Systemic Signaling In The Photosynthetic Induction Phase Of Rumex K-1Leaves

Photosynthetic induction is the course that the net photosynthetic rate of a dark adaptedleaf increasing and reaching the maximal level when the leaf is exposed to effective light.The rapidity of the photosynthetic induction is critical to the utilize of the dynamic light ofplants. The understory leaves are usually overshadowed by the canopy leaves, and merelydependant on the small part of light that went through the canopy layer. However, this part oflight source is always short-termed and weakened, so that the understory leaves have toperform rather fast photosynthetic induction to make the best use of this dynamic andprecious light resource. Former studies around the photosynthetic induction of the understoryleaves are mainly focused on individual leaves, no attention have been paid to the correlationbetween leaves and other part of the plants.In fact, higher plants have developed complicated and efficiency communicationmechanisms between different organs during the long evolution. Organs of a plant are notisolated. During a long time acclimation to various environments, a suite of complicatedmechanisms formed up so the plants can adapt changing environment. As an importantregulation mechanism of plant development and metabolism, systemic signaling has become arecent hot topic and drawn a lot of attention of the researchers. However, former researcheson systemic signaling were generally focused on the regulation of plant development andstress tolerance, few work was published on the link between photosynthetic induction andsystemic signaling.Therefore, in this research, the Rumex K-1was taken as the experimental plant toinvestigate the photosynthetic induction of the leaf that was not pre-illuminated （target leaf）after different number of leaves （system leaves） were illuminated with different PPFD.Possible mechanism and pathway for the transportation were also discussed. The main results acquired are as follows:（1）After the system leaves were illuminated, photosynthetic induction of the target leafwas significantly promoted. The time needed to reach50%of the target leaf’s maximalphotosynthetic rate decreased by about29%, and the time needed to reach90%of the targetleaf’s maximal photosynthetic rate decreased by about38%. This phenomenon significantlyincreased the photosynthetic induction of the target leaf in the first10minutes of theinduction, the photosynthetic accumulation increased by76%compared with the control leaf.With the irradiation time prolonged, compared with the control leaf, the target leaves thecumulative increase in the proportion of photosynthetic decline, but in the first25minutesbefore the photosynthesis, the photosynthetic accumulation of the target leaves are still29%higher than the control. The at the30minutes photosynthetic induction of the target leaf, thephotosynthetic accumulation was only10%higher than the control. The results suggested thatthe regulation makes the target leaf perform the fast photosynthetic induction in the shortestpossible period.（2） After pre-illumination1,2or3system leaves, the photosynthetic induction of thetarget leaf was significantly promoted compared with the control leaf. The promotion extentcaused by1,2or3system leaves showed no significant difference between them. This resultshowed that, when illuminated, the Rumex K-1leaves generate proper systemic signal thatcan promote the photosynthetic induction on the same unit. The promotion extent of theinduction was not increased by the increase of the system leaves, indicating that it is not adose-effect relationship between the number of the system leaves and the promotion extent.Small number of the leaves is sufficient to generate equivalent promotion to increased numberof system leaves.（3）After illumination to the system leaves with100μmol m^-2s^-1，400μmol m^-2s^-1or1000μmol m^-2s^-1PPFD, the photosynthetic induction of the target leaves was significantlypromoted. Illumination with30μmol m^-2s^-1PPFD to the system leaves did not significantlypromote the photosynthetic induction of the target leaf. No obvious differences wereobserved between the promotion extents of the photosynthetic induction of the target leaves after the system leaves were illuminated with different light intensities. The results indicatedthat, the system leaves generate effective systemic signal only when the incident light is abovea threshold value. When the incident light is above the threshold value, there are nodifferences of the promotion extent of the photosynthetic induction between the target leaveswith system leaves illuminated with different light intensities, indicating that the relationbetween the incident light and the promotion level is not dose-effect either.（4）When the upper leaves were taken as the system leaves and illuminated, thephotosynthetic induction of both the lower target leaves and the upper target leaves （the restupper leaves） was significantly enhanced compared with the contro leaves. When the lowerleaves were taken as the systemleaves and illuminated, the photosynthetic induction of lowertarget leaves （the rest lower leaves） was significantly enhanced,however, the photosyntheticinduction of the upper target leaves was not enhanced, which indicates that, the systemicsignal is transported directionally. The signal is transported only from upper leaves to lowerleaves or a leaf on par, yet no signal is transported from a lower leaf to upper leaves.（5）After the phloem in the petiole of the target leaf was destroyed via chemical girdlingtreatment, the photosynthetic induction of the target leaf was no longer promoted, indicatingthat the systemic signal could be blocked by chemical girdling. We determined that thegirdling treatment killed the living organisms in the petiole yet the water transportation wasnot affected because the relative water content of the leaf with girdled petiole did not differfrom that in control leaves, indicating that the systemic signal is probably transported out ofthe system leaf via the phloem tissue.（6）The stomata behavior of the target leaf was studied after the system leaves wereilluminated. We noticed that, after the system leaves were illuminated, the stomatalconductance was not increased in the target leaves without illumination. The stomatalconductance was significantly increased in the illuminated target leaf., meanwhile, theinternal CO₂content （Ci） of the regulated target leaf was generally lower than that in thecontrol leaf. It is concluded that the systemic signal is also affective to the stomata opening ofthe target leaf, the lower Ci and higher stomata conductance of the regulated target leaf indicated that the stomata is not the primary factort limiting the rise of CO₂assimilation rateduring the photosynthetic induction phase, The increase of CO₂assimilation ability is mainlyresponsible for the faster photosynthetic induction of the regulated target leaf.