Carbon Isotope Discrimination, Gas Exchange, and Growth of Sugarcane Cultivars under Salinity.

Physiological features associated with differential resistance to salinity were evaluated in two sugarcane (Saccharum spp. hybrid) cultivars over an 8-week period during which greenhouse-grown plants were drip-irrigated with water or with NaCI solutions of 2, 4, 8, or 12 decisiemens (dS) m-1 electrical conductivity (EC). The CO2 assimilation rate (A), stomatal conductance (g), and shoot growth rate (SGR) began to decline as EC of the irrigation solution increased above 2 dS m-1. A, g, and SGR of a salinity-resistant cultivar (H69-8235) were consistently higher than those of a salinity-susceptible cultivar (H65-7052) at all levels of salinity and declined less sharply with increasing salinity. Carbon isotope discrimination ([delta]) in tissue obtained from the uppermost fully expanded leaf increased with salinity and with time elapsed from the beginning of the experiment, but [delta] was consistently lower in the resistant than in the susceptible cultivar at all levels of salinity. Gas-exchange measurements suggested that variation in [delta] was attributable largely to variation in bundle sheath leakiness to CO2 ([phi]). Salinity-induced increases in [phi] appeared to be caused by a reduction in C3 pathway activity relative to C4 pathway activity rather than by physical changes in the permeability of the bundle sheath to CO2. A strong correlation between [delta] and A, g, and SGR permitted these to be predicted from [delta] regardless of the cultivar and salinity level. [delta] thus provided an integrated measure of several components of physiological performance and response.