Abstract

This study was performed to investigate the effects of water and light intensity on the growth and anatomical aspects (that is, stomatal density and chloroplasts ultrastructure) of cork oak (Quercus suber L.) seedlings from acorns collected from Tabarka (Northern Tunisia). Seedlings, four-month after germination, were planted at 3 L pots with artificial soil and were cultivated for 5 months at the greenhouse. The experiments were divided into two treatments: Water stress (well-watered: 25 to 35% of water contents, moderate drought stress: 15 to 20%, and severe drought stress: 7 to 10%) and light intensity stress (full sunlight: 100% of full sunlight, 1,200μmol m^-2 s^-1; moderate sunlight: 65 to 70% of full sunlight, 800 μmol m^-2 s^-1; low sunlight: 20 to 25% of full sunlight, 300 μmol m^-2 s^-1; and extremely low sunlight: 3 to 4% of full sunlight, 40 μmol m^-2 s^-1). Growth varied among water stress, but this variation was typically smaller than light intensity. Seedlings grown under well-watered treatment showed the increased above ground growth as compared with deep root growth of severe drought stress. In contrast, seedlings grown under severe drought stress increased in root growth than in the above ground part. Q. suber seedlings grown under the shade-exposed low light intensity indicated adaptation such as the increased leaf area and increased specific leaf area than the leaves grown in full sunlight. In different light intensity treatments, the highest specific leaf area indicated at the extremely low light. Seedlings grown under severe drought stress increased in root growth. The palisade parenchyma chloroplasts of well-watered treatment were well developed: their shape was regular, chloroplast envelope membranes and grana were distinct, and an abundance of starch grains and a scarce of plastoglobuli was also visible inside in the chloroplasts. In severe drought stress, the ultrastructural features of the abaxial leaf surface were folded and shrunken, and was observed in a higher stomatal frequency, the thicker palisade and spongy parenchyma (containing three layers of palisade parenchyma). The severe drought stress indicated a swelling and disruption of the thylakoid, large starch grains, increased plastoglobuli, and accumulation of the reactive oxygen species (ROS), hydrogen peroxide (H₂O₂). The main features in the chloroplasts ultrastructure caused by the extremely low sunlight revealed not only the higher grana stacks and the broader grana, but also the higher stacking degree of thylakoids. Furthermore, seedlings grown under the extremely low sunlight showed no starch or exhibited only little starch in the chloroplasts, occupied the intracellular space in all organelles, and presented numerous small vacuoles and lipid droplets in the cytoplasm of these cells as compared with the well-watered and the severe drought stress. These results suggested that Q. suber has a well-developed mechanism to drought compared with shade stress by structural adaptations such as a deeper root system, a smaller reduction in stomatal density, and a smaller leaf area.

Key words: Water stress, light intensity stress, growth, stomatal density, palisade parenchyma, starch grains, plastoglobuli, Quercus suber L.