Submitted to: Weed Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/3/2012
Publication Date: 8/1/2012
Citation: Ratnayaka, H., Molin, W.T., Sterling, T. 2012. Comparison of physiological and antioxidant responses of Anoda cristata and cotton to progressive drought. Weed Research. 52:358-366. Interpretive Summary: Physiological responses of two species of cotton and two accessions of the weed, spurred anoda, were investigated under unstressed and progressive drought conditions that completely suppressed net photosynthesis in the greenhouse. This research was performed at New Mexico State University in Las Cruces, NM, as part of a USDA/CSREES National Research Initiative competitive grant to Drs. Sterling and Molin. The results show that cotton and spurred anoda adapted different physiological mechanisms to survive drought conditions which prevents widespread damage to pigment. These physiological characteristics of cotton may contribute to cotton’s ability to compete with this weed species when grown in close association with cotton.
Technical Abstract: Simultaneous investigation of variables related to gas exchange, photochemistry and antioxidant defenses during water stress is crucial for understanding stress tolerance mechanisms and consequent success of both economically important plant species and their interfering counterparts. This study evaluates responses of these three physiological aspects of individually grown plants of two cotton species (Gossypium hirsutum, cv. Delta Pine 5414, and G. barbadense, cv. Pima S-7) and two accessions of a noxious cotton weed, spurred anoda [Anoda cristata, Mississippi (MS) and New Mexico (NM) accessions] under unstressed conditions and a progressive drought that completely suppressed net photosynthesis (Pnet), and reduced biomass. Shoot biomass was at least 64% less in a given spurred anoda accession than a cotton cultivar, Pnet and photosystem II efficiency were the same in all four plant types, except for the 6% greater Pnet in Delta Pine 5414 than MS accession of spurred anoda. However, both spurred anoda accessions had >25% higher stomatal conductance (gs), >24% higher transpiration rate (E), and >1oC lower leaf temperature than cotton. Chlorophylls, lutein and ß-carotene levels were much greater in either spurred anoda accession than cotton, while a-tocopherol concentration was greater in cotton. While ascorbate peroxidase (APX) activity was higher in cotton especially Delta Pine 5414, glutathione reductase (GR) activity was higher in spurred anoda, particularly MS accession. Under progressive drought, gs was a more sensitive reference of water stress than relative water content (RWC), and use of both references was more informative, in all plant types. In all plant types, Pnet and E were more closely associated with gs than with RWC. In cotton, gs and fresh weight:dry weight ratio dropped more promptly in relation to RWC than in spurred anoda. Association of Ci with each of gs and Pnet suggested a more heterogeneous stomatal closure and less stomatal regulation of Pnet in spurred anoda than cotton during drought. Alternative electron sinks appear to be of greater significance in maintaining assimilation in spurred anoda than cotton during the more severe phase of drought, although cotton resorts to xanthophyll cycle-based nonphotochemical quenching (qE) relatively earlier in drought, compared with spurred anoda. During drought, a-tocopherol appears to be a prominent component of antioxidative defense in cotton but not in spurred anoda. Progressive drought adequate for complete suppression of Pnet, and biomass reduction does not cause widespread damage to the pigment pool in cotton or spurred anoda. Greater chlorophyll and carotenoid concentrations, mesophyll water holding capacity favoring greater gs and evaporative cooling, elasticity in growth habits, prolonged water acquisition and assimilation possibly helped by alternative electron sinks and heterogeneous stomatal closure, may confer water stress tolerance and consequent success in spurred anoda as a cotton weed, although adaptive value of these characteristics must be field-tested.