Irrigation and nitrogen management of African (Siphonochilus aethiopicus (Schweinf.) B.L. Burtt) and commercial ginger (Zingiber officinale Roscoe)

Medicinal plants are important and valuable natural resources. South Africa is well-endowed with very diverse flora and fauna that include a considerable number of medicinal plant species. Most medicinal plants have gained popularity for the treatment or prevention of various ailments. Ginger species (Zingiber officinale and Siphonochilus aethiopicus) are essential natural resources, which provide many useful products for use in food as a spice or as medicine. The two species contain beneficial secondary metabolites useful for treating many diseases and numerous digestive imbalances such as indigestion, vomiting, heartburn, diarrhoea and pregnancy-related nausea. However, the role of different agronomic practices such as irrigation, water stress and nutrient management are crucial for enhancing the yield and quality of ginger species. Due to climate change, rainfall is often less while more erratic, putting more pressure on irrigation resources in agriculture to sustain or even increase food production for a growing population. The major plant factors negatively affected by water limitations are plant growth, quality and crop yield. The second most constraining factor in plant growth and quality is the lack of plant nutrients. Macronutrients, such as N, P and K are most important in plants to complete their life cycle and play a significant role in the growth and development of plants. This study investigated the growth, yield and phytochemical profiling of two ginger species under different maximum allowable depletion levels of soil water content and nutrient management. The soil water study was conducted under a rain-shelter at the Experimental Farm on the Hillcrest campus of the University of Pretoria, South Africa. The experiment was laid out in a randomized complete block design (RCBD) with two factors (ginger species and water regimes) and three replicates per treatment. The two experimental factors included the two species of ginger (commercial and African ginger) and four water levels (irrigated as soon as 20-25% of available soil water (ASW) was depleted, 40-45% of ASW depleted, 60-65% of ASW depleted and 80-85% of ASW depleted). Growth and development parameters such as height, leaf number and stem number were evaluated and varied between species and irrigation treatments. Irrigation treatment effects on plant growth and development were dependent on plant species. Leaf area index and fractional interception of photosynthetically active radiation (FIpar) values were higher in African ginger than commercial ginger in both seasons. Scanning electron microscopy images showed that both ginger species had more stomatal pores and open stomata under well-watered than stressed conditions. The study demonstrated that fresh and dry yields were higher for commercial ginger, compared to African ginger. The fresh and dry matter yields for severely water stressed plants were higher for commercial ginger than for African ginger. Water use efficiency in terms of fresh commercial ginger yield was highest for the moderately water-stressed treatment. The severely water-stressed irrigation regime (i.e. 80-85 MAD) resulted in higher production of total flavonoid content, phenolic content and increased antioxidant activity in both species.