Cellular anatomy of tomato stems in response to far-red light 2 29 2.1 INTRODUCTION With the increasing population and shortage of land area, the huge need for food, fuel, and fiber from field crops cannot be fully satisfied by current agricultural production. Hence, plants are farmed in dense vegetation to increase the yield from the available land area. However, this will not necessarily lead to an increase in yield per plant. On the contrary, dense vegetation increases plant competition for limited resources, one of which is light. When the light captured by the plant is not able to meet the growth demand, plant growth will decrease. The competition for light between neighboring plants is usually triggered by impending shade. Shade from vegetation is a light condition that is characterized by the relative enrichment of far-red (FR) light and overall low light density. FR light is already increased in canopies before true shading occurs by the reflection of light from neighboring plants, and FR enrichment alone can trigger a set of developmental responses known as the shade avoidance syndrome (SAS). Plants that face shade stress can either survive through tolerance to low light, or avoid future shade by SAS; a set of morphological changes to grow away from unfavorable growth condition (Smith and Whitelam, 1997). Plants adapt their growth and photosynthetic activity in response to changes in light conditions, particularly the ratio of red (R) to far-red (FR) light, but also the depletion of blue light. This shift in the light spectrum, which occurs in dense plant canopies, triggers SAS in plants, most commonly making them taller and more erect. Key photoreceptors, such as phytochromes and cryptochromes, play pivotal roles in perceiving these light signals. Phytochromes exist in two forms, Pr (red-absorbing) and Pfr (far-red-absorbing), with Pfr being the active form responsible for mediating SAS. The shade-induced changes, including stem elongation and altered leaf architecture, have been observed across various plant species, such as Arabidopsis, Brassica rapa, and tomato (Osborne, 1991; Pierik and De Wit, 2014; Ballaré and Pierik, 2017; Courbier et al., 2021). While stem elongation is a prominent feature of SAS, the cellular mechanisms underlying these changes have not been extensively studied in tomato. The response to shade varies among tomato species, with some showing a stronger SAS than others. Notably, Solanum pennellii exhibits a robust SAS, while cultivated tomato species like Solanum lycopersicum display a milder reaction. However, the specific SAS mechanisms within different tomato species remain less understood (Bush et al., 2015) In SAS, both stems and petioles undergo elongation, and these tissues share similarities in function and cellular morphology. Cellular changes in response to shade, particularly in the epidermal layer, have been documented in other plant species. For instance,
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