Leaf Growth and Nutrient Element Uptake by Adventitious Roots of Collards in Hydroponic Solution of Variable Composition

Abstract.  With proper sanitation and growth rejuvenation, collards may be maintained in hydroponic culture for nine to twelve months for repeated leaf harvesting, thereby reducing the frequency of starting new crops from seed, and thus avoiding the longer lag time from seedling establishment to first harvest.  Growth rejuvenation involves induction of adventitious roots on stems by carefully timed root pruning.  Leaf growth and leaf macro- and micronutrient uptake of collards (Brassica oleracea var. Acephala ‘Champion’) was determined at 42 days after root pruning 77-day old seedling plants.  Root pruned plants were established in Nutrient Film Technique (NFT) hydroponic culture with three sources of nutrient solution (15N-2.2P-12.5K [F₁]; 15-7.0P-14.1K [F₂]; and 20N-8.7P-16.6K [F₃]).  All nutrient sources supplied 200 mg nitrogen (N) per liter plus micronutrients.  The ratios of nitrate (NO₃^–-N) to ammonium (NH₄⁺-N) and Urea-N were 3.63:1, 1.54:1 and 0.43:1 for F₁, F₂, and F₃; and the Urea-N content were 2.05, 2.60 and 10.10% of the total N supply, respectively.  Whereas F₁ contained 66.7 mg calcium (Ca) per liter, F₂ and F₃ had no Ca.  The rate of adventitious root growth was highest with F₁ and lowest with F₃.  Marketable yield of young fully expanded leaves (leaf– count [LC], fresh weight [LFW] were significantly affected by nutrient source (F₁ > F₃ > F₂; p ≤ 0.002).  With F₁, leaf N and phosphorus (P) were within the sufficiency range for collards while potassium (K) and Ca were below the lower limit of the sufficiency range.  Leaves have sufficient levels of magnesium (Mg) and sulfur (S), regardless of nutrient source.  Leaf macronutrient content was highest with F₁, which promoted higher leaf growth than either F₂ or F₃.  With the exception of iron (Fe), leaf micronutrients were above the lower limit of the sufficiency range.  Across nutrient source, LFW and LDW were positively correlated with N, P, K, Fe and manganese ([Mn]; R ≥ 0.5707, R² ≥ 0.3141; p < 0.0001), but negatively correlated with leaf aluminum (Al) content (R = -0.5383, R² = -0.5106; p < 0.0001).  These observations suggests that leaf growth was limited by lower N, P, K, Ca and Fe in F₂ and F₃ plants, due to reduced growth of adventitious roots compared with F₁.  Subsequent experiments will further assess the effect of low Ca as well as high NH₄⁺–N and Urea-N to NO₃^––N ratio on adventitious root growth of collards.