On how to disentangle the contribution of different organs and processes to the growth of whole plants

P. J. Aphalo
2010 Journal of Experimental Botany  
The growing environment of plants presents a whole array of resource limitations, modulators, and informational signals (Aphalo and Ballaré, 1995) , which naturally vary in time and space. Plants are modular in structure; new organs appear asynchronously during the life of the plant and they have shorter life-spans than the plant as a whole. Understanding the growth of plants mechanistically requires a description of the relationships among these organs and their responses to the environment
more » ... Kroon et al., 2009). The size of a plant is the result of its whole life history. It is the result of carbon assimilation and respiration, and organ senescence integrated over time. The resulting growth depends not only on the intrinsic carbon-exchange rates of different organs but also on their relative sizes (allocation) and their morphology and spatial arrangement. For example, a proportionally larger allocation of photoassimilates to the production of photosynthetic organs will increase the relative growth rate (RGR) of a plant in the absence of limitation by soil resources. RGR will also depend on the morphology of the photosynthetic organs through specific leaf area (SLA) and its effect on leaf area ratio (LAR). The spatial arrangement of leaves will affect shading and the irradiance received at their surface. Finally, the time-course of physiological activity during the lifetime of organs and the length of this lifetime will also determine, in part, the growth rate of a plant. The performance of a plant depends on a hierarchy of structures and functions from biochemistry and anatomy to plant architecture and canopy properties. If growth analysis methods are used to study plant performance, the net assimilation rate (NAR) can be calculated and related to RGR, but NAR is an average rate for the whole plant without information about populations of organs (e.g. Poorter and Remkes, 1990 ). If gas-exchange methods are used, assimilation and respiration can be measured for individual organs, but this only gives us instantaneous values. To obtain a detailed mechanistic explanation of growth, both approaches need to be combined. As Suárez's (2010) paper shows, this can be done by combining leaf demography and frequent gas-exchange measurements on a representative sample of each and every leaf cohort. This approach allows the performance in time and longevity of individual leaves to be related to the performance of the whole plant. Suárez (2010) also measured a limited set of growth and morphological variables: leaf area, leaf dry mass, and their ratio. Suárez's (2010) approach is reminiscent of the structuring of some models simulating the growth of crops. Earlier studies gave the elements needed for this approach. Nilsen et al. (1987) discussed the roles of leaf demography and time-courses of leaf physiological characteristics as determinants of whole plant growth. However, they centred their efforts on leaf dynamics and demography. Kitajima et al. (1997 Kitajima et al. ( , 2002 studied the relationship between carbon assimilation and leaf age in tropical trees, but did not attempt to integrate carbon assimilation over an individual. But now Suárez (2010) grew smaller plants under controlled conditions and chose a species well suited to this type of analysis: Ipomoea pes-caprae is a vine with a growth habit producing little shading among leaves. This simplified the analysis and allowed integration of assimilation in time and over leaf cohorts. In the future, combining the approach used by Suárez (2010) with a more complete growth analysis would address the question of what is more important in the acclimation to nutrient limitation, shade or other stressors: changes in allocation, morphology, longevity or instantaneous carbon balance by particular organs. Measuring photosynthesis light-response curves and the light environment of the leaves and their variation through the day would be 626 | Commentary
doi:10.1093/jxb/erp398 pmid:20118495 fatcat:ejefxe5yfvdwfonsitetw7evsi