| Abstrakt: |
Afin de fournir les ?l?ments d'une optimisation dynamique de la r?gulation des diff?rents param?tres climatiques sous serre, un mod?le m?caniste du fonctionnement d'un couvert en rangs a ?t? ?labor? sur la base du mod?le d'interception du rayonnement solaire de Sinoquet (1989), puis cal? et valid? sur une culture de tomates de printemps. A partir de la connaissance du rayonnement incident au sommet de la culture, de sa r?partition en ses composantes directe et diffuse et de la position du soleil, il calcule l'interception du rayonnement en tenant compte des rediffusions, ainsi que de l'activit? photosynth?tique du couvert. Deux fonctions de r?ponse photosynth?tique foliaire ont ?t? utilis?es : - une fonction hyperbolique classique caract?risant les potentialit?s moyennes de l'ensemble des feuilles de la culture (mod?le 1); - une fonction tenant compte de l'?ge des feuilles, leurs capacit?s photosynth?tiques ?tant d'autant plus faibles qu'elles sont ?g?es (mod?le 2). Le calage a ?t? r?alis? ? partir de mesures de photosynth?se foliaire, la validation avec des mesures in situ de la photosynth?se de plantes enti?res. Les mesures n?cessaires au fonctionnement du mod?le (rayonnement, description g?om?trique de la surface foliaire) ont ?t? faites sur la m?me culture. La comparaison des simulations et des mesures de photosynth?se du couvert montre une assez bonne ad?quation des 2 mod?les. Cependant, le mod?le avec effet ?ge sous-estime l'activit? du couvert, lorsqu'il est soumis ? un fort rayonnement; ce biais peut ?tre attribu? ? une sous-estimation des capacit?s photosynth?tiques des feuilles ?g?es. Nous avons simul? l'exp?rience pr?sent?e par Acock et al (1978) o? l'activit? de couches de feuilles d'?ges diff?rents a ?t? mesur?e par effeuillages successifs. Le mod?le 2, avec effet ?ge, conduit ? une photosynth?se ?quivalente, mais r?partie diff?remment, essentiellement r?alis?e par la couche sup?rieure. La sous-estimation des capacit?s photosynth?tiques des feuilles ?g?es appara?t nettement. Photosynthesis of a greenhouse row crop of tomatoes. Development of an analytical model and mapping of foliage activity. On the basis of the solar radiation interception model of Sinoquet (1989), we developed a mechanistic model of the photosynthesis of a row crop, to be used for the dynamic optimization of greenhouse climate. This model was calibrated and validated on a spring tomato crop. The canopy is divided into units of assumed uniform leaf area density, leaf angle distribution and photosynthetic PAR response. This division can be made along 2 or 3 axes, thus allowing variability along the rows to be accounted for the rows as well as across them (fig 1). From the input of incident diffuse and direct radiation on top of the canopy, the model computes, for each canopy unit, the leaf area distribution with respect to PAR interception, taking into account multiple diffusions, and photosynthetic activity. The canopy carbon gain is then computed as the sum of the unit values. Two leaf photosynthetic light-response functions were used: - one single hyperbolic function to describe the mean activity of all the leaves of the crop (model 1); - one function which includes a reduction factor proportional to leaf age, with the lowest photosynthetic rate for the oldest leaves. Measures of leaf photosynthesis were used to calibrate the model; the validation was made with in situ measures of whole-plant CO2exchanges done under different radiative conditions (fig 2). Incident irradiance and canopy structure measurements were also performed on the same crop, and used in the model. Comparing in situ measures of crop photosynthesis with simulated data shows a good predictive value of both models, although the model with a leaf age effect under-estimates crop photosynthesis under high irradiance (fig 3). We analyzed the interaction between the date and hour of photosynthesis measurement on the residuals of the models (fig 4) and we compared the simulations performed with the two models (fig 5). From this analysis, we concluded that the under estimation of crop assimilation by model 2 is mainly due to a too high reduction factor for the oldest leaves. We simulated the experiment in which Acock et al (1978) measured the photosynthetic activity of canopy layers of different leaf age by successive partial defoliations (fig 6). Model 2, including a leaf age effect, computes a value of crop photosynthesis similar to that mesured by Acock et al (1978) but the distribution among the crop layers is distorted; the under-estimation of the activity of the older leaves is clearly shown. |
