INFLUENCE OF SOIL WATER LEVELS ON ASSIMILATION AND WATER USE EFFICIENCY IN HAZELNUT.

In conditions of warm temperature and intense light, the leaf photosynthesis of non-stressed trees was highest at 9:00h, with an inflection at 12:00h and a recovery at 16:00h.

On trees under water stress, photosynthesis decreased during the day and continuously decreased as water availability approached the wilting point. Photosynthesis remained at high level till 60% available water, then it decreased strikingly.

Transpiration and stomatal conductance are correlated with photosynthesis, while the intercellular CO₂ concentration remains constant. On leaves under water stress, there was a low quanta efficiency with reduction of energy transporter enzymes and of those governing CO₂ fixation.

Water use efficiency decreased during the day and it was constant till 60% available water, after which, it decreased to lower values.

On water stressed leaves, chlorophyll, soluble sugars and starch decreased. On efficient trees, the highest water use was about 2 liters per day and per leaf square meter.

Nearly 45% of the water loss was due to stomatal transpiration, 35% to non-stomatal transpiration and 20% to soil evaporation.

Therefore, to obtain the highest assimilation by the leaves, the soil water content must be over 60% field capacity, but not too much or water will be lost.

Soil water loss should be controlled because it is an important part of total water use.