WATER BEHAVIOR OF THREE COMMERCIAL CULTIVARS AND TWO PRESELECTED CLONES OF J. REGIA TREES

The last years many efforts have been done to select the best adapted walnut cultivar in order to plant the most suitable ones in each Spanish area. The Mediterranean zone shows very good climatic conditions to develop that culture but it has an endemic limitation of water supply. There are few references concerning J. regia water relation and all of them report this specie as sensitive to drought. In 1993 we started an experience comparing the response of three commercial cultivars, ‘Serr’, ‘Hartley’ and ‘Franquette’ and two preselected clones of a Mediterranean Spanish population, ‘MB-T-31’ and ‘MB-T-122’ submitted to two cycles of drying/rehydratation. The trail was carried out using two trees per cultivar or clone aged from eight to ten years and grafted onto ‘Serr’ seedling rootstocks. The evolution of predawn and midday leaf and stomatal conductance (gs) during the growing season showed three significant different behaviors groups. The first corresponds to ‘MB-T-31’, the second to ‘Serr’, ‘Hartley’ and ‘MB-T-122’ and the third to ‘Franquette’. ‘MB-T-31’ maintains higher values of predawn, midday leaf and gs (an average of -0.34 MPa, -0.82 Mpa and 0.27 cm/s respectively) when drought is imposed showing an ability to have a high and constant rate of transpiration and no leaf dehydratation. This clone also shows a good overnight recovering. The drastic response to drought of ‘Franquette’ is in the other extreme and has the lowest values of predawn leaf (-0.81 MPa), midday leaf (-1.4 MPa) and gs (0.027 cm/s) and is unable to recover its usual hydric status. The second group which includes the three others cultivars had an intermediate response to drought conditions. After a long period of drought, only ‘Franquette’ was not sensitive to rain, keeping low levels of gs and leaf water potential. The good correlation obtained in all cases between the leaf water potential and the vapor pressure deficit (r²>0.80) also showed same results as mentioned above.