Changes in tissue-water relations, photosynthetic activity, and growth of Myrtus communis plants in response to different conditions of water availability

The influence of different levels of water deficit on physiological and morphological alterations in Myrtus communis plants was investigated to evaluate their adaptability to such conditions. M. communis plants, growing under greenhouse conditions, were subjected to three irrigation treatments between February and August 2007: a control, and two water deficit treatments. Plants submitted to severe water deficit showed reduced shoot and root dry weights, leaf numbers, leaf areas, and plant heights, while moderate water deficit reduced only plant height. Leaf colour was not affected by either water deficit treatment. Root hydraulic resistance increased in proportion to the level of drought, and lower values of leaf water potential (Psi(l)) were observed at pre-dawn in both deficit treatments. The absence of osmotic adjustment could explain the reduction of leaf turgor potential at midday. Photosynthesis decreased in both water deficit treatments and was related to stomatal factors, since no significant changes in chlorophyll fluorescence, chlorophyll content, or ion leakage were observed. The highest Psi(l) values were found in the early morning, and the lowest at midday, in all treatments. The latter coincided with the minimum values of stomatal conductance. Significant differences in Psi(l) values during the day were noted between treatments, but were always highest in the control. Cell wall rigidity, measured as an increased bulk modulus of elasticity, increased under severe water stress, resulting in a loss of turgor at lower Psi(l) values.