Environmental Stress and Seasonal Changes in Proline Concentration of Citrus Tree Tissues and Juice

Abstract

The purpose of this study was to evaluate seasonal changes in the free proline content of citrus roots, leaves, fruit peel, and juice in response to low-temperature and water stress. Nonirrigated trees generally had higher proline in all tissues than did irrigated trees except immediately after a freeze. At this time, nonirrigated trees were less water-stressed because of the greater amount of freeze-induced defoliation that nonirrigated trees had sustained. Using data from an entire year, proline concentration was not correlated with water stress of leaves or fruit. This lack of correlation probably was due to the interacting effects of water stress and low temperature on proline accumulation. Leaves accumulated proline in response to stress before roots and fruit. These data support the idea that the free proline increases first in the leaves in response to stress and subsequently is transported to other tree tissues. Even though proline content in the juice increased with fruit maturity, proline may not be a good indicator of juice quality since it did not always correspond with Brix:acid ratio and fruit in the most exposed canopy positions tended to have the highest proline content.