Influence of soil drying on leaf water potential, photosynthesis, stomatal conductance and growth in two black pine varieties

The aim of this study was to examine the influence of long-term soil water deficit on growth and physiological processes of two black pine varieties (Pinus nigra ssp. laricio var. Corsicana and Pinus nigra ssp. laricio var. Calabrica). Three-year-old seedlings grown in larges boxes (volume: 1.62 m(3)) were subjected to a prolonged summer drought (99 days from the end of June until the end of September) and photosynthesis (A), stomatal conductance (g(w)), water status and growth were measured. No marked differences arose between Corsican and Calabrican pines feature to drought. At least in their juvenile stage, both varieties exhibited a 'drought-avoidance strategy' characterized by an efficient stomatal control of transpirational water loss. This result is consistent with previous studies on Pinus nigra and confirm the water stress adaptation of this collective Mediterranean species. Because a significant decrease of g, (about 30 %) was observed with no obvious variation in Psi(wp), the data suggested that predawn water potential was not the best indicator to precociously detect water stress. However, both A and g, reduced to nearly zero as soon as the threshold value of Psi(wp) = -1.6 MPa was reached (respective values 0.5 mu mol m(-2) s(-1) and 11 mmol m(-2) s(-1)). Because most fine roots were within the upper 40 cm of the soil, a superficial soil desiccation has probably induced rapid stomatal closure, triggered by a biophysical and/or biochemical signal from the desiccated roots to the leaves. Embolism seems not to be responsible for the effect of drought on physiological processes, because the minimum value of Psi(wp) observed at the end of the drying cycle (-2.5 MPa) remained higher than the threshold inducing a significant xylem cavitation for these varieties (-4 MPa). Summer drought significantly reduced annual stem diameter (-20 %) and needle length (-25 %), but not stem elongation. Total elaborated dry weight was reduced about 45 %. Seedlings grown in the dry regime reduced belowground growth proportionally more than aboveground growth, causing a significant decrease in the R/S ratio. Such a result, which diverges with classical models of whole plant biomass partitioning, might be partially explained by seasonal pattern in the root growth which typically has its most important peak in mid-summer, period of maximum drought in our study. With the parameters studied here, the expression of the genetic characteristics between varieties in drought tolerance appeared to be limited. Thus, further investigations could be undertaken to learn about drought feature at cell and molecular levels. ((C) Inra/Elsevier, Paris.).