Seed germination, tree growth and flowering responses of Moringa Oleifera lam. (horseradish tree) to temperature

Abstract

Moringa oleifera Lam. is a tree with great potential as it is fast growing and drought tolerant. Amongst the tree’s multitude of benefits, it can also be used to produce a biodiesel fuel. However, prior to the establishment of commercial plantations, all cultivation aspects of this promising tree have to be understood. Temperature is a significant climatic factor influencing both geographical plant distribution and growth, and since M. oleifera trees are naturally found in tropical climates around the world, the extent of their adaptability to cooler climates was the main objective of this study. Trees were cultivated from seed and germinated in a controlled greenhouse environment at the Experimental Farm of the University of Pretoria. After germination, 50% of the seedlings were hardened-off by placing them outside under ambient growing temperatures, while the rest remained inside the greenhouse. With trial commencement, 132 of both the hardened and non-hardened seedlings were planted into 10 ℓ plastic bags and randomly placed into three temperature-controlled greenhouses, each with a different fluctuating night/day temperature regime namely; 10/20°C ± 2°C, 15/25°C ± 2°C and 20/30°C ± 2°C. In addition, half the trees within each temperature regime were treated with the growth regulator paclobutrazol to determine its effect on growth/flowering at different temperatures. During the 224-day trial period, biweekly measurements of tree height, stem diameter and leaf area estimates of each individual tree within all three temperature regimes were taken. Despite germination percentages being slightly higher at the low 10/20°C regime, the MGT, germination rate, uniformity and seedling growth were superior at the higher 20/30°C regime. The temperature induced seed dormancy at the 20/30°C regime, could be overcome by an incubation period at lower temperatures, as fewer instances of seed dormancy were observed at the 10/20°C regime. The increase in temperature resulted in significant (P≤0.05) growth rate increases of over 650% between the 10/20°C and 20/30°C and 250% between the 10/20°C and 15/25°C night/day temperature regimes. In addition, the 20/30°C temperature treatment, although fluctuative, consistently had the highest leaf area over the entire trial period. Hardening-off of trees during the seedling stage, significantly (P≤0.05) increased the final tree height by 3.09X, 1.44X and 1.23X, compared to their non hardened-off counterparts under the 10/20°C, 15/25°C and 20/30°C temperature regimes respectively. Leaf thickness decreased by a significant (P≤0.05) 43.1% with increase in temperature between the 10/20°C and 20/30°C regime, mostly due to a thinner mesophyll layer. The efficacy of paclobutrazol on M. oleifera growth was found to be temperature dependant, reducing growth at 10/20°C, while increasing growth at both the higher 15/25°C and 20/30°C regimes. Flowering however remained unaffected by paclobutrazol. The highest instances of flowering and pollen viability were observed at the 15/25°C regime. The absence of inflorescence induction at the 20/30°C regime was responsible for the reduced flowering, signifying the necessity of vernalization prior to flowering. Even though all the results confirm the preference of M. oleifera trees towards a tropical climate, satisfactory growth with possibly improved flowering during the hot summer months in certain sub-tropical climates is achievable. Copyright