The mechanic state of "Inner tissue" in the growing zone of sunflower hypocotyls and the regulation of its growth rate following excision

Spontaneous growth of isolated inner tissue from the etiolated sunflower (H elianthus annuus L.) hypocotyl growing zone was investigated. A new prepara tion technique allowed measurements starting 3 s after excision: Elongation with respect to the turgescent and plasmolized state was quantified in ter ms of relative growth rates, facilitating comparison to growth in situ. Tur gor and turgor-induced strain were determined. Overall longitudinal strain in inner tissues in situ was positive, indicating that compressive forces e xerted by peripheral tissues are outweighed by turgor-dependent tensile str ess. Inner tissue expansion following isolation depended on water uptake. E xtreme plastic extension rates occurred immediately after excision, suggest ing that mechanical parameters of inner tissue in situ cannot be extrapolat ed from the mechanics of excised sections. In the long term, excised inner tissue autonomously established values of turgor, turgor-induced strain, an d relative growth rates similar to values in the living plant. These result s support historic models of tissue cooperation during organ growth, in whi ch inner tissues actively participate in the control of growth rates.