Chloroplastic changes associated with paclobutrazol-induced stress protection in maize seedlings

In plants the chloroplast is a major site of free radical production and pa clobutrazol protects plants by increasing antioxidant defense systems. Leav es from paclobutrazol-treated maize (Zea mays L. cv. Pioneer 3902 and Pride 5) had larger chloroplasts and grana stacks, and more stromal lamellae. Tr eated leaves had more epicuticular wax and were thicker and wider owing to enlarged vascular bundles, epidermal, mesophyll, and bundle sheath cells. P aclobutrazol increased survival rate following chilling (2 degrees C for 8 h) and alleviated damage symptoms owing to chilling and exposure to paraqua t (60 and 120 nn?) in 'Pioneer' and 'Pride', the degree of protection being greater in 'Pride' (a less stress tolerant cultivar). Violaxanthin and lut ein levels were higher in treated etiolated and light-grown plants. During photoinhibitory conditions of 5 degrees C and 1300 mu mol m(-2) s(-1) there was less interconversion of violaxanthin to zeaxanthin in treated leaves. Dissociation of grana, changes in the osmiophillic properties of the thylak oids, decreased Fv/Fm ratio, and ion leakage occurred earlier and at a fast er rate in control leaves. Protection from paraquat and photoinhibitory dam age in maize leaves by paclobutrazol indicates that treated plants have a m ore efficient free radical scavenging system.