Long-Term Alleviation of the Functional Phenotype in Chlorophyll-Deficient Wheat and Impact on Productivity: A Semi-Field Phenotyping Experiment

Abstract

Wheat mutants with a reduced chlorophyll synthesis are affected by a defective control of the photosynthetic electron flow, but tend to recover a wild-type phenotype. The sensitivity of some mutants to light fluctuations suggested that cultivation outdoors could significantly impact productivity. Six mutant lines of Triticum durum or Triticum aestivum with their respective wild-type cultivars were cultivated with a regular seasonal cycle (October–May) in a semi-field experiment. Leaf chlorophyll content and fluorescence parameters were analysed at the early (November) and late (May) developmental stages, and checked for correlation with morphometric and grain-production parameters. The alleviation of the phenotype severity concerned primarily the recovery of the photosynthetic-membrane functionality, but not the leaf chlorophyll content. Photosystem II (PSII) was less photoprotected in the mutants, but a moderate PSII photoinhibition could help control the electron flow into the chain. The accumulation of interchain electron carriers was a primary acclimative response towards the naturally fluctuating environment, maximally exploited by the mature durum-wheat mutants. The mutation itself and/or the energy-consuming compensatory mechanisms markedly influenced the plant morphogenesis, leading especially to reduced tillering, which in turn resulted in lower grain production per plant. Consistently with the interrelation between early photosynthetic phenotype and grain-yield per plant, chlorophyll-fluorescence indexes related to the level of photoprotective thermal dissipation (pNPQ), photosystem II antenna size (ABS/RC), and pool of electron carriers (Sm) are proposed as good candidates for the in-field phenotyping of chlorophyll-deficient wheat.

Thylakoid ultrastructural variations in chlorophyll-deficient wheat: aberrations or structural acclimation?

“…In a first instance, the physiological uniformity of the plant material with previous experiments carried out outdoors was ascertained by evaluating informative parameters in the dark-acclimated state or the light-acclimated steady state (Colpo et al 2023a ). In Table 3 , the values of the maximum quantum yield of PSII photochemistry F V / F M were significantly higher in wheat mutants compared to WT genotypes.…”

“…Chlorophyll a/b is common proxy of the PSII antenna size, although it can be influenced by other factors, particularly the PSI/PSII stoichiometry (Lichtenthaler and Babani 2004 ; Anderson et al 2012 ). At least in young wheat mutants, higher chlorophyll a/b was shown to correspond to a smaller functional antenna size of PSII (Ferroni et al 2022 ; Colpo et al 2023a ). Across wheat lines, an interesting relation links the chlorophyll a/b ratio with the average granum diameter d (Fig.…”

“…Although the reticulum is part of the normal plastid metabolism, it is more easily observed when the chloroplast has an increased transport demand of metabolites to maintain the inner membranes (Lindquist et al 2016 ). Functional and structural maintenance of the thylakoid system in mutants requires very likely an extra biosynthetic effort by the plant and, ultimately, diverts resources away from growth and reproduction (Colpo et al 2023a ).…”

“…Because of the smaller PSII antenna, the mutants show an imbalanced excitation rate of PSI and PSII, which causes a lower PSI/PSII ratio than in the WT lines (Andrews et al 1995 ; Terao et al 1996 ; Brestič et al 2015 ). Moreover, ANDW-7B and ANK-32A are impaired in the cyclic electron transport, which is manifested as a smaller proton motive force and a lower capacity for energy dissipation (Živčak et al 2019 ; Ferroni et al 2020 ; Colpo et al 2023a ). Very characteristic of such mutants is their tendency to over-reduce the electron transport chain.…”

“…This study focuses on the variations of the grana–intergrana organization occurring in ANDW-7B and ANK-32A cultivated outdoors. Based on the previous indoor observations (Ferroni et al 2020 ), it is conceivable that specific structural traits of the thylakoid system, particularly related to the extent and regularity of the thylakoid stacks, could be associated with specific functional alterations known to characterize the mutants also outdoors (Živčak et al 2019 ; Colpo et al 2023a ). To verify such hypothesis, a quantitative grana structure analysis of the light-acclimated samples was carried out in parallel to chlorophyll fluorescence quenching analysis.…”