Turgor and Longitudinal Tissue 12Helianthus annuus

The quantitative relationship between turgor and the pressureexerted by the inner tissues (cortex, vascular tissue, and pith)on the peripheral cell walls (longitudinal tissue pressure)was investigated in hypocotyls of sunflower seedlings (Helianthusannuus L.) In etiolated hypocotyls cell turgor pressures, asmeasured with the pressure probe, were in the range 0·38to 0·55 MPa with an average of 0·48 MPa. In irradiatedhypocotyls turgor pressures varied from 0·40 to 0·57MPa with a, mean at 0·49 MPa. The pressure exerted bythe inner tissues on the outer walls was estimated by incubatingpeeled sections in a series of osmotic test solutions (polyethyleneglycol 8000). The length change was measured with a transducer.In both etiolated and irradiated hypocotyls an external osmoticpressure of 0·5 MPa was required to inhibit elongationof the inner tissues, i.e. the average cell turgor and the longitudinaltissue pressure are very similar quantities. The results indicatethat the turgor of the inner tissues is displaced to and borneby the thick, growth-limiting peripheral cell walls of the hypocotyl. Key words: Helianthus annuus, hypocotyl growth, tissue pressure, turgor pressure, wall stress     

The Micronutrient Boron Causes the Development of Adventitious Roots in Sunflower Cuttings

Three-day-old light-grown sunflower seedlings were de-rootedand incubated in nutrient solutions that either contained orlacked boric acid (B). In the absence of B, in the majorityof the seedlings, no adventitious roots were formed. The micronutrientB caused the development of numerous roots in the lower partof the hypocotyl. The effect of B occurred without the supplyof any phytohormones. A dose-response curve of B-induced rootingyielded an optimum concentration of 0.1 mM boric acid. Histologicalstudies revealed that cell divisions occurred in the controlbut no root primordia developed. In cuttings that were incubatedin B (0.1 m M) root primordia were observed that rapidly developedinto well-differentiated adventitious roots. Sunflower cuttingsthat were planted with their cut end in vermiculite that wasmoistened with nutrient solutions without B degenerated afterseveral weeks. In the presence of B the cuttings formed numerousadventitious roots that entirely replaced the tap root systemof intact seedlings. The rooted cuttings developed into sturdyadult sunflower plants. Our results are discussed with respectto the possible role of B in the evolution of vascular fromprevascular plants.Copyright 1999 Annals of Botany Company Adventitious roots, boron, cuttings, organogenesis, sunflower seedlings.     

Particles Associated with the Outer Epidermal Wall in Internodes of Deepwater Rice

Partial submergence induces rapid internodal growth in deepwaterrice (Oryza saliva L., cv. Habiganj Aman II). The infrastructureof the cell wall/plasmalemma interface of air-grown and submergedinternodes was investigated in the region where cell elongationtakes place. In submerged internodes, electron-dense particlesof about 100 nm diameter were found. These particles were detectableonly at the thick outer wall of the outer epidermis but notat the inner walls. In air-grown control plants, no such granuleswere visible. We suggest that these particles are related tothe process of cell wall growth. The wall weight per unit lengthwas 75% lower in the submerged internode as compared to thatof the air-grown control. This indicates that secondary wallformation is suppressed during submergence of the plant Oryza saliva, deepwater rice, intemodal growth, electron-dense particles