No abstract

The production of sorghum [Sorghum bicolor (L.) Moench], a moderately salt‐tolerant crop, often occurs in areas of relatively low rainfall and on marginal soils containing excess salts. Despite the importance of sorghum to agriculture, relatively less is known about its response to these stressful environments than that of other major cereal grain crops. The purpose of this investigation was to study the effects of moderate levels of salinity on growth, assimilate partitioning, and mineral nutrient relations of a single cultivar of sorghum under both field and greenhouse conditions. Sorghum dry matter production decreased substantially in response to a moderate increase in soil electrical conductivity (2.1 to 5.9 dS m−1). Overall, total shoot and root dry weights of greenhouse‐grown plants decreased to a greater extent by moderate (−0.2 and −0.4 MPa) additions of isosmotic concentrations of NaCl than Na2SO4. However, at higher salinity levels (− 0.6 MPa), growth was more inhibited by Na2SO4. Sheath tissues of the leaves of NaCl‐stressed plants tended to accumulate chloride. Sodium concentrations in the roots of control and NaCl‐treated plants were significantly higher than in shoot tissues at all salinity levels. At higher concentrations of Na2SO4, this exclusion of Na from the shoot broke down and resulted in a dramatic increase in the Na levels of shoot tissue and concurrent decrease in K and Mg concentrations. Seminal roots contained more Ca, Mg, Cl, and Na, while adventitious roots were higher in K.

Chloride distribution in the blade, sheath and midrib of leaves was determined for several grasses in salinised field plots and in nutrient solutions at various NaCl levels in the greenhouse. Chloride preferentially accumulated in the sheaths relative to the blades in all grass species and varieties examined when Cl- concentrations were expressed on a dry weight basis, although to varying degrees. Substantial levels of Cl- sheath partitioning were obtained only for sorghum when Cl- con- centrations were expressed on a fresh weight basis. Partitioning of Cl- in sorghum leaves was found to be ion specific and resulted from a combination of the ability of sheath tissue to accumulate Cl- to high concentrations and blade tissue to regulate Cl- concentrations at moderate levels.

Chloride transport in sheath and blade tissue and the cellular distribution of Cl‐ were investigated in an attempt to determine the physiological basis of the preferential accumulation of Cl‐ in sheaths of salt‐stressed sorghum (Sorghum bicolor L.). Import and export of 36Cl‐ in leaf sheaths and blades of intact sorghum were followed over a 2 week period. X‐ray microanalysis of frozen‐hydrated bulk tissue samples was used to determine the accumulation of Cl‐ and other elements in the vacuoles of sheath and blade cells. Sheath tissue accumulated Cl‐ despite a relatively high Cl‐ turnover rate. Chloride was shown to accumulate in most cell types of the sheath, particularly in adaxial epidermal cells. After an initial increase in the concentration of Cl‐, blade tissue regulated Cl‐ levels within certain limits. Chloride levels in blades were greater in the abaxial and adaxial epidermal cells than in other cell types. The epidermal cells of blades accumulated Cl‐ to approximately the same concentration as sheath epidermal cells. The Cl‐ concentration in the photosynthetically active mesophyll and bundle sheath cells, however, remained low. Thus, the partitioning of Cl‐ previously observed in the leaves of salinized sorghum apparently results from the ability of bundle sheath and mesophyll cells to maintain concentrations of Cl‐ at lower levels than do epidermal cells. In addition, the relatively large sheath parenchyma cells tend to serve as reservoirs for the storage of Cl‐.

Considerable variation exists in recommendations for the use of N during establishment of forage legumes. Abundant literature documents the inhibition of N2 fixation by applied N, but few experiments determined the consequences of its subsequent removal. In this study the effects of NH4NO3 on the growth and N2 fixation of subterranean clover (Trifolium subterraneum L.) were investigated. Plants were grown in a greenhouse from seed with a modified Hoagland's solution containing 0, 2, 4, 6, 8, and 12 mM NH4NO3 for 80 d. The plants were then divided into two groups, the N treatments continued for one (+ N) group and the other group given an N‐free Hoagland's solution (−N) for an additional 21 d. Total dry wt. (DM), apparent N2 fixation (ANF), leaf area (LA), and shoot N concentration were measured and levels of photosynthetically active radiation (PAR) were monitored continuously during daylight hours. Per plant DM accumulation increased exponentially over 101 d. Neither DM nor LA accumulation was influenced by NH4NO3 at 50 and 80 d. At 101 d, both DM and LA were greater (P < 0.01) for the 2 mM treatment than for the control, and each declined linearly (P < 0.01 for ± NPM and −NLA; P < 0.05 for +NLA) with increasing NH4NO3 levels. Differences in slopes were not significant (P > 0.05). Leaf area ratio (LAR) was not influenced by NH4NO3 treatments at any of the sampling dates. Nitrogen per plant was greater than the control (P < 0.01) for both ± N at 2 mM, and it declined linearly (P < 0.05 for +N; P < 0.01 for −N) with increasing NH4NO3 levels. A reduction in per‐plant ANF was significant (P < 0.01) over all NH4NO3 levels at 50 and 101 d and over 6 to 12 mM levels at 80 d. Plants previously grown at 6 to 12 mM levels (−N) recovered much of their ANF ability at 101 d, while ANF for + N plants declined (P < 0.01) across all N levels. Expression of ANF on a per unit DM or LA basis improved interpretation of these data. Nitrogen concentration of shoots for all treatments except 12 mM −N was similar to the control at 101 d. Overall, N yields per plant were best explained by DM accumulation from 80 to 101 d. The ability to rapidly recover N2‐fixing ability following depletion of inorganic soil solution N provides opportunity for early season use of N fertilizer in annual range systems.