CARBON-ISOTOPE DISCRIMINATION, GAS-EXCHANGE, AND GROWTH OF SUGARCANE CULTIVARS UNDER SALINITY

Physiological features associated with differential resistance to sali nity 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 decisiem ens (dS) m(-1) electrical conductivity (EC). The CO2 assimilation rate (A), stomatal conductance (g), and shoot growth rate (SGR) began to d ecline as EC of the irrigation solution increased above 2 dS m(-1). A, g, and SCR of a salinity-resistant cultivar (H69-8235) were consisten tly higher than those of a salinity-susceptible cultivar (H65-7052) at all levels of salinity and declined less sharply with increasing sali nity. Carbon isotope discrimination (Delta) in tissue obtained from th e uppermost fully expanded leaf increased with salinity and with time elapsed from the beginning of the experiment, but Delta was consistent ly lower in the resistant than in the susceptible cultivar at all leve ls 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 activi ty rather than by physical changes in the permeability of the bundle s heath to CO2. A strong correlation between Delta and A, g, and SCR per mitted 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.