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 layers, each 24 cm in diameter and 33 cm high were used to examine the influence of drying different proportions of the root system on the water relations, gas exchange and abscisic acid (ABA) concentration of lupin (Lupinus cosentinii Cuss. cv. Eregulla) leaves. The treatments imposed were (i) all three layers adequately watered (control), (ii) the upper layer unwatered with the remaining layers kept adequately watered, (iii) the two upper layers unwatered with the basal layer kept adequately watered, (iv) all three layers unwatered. The treatments were applied at 56 d after sowing (DAS), and continued for 21 d in the treatment in which the three layers were dried and for 36 d in the other three treatments. After 21 d, the soil matric potential in the layers that were unwatered had decreased to -1.3 MPa, compared to -0.03 MPa in the adequately-watered layers. Within 8 d of cessation of watering, plants 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 osmotic potential decreased within 8 d of cessation of watering, the leaf water potential did not change for the first 15 d after water was withheld. 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 subjected to drying. Compared to the adequately-watered plants, drying up to 50% of the root system for 36 d, in the two partially-droughted treatments, 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 middle 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 supplies of available soil water to maintain leaf gas exchange despite appreciable portions of the root system being in dry soil. In contrast to other studies, the results also suggest that when only a portion of the soil volume is dry and adequate water is available in the wet zone, root signals do not influence stomatal conductance and leaf gas exchange of lupin.