INFLUENCE OF SOIL DRYING ON LEAF WATER POTENTIAL, PHOTOSYNTHESIS, STOMATAL CONDUCTANCE AND GROWTH IN 2 BLACK PINE VARIETIES

The aim of this study was to examine the influence of long-term soil w ater deficit on growth and physiological processes of two black pine v arieties (Pinus nigra ssp. laricio var. Corsicana and Pinus nigra ssp. laricio var. Calabrica). Three-year-old seedlings grown in larges box es (volume: 1.62 m(3)) were subjected to a prolonged summer drought (9 9 days from the end of June until the end of September) and photosynth esis (A), stomatal conductance (g(w)), water status and growth were me asured. No marked differences arose between Corsican and Calabrican pi nes feature to drought. At least in their juvenile stage, both varieti es exhibited a 'drought-avoidance strategy' characterized by an effici ent stomatal control of transpirational water loss. This result is con sistent with previous studies on Pinus nigra and confirm the water str ess adaptation of this collective Mediterranean species. Because a sig nificant decrease of g, (about 30 %) was observed with no obvious vari ation in Psi(wp), the data suggested that predawn water potential was not the best indicator to precociously detect water stress. However, b oth 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 u pper 40 cm of the soil, a superficial soil desiccation has probably in duced rapid stomatal closure, triggered by a biophysical and/or bioche mical signal from the desiccated roots to the leaves. Embolism seems n ot to be responsible for the effect of drought on physiological proces ses, because the minimum value of Psi(wp) observed at the end of the d rying cycle (-2.5 MPa) remained higher than the threshold inducing a s ignificant xylem cavitation for these varieties (-4 MPa). Summer droug ht significantly reduced annual stem diameter (-20 %) and needle lengt h (-25 %), but not stem elongation. Total elaborated dry weight was re duced about 45 %. Seedlings grown in the dry regime reduced belowgroun d growth proportionally more than aboveground growth, causing a signif icant decrease in the R/S ratio. Such a result, which diverges with cl assical 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 ou r study. With the parameters studied here, the expression of the genet ic 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.).