L. KOWITHAYAKORN and L.R. HUMPHREYS

Department of Agriculture, University of Queensland, St. Lucia, 4067, Australia.

Abstract

Macroptilium atropurpureum cv. Siratro was grown at Khon Kaen, northeast Thailand, in swards or on trellises 1.4 m high and 1 m apart. Plants were (1) irrigated twice weekly during the dry season at an average rate of 8.6 and 11. 4 mm d^-1 for swards and trellises, (2) not watered for 10 weeks, when dawn leaf water potential (LWP) decreased to c. –0.76 MPa (medium stress), watered for 6 weeks and stressed again, or (3) not watered for 13 weeks, when LWP reached c. –0.97 MPa (severe stress), rewatered and stressed again.
As water stress developed, the rate of node appearance on this indeterminate plant was decreased. Shoot death and leaf abscission occurred under severe stress, but the ratio of axillary buds which were floral rather than vegetative and their survival to form inflorescences increased. The rate of floral bud appearance and of inflorescence production increased during the first 4 weeks of moisture deficit, but subsequent equilibration of the components of seed yield reduced treatment differences in seed production. The total seed collected over 20 weeks was 273, 323 and 298 g m^-2 respectively in the control, medium and severe stress treatments when sward planted, or 354, 372, and 355 g m^-2 for the same treatments when trellis grown.
Dawn LWP of plants on trellises was 0.04 to 0.14 MPa lower than for plants in swards, and the higher seed production on trellises was associated with higher shoot and floral bud density.
The implication of this study for irrigation economy in Siratro seed production are discussed.