Stress resistance and quality criteria for tree seedlings: analysis, measurement and use.

Stress resistance and seedling quality are considered to be fully defined by the curve of future shoot growth. The factors controlling this curve's starting level, slope, and upper asymptote are analysed in terms of four major subsystems: substrate utilisation, photosynthesis, water, and information. Published equations describing the first three of these subsystems are used to define a necessary and sufficient set of quality criteria. These include functional capabilities such as specific maintenance rate and photochemical efficiency, material properties such as elasticity and hydraulic conductances, environmental coefficients such as the temperature range for root growth, and lethal doses such as frost hardiness. In addition, they include variables describing the current state of the plant, such as leaf area, and water content. The informational subsystem is considered to control the seasonal change, or "acclimation", in parameters of the other three subsystems, but is still too poorly understood for mechanistic description. Quality criteria arising from it include the extent to which chilling requirement has been fulfilled. Applying such analyses to the business of reforestation consists of choosing a subset of the quality criteria according to past and future conditions in the crop and measuring them by methods such as those outlined here. Important methods include carbohydrate and infra-red gas analysis, porometry, the pressure-volume technique, and short-cut procedures derivable from these. Measurements of field-proven quality criteria can be compared with seasonal norms, or with values calculated from mechanistic models to be suitable for given site conditions. Practical decisions can then be made about nursery treatments, site preparation, planting, and genetic selections. INTRODUCTION Seedlings that flourish in spite of the relatively harsh environment into which they are normally transplanted from the nursery, are variously said to be "stress resistant", "hardy", "vigorous", or of "high quality". Producing them consistently and economically * This paper was completed while the author was a Visiting Fellow at the Research School of Biological Sciences, Australian National University, Canberra. Thanks are due to G. D. Farquhar of that institution and G. A. Ritchie of Weyerhaeuser for helpful review N.Z. J. For . Sci. 10(1): 21-53 (1980). 22 New Zealand Journal of Forestry Science Vol. 10 is the nurseryman's prime objective. It is an objective made difficult, however, by the absence of definition of these terms, the complexity of plant-environment interactions embraced by them, and the lack of a rational framework in which to view these relationships. In this paper I show how mechanistic models of various aspects of plant growth can provide such a framework and definition. If the models are sound and operate at the right level of detail, their parameters can be adopted as precisely defined quality criteria. I show how appropriate criteria can then be selected, measured, and interpreted for use in reforestation programmes, and discuss the extent to which each is of proven versus still-hypothetical value. Scope and Definition of Stress Resistance Stress can be an excess of deficiency of any factor needed for growth — light, heat, water, CO2, etc. — as well as the presence of such unneeded factors as browsing animals, disease-causing organisms, or toxic chemicals. Depending on its intensity, rapidity of onset, total duration, and frequency of occurrence, stress may be injurious or merely inhibitory. Or it may act by leading to secondary stresses (Levitt, 1972), as in the case of low temperature, which injures through desiccation by intercellular ice; or flooding, which causes an oxygen deficiency in the roots. One or many growth processes may be directly or indirectly affected, for example in cold storage. Here, the dark and chilly environment has an apparently direct effect on the membranes and/or proteins of early-lifted seedlings (which although still well-hydrated will quickly die), and indirect effects on late-lifted seedlings through desiccation, or respiratory consumption of needed food reserves over a long period. Mechanisms of "resistance" to any one of these stresses are equally diverse, and again have been extensively reviewed by Levitt (1972). They consist of the plant's ability (1) to avoid coming to equilibrium with the stressful component of its environment, e.g., by conserving water through stomatal closure and rapid root extension, or (2) to tolerate the stress within the plant itself. Tolerance can in turn be rigid, plastic, or repairable in nature. In the case of a drought in which the plant has attained a low value of xylem water potential, these would correspond respectively with (1) retention of water by the surrounding tissues due to high solute content of the cell sap, or stiffness of the cell wall, (2) loss of water from the tissue which, due to other cell characteristics, was not harmful, (3) an injurious loss of water, following which the cells could nevertheless be repaired. Two or more of the mechanisms of avoidance and/or tolerance may contribute quantitatively to the plant's total resistance to a particular environmental factor. The effectiveness of any one resistance mechanism may also change (reversibly) under non-injurious levels of stress, such that the plant becomes "acclimated". This may take place over time scales of days, weeks, or even years as exemplified respectively by shade-adaptation of leaves, frost hardening, and changes in tree form following canopy closure. The acclimated or "hardened" seedling is then less adversely affected by further or more severe exposures. In general, there are three types of physiological process relevant to any discussion of stress resistance, which operate over different regions of a particular environmental factor's range. These are shown schematically with respect to temperature in Fig. 1, No. 1 Timmis — Stress Resistance and Quality Criteria 23 together with Levitt's limited analogy of the elastic and plastic strains in a stretched spring. The progresses are: (1) growth, which includes optimal and two non-optimal or inhibitory (elastic strain) regions; (2) injury (plastic strain), from heat or cold in this case, which may be repairable or not; and (3) acclimation, which can lead to a change in (1) and/or (2). The environmental ranges over which any two processes occur may overlap considerably, and the cumulative effects increase with exposure time. Plastic strain Elastic strain Zero Elastic Plastic —--x^w\/\/ \ / \ / \A/\/\AA/\^^ Growth reduction] f Growth ] [ Growth ~|[Heat in jury] Irrepairable"! ["Repairable injury J [ injury Assimi lat ion z. respiration -*starvation [ G r o t 1 I" TH I opt imumj j reduct ion j