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 NaCl 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 SCR 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 C-3 pathway activity relative to C-4 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 SCR permitted these to be predicted from Delta regardless of the cultivar and salinity level. Delta thus provided an integrated measure of several components of physicological performance and response.