Various agronomic strategies were used to produce variability in the extent and distribution of leaf in the canopy of Cyamopsis tetragonoloba [L.] Taub. (guar or cluster bean). The aim of this study was to investigate the influence that such variability in the leaf canopy may have had on the amount and distribution of seed and gum yield. Such information would be of interest to plant breeders and producers in the development of C. tetragonoloba as a new crop. A preliminary glasshouse trial was conducted during 1992 and two field trials conducted in the summers of 1992/1993 and 1993/1994 at The University of Queensland Gatton College, Lawes, Lat. 27° 32' S.
In the glasshouse trial, the variability in yield and morphological characteristics among 35 cultivar/selections of C. tetragonoloba grown in pots was examined and six cultivar/selections were chosen for field trials, based on differences in their yield potential and in their branching ability and height.
In the 1992/1993 field trial, four cultivar/selections (high-yielding, sparsely-branching CP177; an apparently low-yielding sparsely-branching selection from CP177, identified as S9; high-yielding, profusely-branching IC9203 and apparently low-yielding profusely-branching Brooks S36) were sown on three dates (5 December 1992, 5 January 1993 and 15 February 1993) using three row spacings (40, 27 and 20 cm).
In the high-yielding sowing, 5 Jan 1993, the crop canopy was more open than in the early or late sowings and intercepted a significantly greater percentage of photosynthetically active radiation (PAR), as measured at Days 31 and 46.
The early sowing, 5 Dec 1992, resulted in large amounts of vegetative growth but relatively low seed yield, due to a reduction in the number of pods m-2 . Late sowing, 15 Feb 1993, resulted in a short plant with low dry weight and relatively low leaf area index (LAI). This resulted in a severe yield reduction, due to fewer seeds per pod and smaller seed size.
Gum yield was greatest in the 5 Jan sowing due both to higher seed yield and a higher percentage of gum in the seed.
Different cultivars had significant, but lesser effects on canopy characteristics and yield. Differences among cultivars in the distribution of gum yield in the canopy mostly paralleled differences in plant structure. Row spacing had no significant effect on total canopy yield but affected yield distribution.
In the 1993/1994 field trial, the four cultivar/selections used in 1992/1993, as well as the high-yielding, profusely-branching IC9007/P2 and the moderate-yielding, moderately-branching CP66, were sown on three dates (17 December 1993, 5 January 1994 and 27 January 1994) and one half of each plot was regularly debranched.
Seed yield in 1993/1994 was greatest in the early sowing and was least in the late sowing. Reduced yield was associated with a reduction in pod number and seeds per pod.
Cultivar had a significant effect on total plot seed yield.
Debranching was used to separate the influence of leaf canopy per se on yield fi-om that due to genetic and environmental factors.
Debranching reduced total canopy seed yield by 14 %, due mainly to a reduction in pod number, and reduced leaf area as the season progressed, the reduction being 17% at Day 31 and 65% at Day 99.
Debranching resulted in a main stem seed yield increase of 80%, mainly due to increased numbers of clusters, pods per cluster and increased seed size, and a 41% increase in main stem leaf area at Day 61 after sowing.
The relatively small reduction in total yield and the large increase in main stem yield, as a result of the debranching treatment, were considered to be due to increased photosynthetic rates arising both from compensatory mechanisms involving an hypothesized increase in RuDP carboxylase activity in the leaves and from adjustment to the higher levels of radiation experienced by leaves particularly in the lower part of the canopy.
The reduction in total canopy seed yield due to debranching was associated with a significant reduction in the percentage of photosynthetically active radiation (PAR) intercepted at Days 70 and 133.
The reduced seed yield in the late sowing was also associated with a significantly lower percentage of PAR intercepted. Significant differences were observed among cultivars in the percentage of PAR intercepted and for most cultivars corresponded with differences in total plot yield.
At the start of Week 10 in the 1993-4 season, when leaf canopy development would have been at or near its greatest, the percentage of the PAR intercepted reached a maximum at a leaf area index (LAI) of approximately 3.
Debranching in the 5 January 1994 sowing caused a slight decrease in gum percentage. Debranching severely reduced gum yield in the highest yielding cultivar, S9, whilst in the much lower-yielding Brooks S36, debranching resulted in a gum yield increase. Differences among cultivars in the percentage of gum in the seed were relatively small.
The choice of sowing date was shown to be the major factor in determining seed and gum yield, with the effect on leaf area and its vertical distribution being directly responsible for some of the yield differences observed. Cultivar effects on yield were smaller and were less directly influenced by canopy differences. The row spacing differences used in these experiments produced no canopy differences of sufficient magnitude to significantly influence total yields.
The distribution of yield in relation to the distribution of leaf area indicated that assimilate for pod initiation and seed-filling was obtained from a nearby source.