Growth Capacity of ‘Valencia’ Orange Buds on Different Rootstocks during Cold-hardening Temperatures

Abstract

Abstract Initial growth and development of ‘Valencia’ orange [Citrus sinensis (L.) Osbeck] shoots from buds grafted on 7 different rootstocks were determined during cold-hardening and nonhardening temperature regimes for the 1st 4 months after budbreak. Cold hardening included 12 hr each of 15.6°/4.4°C day/night (D/N) in controlled-environment rooms, where 80% fluorescent and 20% incandescent light averaged 450 μmol m-2s-1. Nonhardening included 32.2°/21.1° D/N as well as greenhouse conditions with natural daylight. Rootstocks influenced growth of ‘Valencia’ during cold-hardening temperatures. Cold-hardening temperatures, however, did not stop ‘Valencia’ growth on any of the rootstocks. Total growth was less than 1/20 of the dry weight accumulated during the warm temperatures. During the cold-hardening treatment, ‘Valencia’ grew the most on the relatively cold-tender volkamer lemon [C. volkameriana (Ten. and Pasq.)] rootstock. The least growth was on the more cold-hardy Swingle citrumelo (C. paradisi Macf. × Poncirus trifoliata (L.) Raf.] rootstock. The amount of growth during cold-hardening temperatures generally coincided with the cold hardiness of the rootstocks, but statistical separation was poor. Trees grown at the warm temperatures started to freeze at higher temperatures than trees grown at cold-hardening temperatures. Rootstocks did not influence freeze avoidance, and no differences were apparent in freeze (ice) tolerance. Three layers of palisade cells were found in leaves that were grown at the cold temperatures.