Acclimation of plants to light gradients in leaf canopies: evidence for a possible role for cytokinins transported in the transpiration stream

Abstract

The mechanism of response of plants to vertical light intensity gradients in leaf canopies was investigated. Since shaded leaves transpire less than leaves in high light, it was hypothesized that cytokinins (CKs) carried by mass transport in the transpiration stream would be distributed over the leaf area of partially shaded plants parallel to the gradient in light intensity. It was also hypothesized that this causes the distribution of leaf growth, leaf N and photosynthetic capacity, and possibly chloroplast acclimation as observed in plants growing in leaf canopies. In a field experiment, the distribution of Ca, N and CKs in a bean leaf canopy of a dense and an open stand supported the concept of a role for CKs in the response of N allocation to the light gradient when a decreasing sensitivity for CKs with increasing leaf age is assumed. Both shading of one leaf of the pair of primary bean leaves and independent reduction of its transpiration rate in a growth cabinet experiment caused lower dry mass, N and Ca per unit leaf area in comparison to the opposite not treated leaf. Shading caused a parallel reduction in CK concentration, which supports the hypothesis, but independent reduction of transpiration rate failed to do the same. Application of benzylaminopurine (BA) counteracted the reduction caused by shade of leaf N, photosynthetic capacity and leaf area growth. The experiments show an important role for the transpiration stream in the response of plants to light gradients. Evidence is presented here that CKs carried in the transpiration stream may be important mediators for the acclimation of plants to leaf canopy density.

CN-Wheat, a functional–structural model of carbon and nitrogen metabolism in wheat culms after anthesis. II. Model evaluation

“…An illustration of the simulated dynamics and distribution of cytokinins is available in Supplementary Data SI 3. Although the submodel of cytokinins remains exploratory, it allows us to account for experimental evidence related to (1) the inhibition of tissue death by cytokinins (Mok and Mok, 1994) through (2) the down-regulation of protein degradation (Wingler et al, 1998;Criado et al, 2009;Koeslin-Findeklee et al, 2015), (3) the role of roots in shoot senescence through the synthesis of cytokinins that are exported by the transpiration stream (Badenoch- Jones et al, 1996;Pons et al, 2001) and (4) the regulation of cytokinin synthesis by root concentrations in carbohydrates and nitrates (Sakakibara et al, 2006). The dynamics of cytokinin concentration, by regulating the rate of protein degradation, played a pivotal role in determining the difference in organ death between treatments.…”

Dynamics of Light and Nitrogen Distribution during Grain Filling within Wheat Canopy

“…There is also some evidence that light quality (in particular the red to far-red ratio) influences N distribution in the canopy (Rousseaux et al, 1999;Frak et al, 2002), but the spectral component of the light gradient is probably less important than the total irradiance component (Pons and de Jong-van Berkel, 2004). It has clearly been demonstrated that accumulation of cytokinins imported through the xylem is involved in the regulation of vertical leaf N distribution (Pons et al, 2001;Boonman et al, 2007). Unexpectedly, in this study, SLN was constant over the length of the flag leaf laminae, although the difference in PPFD between the top and the bottom of the flag leaf laminae was 40%, which was probably associated with similar gradients of transpiration (and thus cytokinin import) and red to far-red ratio.…”

Acclimation of Leaf Nitrogen to Vertical Light Gradient at Anthesis in Wheat Is a Whole-Plant Process That Scales with the Size of the Canopy

“…However, the impact of each of these factors has been shown to be much less than that of the PPFD gradient (Werger and Hirose, 1991;Pons and de Jong-van Berkel, 2004), although for the grass species Brachypodium pinnatum other factors than light might be involved (Pons et al, 1993). At the molecular level, the process could be driven by the import of compounds such as cytokinins transported in the transpiration stream (Pons et al, 2001;Boonman et al, 2007). Although the actual N distribution usually follows the light gradient, in all studies it is less steep than the calculated optimal N profile maximizing canopy photosynthesis (Pons et al, 1989;Yin et al, 2003).…”