Abstract:
Efficient agronomic practices are vital for achieving sustainable management of water
resources and N for producing highly nutritious leafy vegetables to curb malnutrition and
poverty. The importance of proper N and irrigation of sustainable crop production is well
recognised in literature, although irrigation and N application rate guidelines for ALV’s
might not be sufficient for advisory purposes, especially for smallholder and commercial
farmers. The limited access is attributed by factors such as the lack of commercialisation as a
result of limited agronomic information describing optimum management options for S.
retroflexum. Availability of such information would contribute to successful
commercialisation of this crop. The primary objective of this study was to establish optimum
agronomic management practices for S. retroflexum for smallholder farmer as well as
commercial production in South Africa.
This project consists of three main components with the overall objective to evaluate
agronomic management practices of irrigation and N application on the marketable yield,
nutritional water productivity and 1H-NMR metabolic profiling under a rainshelter. The
results suggested that S. retroflexum responded positively to N application rates until an
optimum marketable yield was obtained at 150 kg N·ha-1 with I66%FC, followed by the
I100%FC with 150 kg N·ha-1,while, the lowest biomass was recorded in the I33%FC
irrespective of the N application rate. This also indicates of how S. retroflexum is affected by
insufficient irrigation, even at sufficient N application rates. N and irrigation are key factors
limiting plant survival and growth and low applications has adverse effects on the marketable
yield of S. retroflexum. Dry matter production increased with the higher N application and a
linear increase was observed with N application having a significant effect on the dry matter
production.
Maximum irrigation use efficiency (IUE), was obtained in the moderately irrigated treatment
(I66%FC) and the 150 kg N·ha-1, followed by the I100%FC and 150 kg N·ha-1. The stressed irrigation and N treatments showed sustainably low irrigation use efficiency as compared to
the well irrigated treatment. This suggests the competitive capacity of S. retroflexum roots to
draw water from deeper parts of the soil profile, during stressed conditions to maintain the
turgor pressure, indicating that production of S. retroflexum is possible in arid areas where water could be a limiting input, but might not be profitable for farmers. However, the
biomass yields obtained under the less irrigated soil water conditions may lack the quality
needed to market the produce.
The NWP for Zn and Fe showed significant differences among the irrigation and N
treatments. Although the NWP of Mg in S. retroflexum was neither influenced by irrigation
nor N application, with no statistical differences between the irrigation and N application
observed. Among the different irrigation and N treatments evaluated, I33%FC obtained the
lowest NWP irrespective of the N application rate, followed by I100%FC. The NWP
increased linearly with an increase in N application. Optimum Fe NWP was obtained with
150 kg N ha-1, but declined significantly at application 225 kg N ha-1 in I33%FC and
I100%FC. Maximum Fe NWP was obtained with I66%FC with 150 kg N ha-1. The NWP Fe
for S. retroflexum in this study, are acceptable when compared to those obtained in literature.
Therefore a significant interaction between N and irrigation application exist, with respect to
Fe and Zn availability of S. retroflexum, which could be ideal for Fe and Zn malnutrition
alleviation in resource poor households.
1H-NMR based metabolomic profiling was utilized for compound annotation as effected by
irrigation and N. Chlorogenic acid, proline, sucrose and trigonelline were associated with
separation in the irrigation treatments. Since no separation of the irrigation or N treatments
was observed with the PCA, an OPLS-DA plot was constructed. A pairwise comparison of
the I33%FC and I100%FC samples was done, which provided better separation between the
clusters for the first harvest. Even better separations were observed with a pairwise OPLSDA
analysis of the I33%FC and I100%FC samples for the second harvest. Primary
metabolites such as sucrose, and proline and secondary metabolites trigonelline and
chlorogenic acid were responsible for grouping of the stressed irrigation treatment as
compared to the well irrigated treatment. Main findings of the study suggest that S. retroflexum is highly nutritious in Fe, Zn and Mg, which might play a significant role in
malnutrition alleviation. Solanum retroflexum requires sufficient soil water content, for
achieving high nutrient yield and nutritional water productivity.
The results clearly illustrate that the perception that S. retroflexum grow well on low soil
fertility mostly practised in rural and smallholder farming is incorrect and misleading.
Moreover, S. retroflexum can be an ideal cropping system for smallholder and commercial
farmers in water scarce areas such as South Africa, although marketable yield is severely
affected.
