driven by the maintenance of lower T' by net solute increase in the expanding cell. The lowering of Tr, by OA also minimizes the opportunity for significant water loss to occur from leaftissue.Passive concentration of solutes due to dehydration can arise from a decrease in WC per unit DW, reduced leaf tissue volume due to cellular shrinkage, change in leaf tissue elasticity, or relative partitioning of water between the symplastic and apoplastic fractions.

Lowering of the solute potential by osmotic adjustment (OA) has been proposed to allow maintenance of leaf turgor potential (*p), stomatal conductance (g), and photosynthesis (A) at low leaf water potential. However, literature conceming the role of OA in the maintenance of g and A under water stress is limited and often contradictory. The objective of this experiment was to examine the association of OA with g and A in grain sorghum (Sorghum bicolor L. Moench). A single sorghum hybrid (cv ATx623 x RTx430) was studied under field conditions using four different water supplies. Diumal and midday water potential, solute potential, lp, OA, g, and A were measured during preflowering and grain-filling growth stages. A second experiment was conducted under greenhouse conditions. Two sorghum genotypes (BTx623 and BTx378) differing in their g and A responses to plant water stress were compared for their OA capacity during a water deficit cycle imposed from the beginning of panicle initiation through flowering. Under both field and greenhouse conditions, g and A rapidly declined with increased water stress despite the occurrence of OA. Under greenhouse conditions, BTx623 maintained significantly higher g and A than BTx378 during the water stress cycle. However, no significant differences in OA or Ip existed between the two genotypes, indicating that OA was not associated with differences observed in g and A between these genotypes. We conclude that the response of g and A to water stress was not directly associated with OA and certainly was not maintained by OA.

The presence of an extranuclear genome in the chloroplast (ctDNA) suggests possible cytoplasmic inheritance of photosynthetic characteristics of leaves. Recent evidence indicated a strong maternal influence on photosynthetic rates (A) of grain sorghum [Sorghum bicolor (L.) Moench] hybrids. Since essentially all commercial grain sorghum hybrid seed production is accomplished using cytoplasmic male‐sterility (CMS), we were concerned about the possible effects of CMS on photosynthetic rates of sorghum hybrids and about potential nuclear—cytoplasmic interactions. The purpose of this experiment was to evaluate the effect of different sources of CMS and potential nuclear—cytoplasmic interactions on photosynthetic rate (A) and stomatal conductance (g) of H2O for sorghum leaves under field and greenhouse conditions. Five sources of male‐sterility‐inducing cytoplasm (A1, A2, A3, A4, and 9E) and a male‐fertile line (B) were compared in a common nuclear background (TX398) to test the effect of source of cytoplasmic genotype on leaf gas exchange parameters. Additionally, eight commonly used sorghum lines with different nuclear backgrounds were used to determine nuclear—cytoplasmic interaction effects by comparing fertile with male‐sterile counterparts. No significant differences were observed among lines with different sources of CMS in a common nuclear background (TX398) for A or g. Significant genetic variation in A and g was observed within the eight lines having different nuclear backgrounds. However, no significant differences were observed in any of the gas exchange parameters between fertile and sterile lines within each nuclear genome. The interaction of cytoplasm with genotype or environment was not significant. Results from this study suggest that the genetic expression of leaf gas exchange rates was not influenced by CMS, nor were there any differences among the sources of CMS.