LEAF GROWTH, PHOTOSYNTHESIS AND TISSUE WATER RELATIONS OF GREENHOUSE-GROWN EUCALYPTUS-MARGINATA SEEDLINGS IN RESPONSE TO WATER DEFICITS

Leaf growth, rate of leaf photosynthesis and tissue water relations of shoots of Eucalyptus marginata Donn ex Sm. (jarrah) seedlings were st udied during a soil drying and rewatering cycle in a greenhouse experi ment. Rates of leaf growth and photosynthesis were sensitive to water deficits. The rate of leaf growth decreased linearly with predawn leaf water potential to reach zero at -1.5 MPa. Rate of leaf growth did no t recover completely within the first three days after rewatering. Mid day photosynthetic rates declined to 40% of those of well-watered seed lings at a predawn leaf water potential of -1.0 MPa and reached zero a t -2.2 MPa. Photosynthetic rate recovered rapidly following rewatering and almost fully recovered by the second day after rewatering. All ti ssue water relations parameters, except the bulk modulus of elasticity , changed significantly as the soil dried and recovered completely by the third day after rewatering. Changes in osmotic pressure at full tu rgor of 0.4 MPa indicated considerable capacity for osmotic adjustment . However, because there was little osmotic adjustment until predawn l eaf water potential fell below -1.5 MPa, this capacity would not have enhanced seedling growth, although it may have increased seedling surv ival. The sensitivity of photosynthesis and relative water content to water deficits suggests that greenhouse-grown E. marginata seedlings b ehave like mesophytic plants, even though E. marginata seedlings natur ally grow in a drought-prone environment.