Cochard, Herv\ue9

Delzon, Sylvain

Petit, Giai

Torres-Ruiz, Jos\ue9

English

Hervé Cochard

Giai Petit

José Torres-Ruiz

Sylvain Delzon

Archivio istituzionale della ricerca - Università di Padova

                                                        Journal of Plant Hydraulics 6: e-002 XIM4 meeting report, Sept. 25-27 2019, Padua (Italy)The  hydraulic  community  has  taken  a  step  forward  towards  non-invasivemeasurementsHervé Cochard1, Giai Petit2, José M. Torres-Ruiz1 and Sylvain Delzon3 1Université  Clermont-Auvergne,  INRA,  PIAF;  2  Dept.  TeSAF  -  University  of  Padua;  3Université  deBordeaux, INRA, Biogeco. Corresponding author: Hervé Cochard, herve.cochard@inra.frAbstractThe fourth edition of the international xylem meeting was held for the first time outside France, where thethree first editions were held. This represents an important step forward for the meeting and attests to theresolutely international dimension of this symposium. The conference was organized by the University ofPadua in the green setting of the botanical garden. No less than 140 researchers from more than 21 countriesattended these three intense days of seminars and discussions. For logistical reasons, the number of placeshad been limited by the organizers, which forced us to decline a number of registrations. This tends to prove,if there were any need, that the community of researchers in xylem physiology and the hydraulic functioningof plants is booming, and that these meetings are becoming an essential biennial event for our discipline.For this edition, no less than a hundred communications were presented, including about fifty plenary talks.It is difficult to summarize all the richness of this work in a few lines, so we propose here to highlight someof the most salient points. A scientific community in tune with its societyWhile we were discussing our scientific work in session, more than 20,000 peaceful demonstrators marchedfor climate in the streets of Padua. In a manifesto published on social networks and signed by all congresses,we wanted to remind that: “We,  as  an  international  community  of  plant  scientists,  support  climate  change  mitigation.  Ourinvestigations  of  trees,  forests  and  climate  dynamics  clearly  show  that  critical  ecosystems  are  beingthreatened by increasingly frequent extreme climatic events.  Large-scale tree mortality across the globedemonstrate the severity of the threat that we are facing and demands actions.”Our work on the hydraulic functioning of plants allows us to better understand the response of trees to futureclimate  change,  especially  to  drought  events,  and their  ability  to  adapt  or  migrate.  We must  also  takeadvantage of these scientific meetings to better communicate with the general public, the media and politicaldecision-makers, and to explain what will be the consequences of the ongoing climate change based onscientific evidences. It is our responsibility to alert people of the serious risks that our current lifestyles poseto natural and anthropized ecosystems. We will try not to miss this opportunity during our next meeting. The long history of plant hydraulicsThe symposium began with a presentation by Mel Tyree on "A brief history of 'our' science". Mel Tyree, oneof the founding fathers of the modern history of plant hydraulics, honored us with his presence this year. Hispresentation,  punctuated  by  numerous  anecdotes,  made  young  researchers  aware  of  the  importance  ofplacing our research in a more global historical context. From its origins in the 15th century until today, 'our'science has been marked by controversies about the mechanism of sap ascend in trees. Erroneous methodsor observations are most often at the root of these controversies, but when rigorous experiments have beenconducted independently,  Dixon's  (1914) cohesion-tension  theory has  always  been reinforced.  We must© The Authors Journal of Plant Hydraulics 6: e-002 therefore look at current controversies in the light of this historical dimension and be convinced that they area sign of a very dynamic field of research.“Nothing in life is certain except death, taxes, and the second law of thermodynamics” (Seth Lloyd)The remarkable feature of sap ascent in plants is that it occurs under negative pressure in a conductive tissueformed by dead cells. This physically very unstable state is likely to evolve into a more stable gaseous statethrough a cavitation process when the sap tension overpasses a threshold value.  Living cells  are  underpositive pressure due to the presence of a plasma membrane that establishes an osmotic barrier. As can beseen from these selected examples, physics, and thermodynamics in particular, are at the heart of the waterrelations of plants. And you cannot escape the laws of thermodynamics! Mel Tyree's many interventionsduring this conference often alerted us to a lack of knowledge of the physics of the systems we are studying.It is therefore critically important to associate physicists to our projects, especially when we are to explorethe controversial topics dealing with the ascent of sap and the process of xylem embolism and repair. Thetalk of Missy Holbrook on the collapse of minor leaf veins was another great illustration of the close linkbetween physics (biomechanics) and hydraulics.  It could be interesting to attract more physicists to ourconference in future editions to benefit from their critical view of our work.“If it matters, it produces controversy” (Jay Green)As a matter of facts, if we can judge by the number of controversies that animate our discipline since itsearly hours, plant hydraulics matters! Three main controversies have driven our debates: - The “Miraculous” refilling of xylem conduits Science is progressing in small steps, but perhaps the most significant advance of this symposium is theconsensus in our community that the "miraculous" ability to resorb embolism under negative pressure iscertainly  elusive.  It  is  thanks  to  the  development  of  new  non-destructive  observation  methods  (X-raytomography,  NMR,  Optical  technique)  that  we  are  now  reaching  this  consensus.  In  other  words,  aconductive system that is highly vulnerable to embolism and that would lead to high exposure to the risk ofcavitation is unlikely to have evolved. As Marciej Zwieniecky mentioned in his presentation, just becausexylem refilling capacities are low does not mean that we should not focus more on the plants' ability torecover from stress. Plants are exposed to many drought events during their lifetime, so their  ability torecover and how they prepare to recover more quickly and effectively should be carefully studied. - Carbon starvation vs hydraulic failure The mechanisms by which water stress leads to plant mortality have yet to be clarified. Nate McDowell inhis presentation recalled several important points to consider in moving this issue forward. If hydraulicfailure is detected in almost all drought-induced mortality events, correlation is not necessary causation. Thetwo mechanisms are therefore not exclusive of each other and can interact, especially over long periods oftime. However, the mechanical link between hydraulic failure and carbon starvation remains to be identified,as the "refilling cost" hypothesis now seems to be excluded. We must better take into account this carbonstarvation hypothesis in our work, and in particular evaluate the carbon status of the key organs involved inplant survival (primary and secondary meristems). Jess Gersony showed in her presentation that Raman-spectroscopy technology could be used to measure several sugars at the cell level, which should lead tosignificant progress on this issue. - News direct observational methods for less artefacts In the history of our discipline, researchers have always been confronted with methodological biases thathave led them to misinterpret their observations and therefore misunderstand the hydraulic functioning ofplants. This is largely due to the very particular water status of the water present in the conductive tissue ofplants (negative pressures) and therefore to the risk of strongly perturbing observations by too invasivemethods. This explains why in many presentations, researchers have implemented less or non-invasive andmore direct methods of observing embolism. Some techniques require very sophisticated and non-easilyaccessible equipment (NMR, micro-CT), others, such as the optical method, are much more accessible. Theelectrical resistivity tomography method presented by Stefan Mayr is also a promising and affordable non-invasive technique which has also the merit of being able to be used in situ and on large trees. We are onlybeginning to use these new techniques and we must still be cautious in our conclusions. Nevertheless, and at2Journal of Plant Hydraulics 6: e-002 this  stage of  our knowledge,  these non-invasive methods reveal  a  conductive tissue that  is  much moreresistant to cavitation than can be suggested by  invasive techniques (i.e. hydraulic methods). In particular,for  all  the  species  that  have  been  studied  with  these  new  methods  and  presented  at  this  symposium,cavitation events never  occur under the normal operating range of water potential, but  only under severewater stress. The hydraulic techniques (benchtop dehydration, air injection) no longer appear to be the goldstandard methods to study species with long pipes and techniques allowing to work on intact plants must bepreferred.  Significantly,  cavitation  occurs  after  the  closing  point  of  the  stomata,  which  also  oftencorresponds to the point of loss of turgor of the leaves. Missy Holbrook and Scott McAdam showed in theirpresentation that even Bryophytes or earliest vascular plants like Lycophyte have conductive tissues that areremarkably resistant to cavitation. This very conservative nature of hydraulic functioning is therefore veryold, and probably constituted an evolutionary lock to be lifted for the colonization of emerged lands. This  should  lead  us  to  be  extremely  critical  when  we  encounter  R-shaped  curve  in  our  work  as  wasenthusiastically demonstrated by B. Schuldt during his presentation on root hydraulic safety and efficiency.Without wishing to reject a priori all these curves, we must collectively require a minimum of experimentalguarantees on the validity of such curves before publication. When these curves are produced by a newmethod (as with the Pneumatron currently), it should be cross-validated with a reference and non-invasivemethod. In all cases, and whatever the method used, it is essential to test the consistency of the curvesobtained with the water relations of the plant. In particular, the cavitation pressure threshold (P12) must bemore negative than the sap pressure measured on a plant that is well supplied with water and transpiringheavily. Similarly, this P12 is always  less than or equal to the sap pressure inducing stomatal closure (Pgs90)or the point of loss of foliar turgidity (Ptlp). If these conditions are not met, and therefore if a significant rateof embolism is predicted based on the vulnerability curve, then this rate should be validated by robust in situembolism measurements. With these simple rules, we will produce more robust data and avoid repeating themistakes we made in the past and stop blocking the progress our discipline because of these avoidablemisconception.Trees hiding the forest.Since the aim was to understand the rise of sap beyond the theoretical 10-metre barrier, the first work onplant hydraulics historically focused on trees.  The development of our discipline at  the end of the 20thcentury was also undertaken in institutes closely linked to trees, such as the Harvard-Forest department, theUS-Forest  Service  and  the  INRA Forest  department.  The  amount  of  information  accumulated  on  treehydraulics  is  now  considerable,  which  has  led  to  major  cognitive  advances  in  tree  physiology,  stressresponse and ecology.  However, work on the hydraulics of non-woody plants is still very marginal, butsome studies presented at this conference show that things are moving forward. We have seen work on thehydraulics of early vascular plants (Mosses, Ferns) and on wild or cultivated herbaceous plants (maize,wheat). We must intensify this work and encourage the crops and herb scientists communities to join ourdiscipline.  It  is  a  mutually  beneficial  process.  Tree  hydraulics  specialists  will  benefit  from  a  betterunderstanding  of  how  their  species  work  by  placing  their  work  in  a  broader  phylogenetic  context.Conversely, as pointed out by Sean Gleason in his presentation, considering hydraulic traits is a promisingnew way to improve the agronomic performance of crop species in a changing climate context.A second important point that has been mentioned in several communications is the need to develop work onplants in situ, under natural growing conditions, and not only ex situ on potted plants placed under controlledbut artificial conditions. The dehydration kinetics of potted plants are often not comparable to that of plantsin situ, so the responses observed may not be representative of the natural functioning of these plants. If, inaddition, the hydraulic features are more plastic than we currently think, as in the case of vines for example,pointed by Uri Hochberg and Laurent Lamarque, then we would underestimate in part the plants' ability toacclimatize  to  the  constraints  of  the  environment  through  these  ex  situ experiments.  An  importantexpectation of our work concerns our ability to understand and predict the risks of tree mortality in forests.Surprisingly enough, very little of the work of this symposium concerned the response to drought in maturetrees under natural conditions. This type of study represents higher technical constraints and requires higherinvestments,  but  they remain  unavoidable  and must  therefore  be encouraged,  especially to  validate  thepredictions of some of our models.On a completely different scale, there was remarkably little work presented on the molecular and geneticbasis of the hydraulic functioning of plants (but see Grace John's presentation). There is a strong challenge3Journal of Plant Hydraulics 6: e-002 in attracting researchers working on this scale to our discipline. Lukas Schreiber's presentation on cuticles isa  good  example  of  how  work  on  well-targeted  gene  mutants  could  advance  our  understanding  ofstructure/function  relationships  and,  more  importantly,  establish  causal  and  not  simply  correlativerelationships. It is also surprising that researchers working on aquaporins do not participate more in oursymposium and the scientists working on variable extra-xylary pathways did not incorporate more thesewater-channels in their study. We must ensure that we can better attract molecular biologists to our meetingsin the future.“All the models are wrong, but some are useful” (George Box)Modelling has always been an integral part of the work on the hydraulic functioning of plants. It is often anessential tool for integrating processes in space or time or for understanding the interaction of complexprocesses. Surprisingly, modelling was not well represented in the work presented in this conference. Forexample, one would expect to see more attempts to integrate hydraulic traits into plant functioning models inorder to better reconstruct or predict tree mortality in situ. Chris Blackman's presentation made it clear thatthe key functional traits of this mortality can now be identified and measured. Collaborations with modelersare now timely to predict mechanistically and globally the response of ecosystems to climate change. Thepeople who walked this Friday in the streets of Padova are waiting for such work and we now have the toolsto carry it out.XIM4 was  undoubtedly  a  new  important  step  in  the  animation  and  cohesion  of  researchers  around  acommon discipline. One of the challenges for the future of our discipline will be its ability to open up toother related field of research.AcknowledgmentsThe meeting was supported, in part, by the University of Padua, Dept. TeSAF (University of Padua), Dept.DAFNAE (University of Padua), INRA-EFPA department, the Federation of European Societies of PlantBiology (FESPB), the Italian Society of Plant Biology (SIBV), the Italian Society of Silviculture and ForestEcology (SISEF) and the New Phytologist Trust.4

XIM4 meeting report, Sept. 25-27 2019, Padua (Italy)

https://www.research.unipd.it/bitstream/11577/3312961/1/General%20et%20al%202019_JPH_XIM4.pdf

XIM4 meeting report, Sept. 25-27 2019, Padua (Italy)

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