Rhythmic stem extension growth and leaf movements as markers of plant behaviour: The integral output from endogenous and environmental signals

Abstract

With the model systems Chenopodium rubrum (short-day plant) and Chenopodium murale (long-day plant), growth and behaviour have been studied in response to photo- and thermoperiod. With time-lapse photography, rhythmic integration of the plant as a whole could be monitored. Upon photoperiodic flower initiation, rhythmic stem extension rate (SER) and leaf movement (LM) change their phase relationship in a specific way. Flower induction correlates to a threshold value for the ratio between integral growth during the dark time span and integral growth during the light time span. This precise output displayed in the growth pattern of the plant is therefore an accurate reflection of all available environmental inputs. Analysis of flower induction in Chenopodium spp. showed that, 2 h after the end of the critical dark period, the patterns of cytoplasmic pH and Ca2??change at the shoot apical meristem (SAM), possibly indicating the arrival of the flower-inductive signal. Changes in LEAFY and aquaporin expression can also be recorded during this phase. The perception of a flower-inducing dark period probably leads to a change in electrochemical, hydraulic signalling between the leaves and SAM, thereby determining polarity in the whole plant and paving the way for "florigen", the flower-inducing hormone postulated in 1936 but still undiscovered. A rhythmic integration over the whole plant, as seen for SER and LM, most likely involves modulation of turgor pressure via stretch-activated ion channels and concomitant changes in membrane potential, making the plant a hydro-electrochemical signal transducer. Regulation of hydraulics and electrochemistry, two coupled physicochemical processes, was an achievement of early evolution as well as metabolic circadian regulation of transcriptional translational control loops (TTCL). Circadian rhythms (CRs) in energy metabolism are gating inputs and outputs to the TTCL, resulting in a CR of protein synthesis and turnover. Evolution of latitudinal ecotypes with different CR period lengths will depend on specific proteins, as is evident from early crossing experiments. The control of the ionic composition of the cell is crucial for survival and requires energy to maintain a resting potential of the plasma membrane. This, in turn, enables the generation of action potentials and, hence, a fast systemic communication between plant organs.