Tomato in the spotlight: light regulation of whole-plant physiology

Abstract

The introduction of light-emitting diodes in plant research and controlled-environment agriculture has given a boost to understanding how light regulates physiology. Here, we review the regulation of whole-plant physiological processes by light in tomato (Solanum lycopersicum), with emphasis on morphogenesis, light interception, photosynthesis, source-sink interactions, assimilate partitioning, fruit set, fruit development, and plant-water relations and how this controls plant growth and fruit quality. Five key aspects of light determine the ultimate plant response, namely intensity, photoperiod, spectrum, directionality, and energy. Tomato possesses five phytochromes, four cryptochromes, two phototropins, one zeitlupe, and one UV-B photoreceptor. Via spectral sensing and photosynthesis, light affects plant morphology, which in turn affects the light interception and consequently whole-plant carbon assimilation. Photosynthesis and carbon partitioning are dynamic processes affected by light. Furthermore, light plays a pivotal role in regulating plant-water-nutrient dynamics by influencing transpiration, stomatal conductance, hydraulic conductance, and cell-wall properties. Changes in light intensity and spectrum can also increase contents of ascorbate, carotenoids, sugars, and volatiles, thereby improving fruit quality. The complex physiological responses of tomato plants to the five aspects of light and their interactions create effectively endless opportunities for future scientific research aimed at improving light-use efficiency, yield, and quality.

Keywords: Solanum lycopersicum; Assimilate partitioning; cryptochrome; fruit quality; morphology; photobiology; photosynthesis; phytochrome; plant–water relations; tomato; transpiration.

Fig. 1.

Yield component analysis. The fresh fruit yield of a tomato crop can be expressed as the product of total crop dry matter production and the fraction of dry matter partitioned to the fruits (harvest index), divided by the fruit dry matter content. The latter parameter determines how much fruit fresh mass (yield) results from the dry matter partitioned into the fruit. Dry matter production is primarily determined by crop photosynthesis, while photosynthesis to a large extent depends on light interception, which differs with leaf area index (LAI) and canopy light-extinction coefficient. Light-use efficiency is the ratio between dry matter production and the amount of intercepted light. High dry matter production only results in a high yield when a large fraction of assimilates is partitioned to the fruits. Partitioning to the fruits depends on the number of fruit (fruit set) and the capacity to import assimilates (sink strength) of individual fruits.

Fig. 2.

Schematic representation of the five key aspects of light that regulate whole-plant physiological processes: intensity, photoperiod, spectrum, directionality, and energy (heat). All five affect processes such as photosynthesis, morphology, assimilate partitioning, plant–water relations, and their interrelationships. All these processes ultimately affect plant growth and quality of the fruits. UVR8, ZTL, Cry, Phot, and Phy are photoreceptors acting in different regions of the spectrum (see Fig. 3).

Fig. 3.

Schematic representation of the different photoreceptors in tomato, their respective absorption spectra, corresponding chromophores, and main functions. FAD, flavin adenine dinucleotide; FMN, flavin mononucleotide; PHY, phytochromobilin; Trp, tryptophan residues.

Fig. 4.

Effects of different levels of additional far-red (FR) radiation or different fractions of blue light on the growth of tomato plants. (A, B) At 15 d after sowing, plants were treated with 0, 50, 112, or 150 μmol m⁻² s⁻¹ FR (resulting in phytochrome photostationary state values of 0.88, 0.80, 0.74, 0.70, respectively). FR was added to a background of 150 μmol m⁻² s⁻¹ red plus blue light. In addition, 17 μmol m⁻² s⁻¹ FR was added for 15 min as an ‘end-of-day’ (EOD) treatment. Plants were photographed after (A) 7 d or (B) 28 d. From Kalaitzoglou et al. (2019). (C) Plants were grown for 23 d under solar-like white light from plasma lamps containing 27% blue, and 0, 5, 30, or 50% of the white light was replaced by blue, resulting in blue-light fractions of 27, 31, 43, and 61%, respectively. The total photosynthetically active radiation was 100 μmol m⁻² s⁻¹, and the light treatments started at 7 d after sowing. From Kalaitzoglou et al. (2021).

Fig. 5.

Schematic illustration of the responses of carbon assimilate partitioning among plant organs to different light regimes in fruiting tomato plants. Low light intensity can lead to fruit abortion, resulting in a low fraction of carbon assimilates partitioned into fruits. Low light also decreases the fraction of assimilates partitioned to the roots. Far-red (FR) increases the fruit sink strength, resulting in an increase in the assimilate partitioning to the fruits. FR also increases the relative amount of assimilate partitioned to stems at the expense of leaves and roots. Blue light results in more compact plants, while strong effects on carbon partitioning have not yet been documented.