WATER RELATIONS, GAS-EXCHANGE AND ABSCISIC-ACID CONTENT OF LUPINUS-COSENTINII LEAVES IN RESPONSE TO DRYING DIFFERENT PROPORTIONS OF THE ROOT-SYSTEM

Soil columns in which the root system was divided into three equal lay ers, each 24 cm in diameter and 33 cm high were used to examine the in fluence of drying different proportions of the root system on the wate r relations, gas exchange and abscisic acid (ABA) concentration of lup in (Lupinus cosentinii Cuss. cv. Eregulla) leaves. The treatments impo sed were (i) all three layers adequately watered (control), (ii) the u pper layer unwatered with the remaining layers kept adequately watered , (iii) the two upper layers unwatered with the basal layer kept adequ ately watered, (iv) all three layers unwatered. The treatments were ap plied at 56 d after sowing (DAS), and continued for 21 d in the treatm ent in which the three layers were dried and for 36 d in the other thr ee treatments. After 21 d, the soil matric potential in the layers tha t were unwatered had decreased to -1.3 MPa, compared to -0.03 MPa in t he adequately-watered layers. Within 8 d of cessation of watering, pla nts with the entire root system in drying soil had significantly lower stomatal conductances, lower rates of net photosynthesis, and higher leaf ABA contents than did adequately-watered plants. While the leaf o smotic potential decreased within 8 d of cessation of watering, the le af water potential did not change for the first 15 d after water was w ithheld. After withholding water from all layers, the shoot dry matter was 63% lower than that in the adequately-watered plants. In the two partially-droughted treatments, 17% and 48% of the root length was sub jected to drying. Compared to the adequately-watered plants, drying up to 50% of the root system for 36 d, in the two partially-droughted tr eatments, did not reduce stomatal conductance, net photosynthesis, or plant growth. Similarly, there was no significant effect on leaf water potential or osmotic potential. When either the upper or upper and mi ddle layers of soil were dried, the ABA content of the leaves for most of the drying period was slightly, but not significantly, higher than in leaves of the adequately-watered plants. The results suggest that lupins with a well-established root system can utilize localized suppl ies of available soil water to maintain leaf gas exchange despite appr eciable portions of the root system being in dry soil. In contrast to other studies, the results also suggest that when only a portion of th e soil volume is dry and adequate water is available in the wet zone, root signals do not influence stomatal conductance and leaf gas exchan ge of lupin.