The greater sensitivity of B. carinata to salinity in comparison to B. napus has been linked to a greater reduction in net assimilation rate. Apparently this is not due to ion toxicity; the cause is unknown. In this report, we test the hypothesis that increases in abscisic acid (ABA) are involved in the reduction of growth by salinity. Salinity (8 dS m-l) caused an increase of ABA concentrations in the shoot, root and callus of both species. ABA concentrations were lower in the salt-tolerant species, B. napus, than the salt-sensitive species, B. carinata, both in the whole plant and callus. Leaf expansion for both species was equally sensitive to ABA; salt stress did not significantly alter sensitivity to applied ABA. The growth inhibition increased in a hyperbolic manner with an increase in endogenous ABA concentration. These results indicate that ABA in salt-stressed plants may play a role in the inhibition of growth. The photosynthesis of salt-sensitive species, B. carinata, was also decreased by salinity, corresponding to the reduction in growth. The decreased photosynthesis does not appear to be the cause of the growth reduction, because photosynthesis was not inhibited by short-term exposure to salinity and photosynthesis was poorly correlated with endogenous ABA concentrations.

The growth of six rapid-cycling lines of Brassica species, B. napus, B. campestris, B. nigra, B. juncea, B. oleracea and B. carinata was inhibited by seawater salinity. Based on the change in dry matter reduction relative to the control at varying concentrations of salts (4, 8 and 12 dS m-l), the relative salt tolerance of these species was evaluated. B. napus and B. carinata were the most tolerant and most sensitive species, respectively, while the other four species were moderately tolerant. The influence of seawater on the concentrations of 12 elements including macronutrients and micronutrients in the shoots of these Brassica plants was characterized to determine the relationship between nutritional disturbance and relative salt tolerance. It was found that seawater salinity had a significant effect on the concentrations of Ca, Mg, K, CI, Na and total N in the shoots of these plants but only the change in Ca concentration was significantly related to the relative salt tolerance of these six rapid-cycling Brassica species according to a rank analysis of the data. This finding indicates that Ca may play a regulatory role in the responses of Brassica species to saline conditions.

Cellular responses of two rapid‐cycling Brassica species. B. napus and B. carinata, to seawater salinity were characterized to determine whether callus showed a tolerance similar to that of whole plants. Callus was initiated from the leaves of 7‐day‐old seedlings of B. napus and B. carinata and then subcultured with two different levels of seawater salinity (2.3 and 5.2 g l−1 Instant Ocean. Aquarium Systems, Inc. Mentor. OH, USA) for 14 days. Callus growth of both species was reduced by seawater salinity. Based on the percentage of the reduction in the relative fresh weight gain. B. napus was more salt‐tolerant than B. carinata. consistent with the response of whole plants of the same species to seawater salinity. Seawater salinity caused changes in the concentrations of Na, K. Ca, Mg and Cl in both B. napus and B. carinata. The growth expressed as the percentage of control was significantly (P = 0.05) positively correlated with the concentration of Ca. and K/Na and Ca/Na ratios. It was also negatively correlated (P= 0.01) with the concentrations of Na. Cl and Mg. In comparison with B. carinala. the salt‐tolerant species, B. napus, showed a small reduction in the concentrations of Ca and K in the salt‐stressed plants relative to the control.

The potassium/sodium (K/Na) ratio in the shoots of six rapidcycling Brassica species was greatly reduced by seawater salinity. It proportionally decreased with increasing salinity from 4 dS/m to 12 dS/m. This decrease was correlated with salt-induced growth reduction (expressed as the percentage of control) within a species. The change in K/Na ratio with increasing salinity, however, was not correlated with K-Na selectivity. The change in K/Na ratio at the callus level corresponded to that at the whole plant level. Both K/Na ratio and K-Na selectivity were not found to be correlated with the relative salt tolerance of these Brassica species, indicating that the maintenance of a high K/Na ratio as a mechanism for salt tolerance in these Brassica species was not operative. These results also suggest that a high shoot K/Na ratio or K-Na selectivity may not be reliable selection criteria for salinity resistance in some species. INTRODUCTIONA condition of "ion excess" within the plant has been considered an important factor that limits plant growth under saline conditions (1). Studies on the effects of an excessive ion concentration on membrane condition (2,3,4), enzyme activity (5,6), protein synthesis (7) and glycolysis (8) have shed some light on the mode of salinity injury, but the exact mechanism by which an excessive ion concentration suppresses plant growth is not clear.

The response of two rapid-cycling Brassica species differing in tolerance to seawater salinity was studied over a period of 24 days. In response to 8 dS m 1 salinity, the two Brassica species showed clear differences in the changes in relative growth rate (RGR), net assimilation rate (NAR) and leaf area ratio (LAR). The RGR of B. napus was slightly reduced by salinity, whereas the RGR of B. carinata was largely reduced in the early stages of salinization. LAR of B. napus was affected by salinity in the later stages of growth and significantly correlated with the reduction in RGR. On the other hand, the NAR of B. carinata was decreased by salinity, corresponding to the decrease of the RGR of B. carinata. The NAR of B. napus was not significantly affected by salinity according to analysis of covariance. The shoot concentrations of Na, Mg and C1 increased while the concentrations of K and Ca decreased sharply during the first 5 days of salinization; subsequently, all ion concentrations remained relatively constant. The concentrations of Na, K, Ca, Mg and C1 in the root were similarly affected by salinity. There were no significant differences of ion concentrations between species that could be related to the differences in salt tolerance. Thus, the differences in salt tolerance between species can not be related to differences in specific-ion effects, but may be related to some factor that reduces the NAR of B. carinata during the early stages of growth.