A Transpiration-Driven Mass-Balance Framework for Quantitative Analysis of Nutrient Uptake in Cannabis sativa L.

Abstract

ABSTRACT A Transpiration-Driven Mass-Balance Framework for Quantitative Analysis of Nutrient Uptake in Cannabis sativa L. Nutrient management that replenishes plant uptake from solution begins with a knowledge of the amount of nutrients roots extracted from the nutrient solution and how that quantity changes over the plant’s lifecycle. This thesis tests a physiology-informed mass-balance framework for estimating Cannabis sativa L. nutrient requirements by coupling whole-plant nutrient uptake with water removed by transpiration. Changes in the amount of nutrients taken up with water are determined by multiplying the concentration of nutrients in organ tissue, by fluctuations in plant water use efficiency (WUE). WUE and variations in the nutrient requirement, therefore, adjust the nutrient replenishment calculated concentrations in relation to the environment and plant genetics. Electrical Conductivity (EC) reports ionic strength, not ion identity or balance, so equal EC values can mask very different nutrient profiles. Similarly, fixed nutrient input setpoints can obscure ion selective uptake and the accumulation of unused ions in solution can result. Furthermore, determining nutrient replenishment solutions using leaf-only nutrient concentrations bias element input prescriptions because organs differ in element composition and storage in addition to uptake amounts that change with phenology. We found that flowers often accumulate potassium [K] and phosphorus [P] in excess, while stems and roots contain proportionally more calcium [Ca] and magnesium [Mg]. Element mobility and transport pathways also vary with sink strength, which was an additional impetus for us to investigate organ biomass-weighted nutrient uptake predictions validated against leachate and tissue concentrations. We implemented this framework for two cultivars, CJ2 and First Light, in containerized substrate culture across the Cannabis sativa L. vegetative and reproductive phase, to estimate nutrient uptake from plant tissue concentrations using three physiologically driven observations: continuous gravimetric measurement of transpiration, pour-through leachate analysis of residual solution element composition, and organ-level analysis of tissue nutrient concentrations. In the vegetative phase, nutrient uptake tracked canopy growth and water use. Nitrogen [N] and potassium [K] uptake increased with leaf area, while calcium [Ca] and magnesium [Mg] uptake escalated as vascular transport matured and sinks shifted toward structural growth. Organ biomass-weighted mean element input calculations aligned more closely with solution nutrient drawdown than single organ input calculations during the vegetative stage, which prompted us to investigate nutrient uptake during the flowering phase. Because the biomass-weighted mean among organs outperformed single-organ proxies in vegetative plants, we next tested whether that advantage persisted through the reproductive phase, when sinks and transport pathways shift toward regenerative growth. In the regenerative phase, N leaf tissue element concentration calculations overpredicted uptake by 34.3% in CJ2 and 25.6% in First Light, roots underpredicted by 19.4% and 5.9%, and stem nutrient input estimates were lower than uptake by 36.3% and 37.3%. For P, root tissue nutrient concentrations in conjunction with WUE were closest to nutrient uptake from solution, slightly higher in CJ2 (+7.5%) and lower than observed uptake in First Light (−20.3%), while stem concentrations underpredicted uptake by about 60% in both cultivars. For Mg, the organ biomass-weighted mean input estimate was between other organ tissue uptake calculations, resulting in 38.3% and 49.8% less than predicted uptake compared to actual observed uptake from solution. For micronutrients, agreement was element specific once tissue values were adjusted per liter of transpired water. Several micronutrient calculations of input concentration were a near match with that of uptake, whereas others showed modest underprediction. Excluding flowers from the organ biomass-weighted mean during periods of inflorescence luxury accumulation improved whole-plant nutrient uptake prediction accuracy. Our results show that nutrient uptake scales with whole-plant WUE. Periodic validation against leachate and tissue nutrient profiles can adjust for phenological shifts in nutrient uptake and partitioning to improve nutrient input predictions. Plants take up only a fraction of the nutrients supplied. By quantifying baseline uptake in Cannabis sativa L. and coupling it to water use and whole-plant nutrient partitioning, the result of this research supports the adoption of a physiologically based mass-balance framework for estimating nutrient prescriptions. In addition, we found that calculations can reproduce whole-plant nutrient requirements in different cultivars and throughout distinctly different plant growth stages. The result is greater efficiency of resources, less waste, and a nutrient input supply that mirrors uptake requirements in semi-closed substrate systems.