The nonclimacteric guava cultivar ‘Jen-Ju Bar’ is defective in system 2 1-aminocyclopropane-1-carboxylate synthase activity

The ripening characteristics and physiology of two guava cultivars, ‘Jen-Ju Bar’ (‘JJB’) and ‘Li-Tzy Bar’ (‘LTB’), were investigated and compared because of their divergent storability. During storage at 20°C, mature-green ‘LTB’ fruit exhibited ripening behavior typical for a climacteric fruit, including increases in respiration and ethylene production (3.75mmolCO₂kg⁻¹h⁻¹ and 1.37×10³nmolC₂H₄kg⁻¹h⁻¹ at peak levels, respectively), yellow color development, flesh softening, and volatile production. By contrast, ‘JJB’ is considered a nonclimacteric fruit because it features low and stable rates of respiration (0.61–0.73mmolCO₂kg⁻¹h⁻¹) and ethylene production (less than 4nmolC₂H₄kg⁻¹h⁻¹) while retaining its pale-green color and firmness throughout a 20d observation period. Furthermore, the nonclimacteric nature of ‘JJB’ fruit was revealed through its responses to various levels of applied ethylene, as its respiration rate increased in a concentration-dependent manner. This respiration enhancement occurred only when exogenous ethylene was present and could be evoked repeatedly in the same ‘JJB’ fruit. The ripening characteristics of ‘JJB’ do not result from a defect in ethylene signaling, but instead result from a lack of autocatalytic ethylene synthesis, as propylene ventilation stimulated normal fruit ripening and an upsurge of respiration without a massive endogenous ethylene release. However, a cutting treatment of ‘JJB’ was able to induce stress-responsive ethylene production expected from system 1 ethylene biosynthesis. The finding that 1-aminocyclopropane-1-carboxylic acid (ACC) content and ACC synthase (ACS) activity were significantly lower in ‘JJB’ than in ‘LTB’ fruit after harvest, together with the finding that treatment with ACC (rather than S-adenosyl-l-methionine) led to both fully ripened ‘JJB’ fruit and endogenous ethylene production, strongly suggest that insufficient ACC synthesis, i.e., the reaction catalyzed by ACS, prevents system 2 ethylene biosynthesis in ‘JJB’ guava fruit.