Compost helps tomato plants preserve photosynthetic function under severe drought

Tomato plants grown with food-waste compost maintained the highest measured photosynthetic efficiency during a two-week severe water-restriction experiment, outperforming plants given sterilized compost or a mineral fertilizer supplying equivalent amounts of major nutrients. Yet compost did not produce the largest plants, and its advantage disappeared under milder water restriction.

The findings, reported by Rosalba Cipriani and colleagues in a bioRxiv preprint, point to a combination of greater substrate water availability and potentially beneficial microbial effects. The work has not been peer reviewed, and the experiments cannot cleanly separate those explanations.

Comparing nutrients, organic matter and living compost

The researchers used Micro-Tom tomatoes grown under controlled conditions. Their treatments included industrially produced biowaste compost, the same compost sterilized by autoclaving, and a mineral fertilizer formulated to match the compost's nitrogen, phosphorus and potassium inputs. Compost was added at 33 grams per litre of substrate.

An initial growth experiment also included an unfertilized, nutrient-poor substrate. After one month, all three fertilized groups had greater leaf dry biomass and higher photosynthetic efficiency than that baseline. Mineral-fertilized plants had more leaf biomass than compost-treated plants, while sterilized compost produced an intermediate result. Photosynthetic efficiency was comparable across the fertilized treatments.

That distinction matters: the experiment did not show that living compost was generally the best treatment for growth. Instead, it showed that broadly similar leaf photosynthetic performance could accompany different growth outcomes and root molecular responses.

Roots responded differently before drought

The team sequenced root RNA from five plants in each fertilized treatment, sampling plants grown without imposed drought. They reported 910 differentially expressed genes across the pairwise comparisons, including 612 in the comparison between ordinary and sterilized compost.

Compost-treated roots showed higher expression of several genes associated with nitrogen metabolism, hormone signaling and antioxidant responses. These included genes involved in nitrate transport, nitrate reduction and nitrogen assimilation. Genes associated with perception and metabolism of abscisic acid, a hormone involved in water-stress responses, also generally had higher expression under compost treatment.

These are clues about how roots respond to different amendments, not direct measurements of improved nutrient uptake or proof of a drought-protection mechanism. Because the RNA samples came from plants grown without drought, the study cannot establish whether those expression patterns caused the later physiological advantage under severe water restriction.

A benefit that depended on drought severity

For the drought experiment, the researchers initially established 300 plants across the three fertilization treatments. After three weeks of growth, plants received full hydration, 35 millilitres of water every two days, or 20 millilitres every two days for a further fortnight.

Under the milder restriction, compost-treated plants had significantly lower photosynthetic efficiency than plants receiving sterilized compost or mineral fertilizer. Under severe restriction, the ranking reversed: ordinary compost produced the highest values, mineral fertilizer the lowest, and sterilized compost an intermediate response.

The measurement, Fv'/Fm', assesses the photochemical efficiency of photosystem II in light-adapted leaves. It is an indicator of photosynthetic function, not a direct measurement of carbon fixation, fruit production or harvest yield.

Leaf biomass provided a less decisive result. Under severe stress, biomass did not differ significantly among fertilization treatments. Compost was the only treatment without a statistically significant biomass reduction across watering conditions, but that does not establish a biomass advantage over the other fertilizers.

Water storage complicates the microbial explanation

Separate substrate measurements showed that ordinary compost had the highest estimated available water content. Sterilized compost and mineral fertilizer were intermediate, while the unamended substrate had the lowest value. Greater water availability offers a physical explanation for at least part of the severe-stress response.

The sterilization comparison does not isolate microorganisms perfectly. Autoclaving can alter substrate properties as well as remove viable microbes—a possibility the authors acknowledge. Although substrate pH did not differ significantly after sterilization, that check does not rule out other physical or chemical changes. Matching total macronutrient inputs also does not guarantee identical nutrient availability.

The study therefore supports a treatment-level benefit under its severe-stress conditions, while leaving the microbial contribution unresolved. It tested one tomato cultivar, one compost source and a short pot-based drought period, without measuring fruit yield or post-drought recovery. Experiments that restore microbes to sterilized compost, examine roots during drought and test field performance would help determine how far the proposed explanation and practical benefits extend.