Daily Temperature Amplitude Affects the Vegetative Growth and Carbon Metabolism of Orange Trees in a Rootstock-Dependent Manner

Both instantaneous and average growth temperatures affect plant metabolism, and the physiological importance of daily variations in temperature is frequently underestimated. To improve our understanding of the environmental regulation of citrus trees, we hypothesized that vegetative growth would be stimulated in orange plants subjected to large daily temperature variations, even without changes in the average daily air temperature or the amount of energy given by degree-days. This hypothesis was tested with orange plants grafted onto Rangpur lime or Swingle citrumelo rootstocks and grown for 20 days under thermal regimes (day/night) of 25/25°C or 32.5/17.5°C. Such regimes imposed growth conditions with daily temperature variations of 0 and 15°C. Plant growth, photosynthesis, respiration, and carbohydrate availability in leaves, stems, and roots were measured under both thermal conditions. The daily temperature variation affected the carbon metabolism of young citrus trees; plants grown under daily variation of 15°C used more of the carbon stored in mature leaves and roots and the energy generated by respiration for the biosynthesis of vegetative structures, such as leaves and branches. Thus, there was a significant increase in the leaf area of plants subjected to high daily temperature variation. Current photosynthesis was similar in the two thermal regimes; however, the photosynthetic rates increased under the 15°C variation when measurements were normalized to 25°C. In addition to the stimulatory effect of the source–sink relationship on photosynthesis, we suggest a probable involvement of hormonal regulation of plant growth through gibberellin metabolism. The rootstock affected the response of the canopy to daily temperature amplitude, with the Rangpur lime improving plant growth through higher carbohydrate availability in roots. This is the first report that highlights the importance of daily temperature variations for citrus growth and physiology under nonlimiting conditions.