Effects of Air and Soil Temperatures on Vegetative Growth of Citrus1

Abstract

Abstract Interactive effects of air and soil temperatures on vegetative growth of 4- and 8-year-old ‘Frost Navel’ orange [Citrus sinensis (L.) Osbeck] budded to trifoliate orange rootstocks [Poncirus trifoliata (L.) Raf.] were determined in controlled environments. Quiescent trees were subjected to 1 of 4 environmental treatments in 2 glasshouses consisting of 2 day/night air temperature regimes and 2 constant soil temperatures for 2 months in each experiment. Shoot initiation was doubled with warm soils (25°C) compared with cool soils (15°C) in all experiments. This effect of soil temperature on bud break was observed with both hot (35°C) and warm (25°C) day air temperatures (night air temperature was 15°C in both glasshouses) and with warm (25°C) and cool 15°C) night air temperatures (day air temperature was 30°C in both glasshouses). These large differences in shoot initiation resulted in a greater vegetative flush with respect to total shoot length, branch dry weight and leaf dry weight with warmer soils. Warmer day temperatures resulted in longer shoots with greater dry matter, whereas, warmer night temperatures had little effect on shoot length. Only small differences were observed among treatments in leaf water status and inorganic nutrient status. This suggests that soil temperature effects on root uptake of water and minerals may not have been responsible for the effects of soil temperature on shoot initiation and growth. Warmer soils may have increased bud break by influencing other processes such as the hormonal balance in the shoot.