Soil fertility effects on growth,nitrogen fixation,nodule enzyme activity and xylem exudates of lablab purpureus (L.) sweet,grown on a typic eutrustox

Abstract

The Lablab or Hyacinth bean, Lablab purpureus (L.) Sweet, is a drought tolerant grain‐forage legume widely grown within a wide range of neotropical regions of the world. It has been an important cultivated food grain crop for many centuries within extensive warm humid‐subhumid climatic areas and grown on widely different Ultisol and Oxisol soil types. The objective of these studies was to determine soil fertility effects of Ca, P, and K factorial combinations applied to a Typic Eutrustox on growth, nodulation, and nitrogen fixation of Lablab bean. Stem and nodule xylem components were determined along with soil pH and nitrification within the nodule rhizosphere. Highly significant increases resulted with P fertilization for stem, leaf, seed, nodulation, and nitrogenase activity. With the exception of nitrogenase, all were also increased with K additions. Both stem and seed were significantly increased with Ca treatments. Highly significant increases for total plant N content resulted with P, K, Ca, and P x K interaction. Allantoinates were initially dominant components of stem xylem exudate following decapitation but were not detected after 5 days. Amino‐amide composition remained relatively constant. Nitrates increased from 1.6 to 81.8 ug N ml^‐1 exudate during the 8 day collection period. Soil nitrate levels adjunct to decomposing nodules increased from 4.2 to 1661.1 ug g^‐1 soil. Initial high levels of ureide transforming enzymes ALTN, ALTC, and URC were not detected after 4 days. Amine‐amide enzymes AST, GS, GOGAT, and GDH were relatively constant with nitrate reductase increasing from 0.12 to 9.35 IU ml^‐1 xylem exudate during the 8 d period. Nodule xylem export components were dominated by ureides, 429.8 umol, and amines 30.3 umol ml^‐1. Enzyme activity levels were highest for AST 22.17 and GS 13.25 IU ml^‐1 with the ureide enzymes URC 2.24, ALTN 0.26, and ALTC 0.15 IU ml^‐1 . Nodule exudate composition was K 422.0, P 63.4, Ca 53.8, Mg 42.8, and Na 25.6 ug ml^‐1.