The Influence of Root Temperature on Apple Trees. III. The Effect on Photosynthesis and Water Balance

Abstract

SummaryOne-year-old M 2 apple trees were grown at three constant root temperatures (22°, 29°, and 36°C) in containers filled with either clay soil or a sandy loam.Rates of photosynthesis and transpiration measured four months after the start of the experiment decreased with rising soil temperature; the former more sharply in the sandy loam than in the clay soil. The potassium and chlorophyll content of the leaves decreased significantly with increasing soil temperature only from 29° to 36°C. Leaf water potential decreased, while the resistance of the leaf to the loss of water vapour, and the shoot : root dry weight ratio increased with rising temperature. The water content of the roots increased with rising temperature and that of the leaves decreased. The water content of both roots and leaves was as a rule lower in clay soil than in sandy loam.All these effects at 36°C soil temperature were significantly reduced by the application of potassium sulphate to the soil at the start of the experiment. The reducing sugar content of the leaves was not influenced by any of the treatments applied.The role of ethanol in apple leaves of plants subjected to supraoptimal root temperatures was investigated by immersing the root system of intact apple plants for 24 hours in a 5% ethanol solution. Compared with water-treated controls, the ethanol treatment reduced the rates of net photosynthesis, but did not affect the transpiration rate or the resistance of the leaf to loss of water vapour. Ethanol was recovered from the leaves of the treated plants.The reduction in the photosynthetic rate caused by supraoptimal root temperature was attributed to the direct effect of ethanol and to the closure of the stornata, caused by a low water potential of the leaves.