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sur le développement des zones arides et semi-arides

Titre : Efecto de un factor ontogénico en la respuesta fisiológica de cladodios maduros de Opuntia ficus-indica en condiciones de humedad y sequía

EFFECTS OF AN ONTOGENIC FACTOR ON THE PHYSIOLOGICAL RESPONSE OF MOTHER CLADODES OF Opuntia ficus-indica UNDER DRY AND WET CONDITIONS

Auteur : Zañudo Hernández Julia.

Université de soutenance : Universidad de Guadalajara

Grade : DOCTOR EN CIENCIAS (AREA ECOLOGIA) 2005

Résumé
Drought affects C3 and C4 crops as well as cultivated CAM plants in the semiarid lands of central Mexico. Damage by drought to cultivated CAM species of Opuntia is commonly observed when mother cladodes are planted during the dry spring season, these species are resistant to drought, but their responses can be affected by development of new organs, such as young developing daughter cladodes that behave like C3 plants, with daytime stomatal opening and water loss. In contrast, wild populations of Opuntia are less affected by drought, because they do not develop daughter cladodes under extreme drought conditions. The main objective of this work is to evaluate the effects of the number of daughter cladodes on gas exchange parameters of mother cladodes of Opuntia ficus-indica exposed to varying soil water contents growing in spring under dry and wet conditions. Rates of net CO2 uptake, stomatal conductance, intercellular CO2 concentration, chlorophyll content, and relative water content were measured. Daily net CO2 uptake by mother cladodes of O. ficus-indica was significantly reduced as the number of daughter cladodes increased under dry conditions but was less affected under wet conditions. This was accompanied by decreased mother cladode relative water content, suggesting movement of water from mother to daughter cladodes. Stress caused by daughter cladodes and by dry soil affected the expression of CAM phases in mother cladodes, curtailing CO2 uptake in the late afternoon (phase IV) and reducing carbon gain during the night (phase I) and early morning (phase II), the reduction being most marked under dry conditions. Daughter cladodes significantly hasten the effects of drought on mother cladodes by competition for the water supply and thus decrease daily carbon gain by mother cladodes mainly by inhibiting phase IV of CAM.  Under wet conditions the relative water content of mother cladodes was only slightly affected by daughter cladodes ; nevertheless, daughter cladodes affected the recovery of phases I, II and IV after substantial rainfall interrupted drought, and affected also the net CO2 uptake in phases II and IV ; this is one of the first times that an ontogenic factor has been shown to affect net CO2 uptake under well watered conditions an dry conditions. The increase in the number of daughter cladodes did not increase the photosynthetic capacity of mother cladodes under wet or dry conditions as has been shown on C3 and C4 plants.

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