Physiological responses of sugarcane to nitrogen and potassium availability

Abstract

This study was conducted to evaluate the effects of N and K availability on physiological characteristics of two sugarcane (Saccharum spp. hybrid) cultivars which were previously shown to differ in their resistance to drought and salinity. The plants were hydroponically grown in a greenhouse and subjected to varying levels of N and K. Photosynthesis and growth were reduced by both N and K stress. Reduced stomatal conductance and reduced ribulose 1,5-bisphosphate carboxylase (Rubisco) and phosphoenolpyruate carboxylase (PEPC) activities all contributed to reduced photosynthesis under N stress. The Rubisco:PEPC activity ratio also decreased under N stress resulting in an increase in bundle sheath leakiness to CO2 which in turn reduced the efficiency ofC4 photosynthesis. This was reflected in higher carbon isotope discrimination values in leaves of plants subjected to N stress. N stress increased photosynthetic nitrogen-use efficiency (PNUE) and intrinsic water use efficiency. Leaf water potential and hydraulic conductance decreased in N-stressed plants. In contrast, K-stress increased the leaf water potential and hydraulic conductance. N and K stress increased root affinity for N03- and K+ absorption by increasing Vmax and decreasing Km for N03- and K+ uptake. In addition, dry matter allocation to roots increased with decreasing N and K supply. N and K stress substantially altered the composition of the xylem sap. Xylem sap N03- and K concentrations were highly correlated with the external solution N and K concentrations, respectively, suggesting the potential for using xylem sap composition as a guide in sugarcane fertilization. The more drought and salinity resistant cultivar also showed higher resistance to N and K stress. Greater resistance to N stress was associated with greater allocation of leaf N to Rubisco relative to PEPC. This increased photosynthetic efficiency by reducing bundle sheath leakiness to CO2 resulting in a higher photosynthetic rate and PNUE. The more stress resistant cultivar also had higher affinity for N03- and K+ absorption under N and K stress. These and previous results of drought and salinity stress studies suggest that the resistance of sugarcane to multiple stresses may involve a general stress response system.