Does High Soil Nitrate and Potassium Availability Drive Plant Metabolic Processes Towards Maximizing Growth at the Expense of Drought Tolerance?.

Strategies to increase yields, fertilizer use efficiency, and drought stress tolerance of organic and conventional crops are urgently needed to meet future global food demands, especially in the context of a changing climate. The individual effects of drought and fertilization on agricultural yields have been researched extensively, however the combined impacts of these two factors on leaf sap composition are not well understood. High soil nutrient availability promotes rapid plant growth and increases the ratio of inorganic (i.e. NO₃, K) to organic (carbon-based) solutes in sap extracted from pressed leaves. Field observations suggest that this solute composition ratio change may affect crop plant stress tolerance to drought. We used a greenhouse experiment to investigate the effects of nitrogen and potassium fertilizer (NK) additions at three levels, low (50%), medium (100% - equivalent to typical agricultural applications), and high (150%) on shoot growth, leaf sap composition, and soil microbial nutrient pools of replicate Collard greens (Brassica oleracea, n=9) before and after a water limitation treatment. Shoot biomass increased with NK addition, reaching an asymptote at the medium level, during the first phase of the experiment under normal growing conditions, and during the subsequent three-week water limitation period. Final leaf sap carbon and nitrogen levels were inversely related across the NK fertilizer treatments, with the highest levels of carbon and lowest level of nitrogen occurring in the controls, suggesting that the medium treatment may have avoided or balanced metabolic limitation by either carbon or nitrogen. We conclude that balanced leaf sap carbon to nitrogen ratios may optimize the trade-off between growth and drought stress tolerance, and provide a conceptual and management pathway to meet the challenges of maximizing crop production under future climates.