Sensitivity of frost resistance and growth in citrus and avocado to chronic ozone exposure

Abstract

summaryThe effects of elevated levels of ozone on growth and frost resistance were studied using broadleaf‐evergreen citrus and avocado trees. ‘Ruby Red’ grapefruit (Citrus parodist L.) trees on either Volkamer lemon (Citrus volkanieriana Ten. & Pasq.) or sour orange (Citrus aurantium L.) rootstocks and ‘Simmonds’ or ‘Pancho’ avocado trees (Persea americana Mill.) on the rootstock ‘Waldin’ were exposed to ozone in open‐top chambers for 4 months in 1988 and for 8 months in a second experiment in 1989. The ambient ozone concentration, averaged over a 12 h day, was 30‐1 nl 1−1 in 1988 and 39‐1 nl 1−1 in 1989 during the exposure period. Citrus tree growth, estimated by total leaf mass, was unaffected by ozone concentrations of 3 × ambient in either year but the growth of avocado in 1989 was reduced 20 and 61 % by ozone concentrations at 2 × and 3 × ambient, respectively. Frost resistance, estimated by electrolyte leakage from leaf disks and from survival of leaves, stems and whole plants following exposure to freezing temperatures, was often diminished in avocado and citrus at 3 × ambient ozone, but occasionally was increased at 2 × ambient. The effects of chronic ozone exposure on frost resistance were generally subtle; 3 x ambient ozone exposure in 1989 compared to 0.3 × only caused the temperature for 50% electrolyte leakage (LT30) to increase less than 1 °C (− 5.3 to −4.8 °C) in citrus and about 2 °C (− 8.0 to −5.8 °C) in avocado. Nevertheless, the tests for frost resistance on whole plants indicated that these differences could have a substantial impact on stem and plant survival if the frost was at a critical temperature. The likelihood that chronic ozone exposure currently affects citrus frost resistance in Florida appears slight.