Influence du poids de la semence et de la profondeur de semis sur la croissance et le développement de la plantule de maïs

Résumé - On a étudié, en conditions semi-contrôlées sous serre, la croissance et le développement de plantules de maïs selon le poids de la semence, la profondeur de semis et la variété (4). Pendant la phase hétérotrophe, la croissance de l'hydride DEA est, à peu de choses près, directement proportionnelle à la quantité de réserves de la graine utilisées depuis la germination, dans un rapport peu variable suivant les conditions (coefficient de variation de 3,1%). Les plantules issues de grosses semences présentent une croissance nettement plus rapide, mais une efficience légèrement moindre pour l'utilisation des réserves de la graine. Les stades foliaires ou racinaires sont déterminés par l'âge de la plantule depuis sa germination, indépendamment de la taille de la semence d'origine, et, après l'apparition de la 2^e feuille, indépendamment de la profondeur de semis. Les plantules issues de semis profonds utilisent un peu plus rapidement les réserves de la semence et montrent un taux de croissance supérieur. La fraction des réserves mobilisées jusqu'au passage à l'autotrophie est peu variable, autour de 65% : les plantules issues de grosses semences débutent donc leur période photosynthétique avec une plus grande quantité de réserves encore disponibles dans la semence et avec un poids supérieur d'organes chlorophylliens.

Abstract - Effects of seed weight and sowing depth on growth and development of maize seedlings. Growth and development of maize seedlings according to seed weight, depth of sowing and variety (4) were studied in a greenhouse. Up to 240° per d, the growth of the DEA variety is directly proportional to the amount of seed reserves which have been used since germination: the ratio shows little variation according to seed weight and sowing depth (the variation coefficient is 3.1 %, figs 1 and 2). The slight advantage of seedlings originating from light seeds in relation to the relative amount of reserves which have been utilized during a given period of time (table I) is quite insufficient to counterbalance the much larger absolute amount used from heavy seeds. As a consequence, the seedlings originating from heavy seeds displayed rapid growth (fig 3). We found a genetical variation in the rhythm of seed reserves utilisation as follows: Monarque was more rapid, followed by DEA, F₇ x F₂ then Mohican (table V). These differences have consequences on the rate of growth during the heterotrophic stage, which is higher with Monarque. Unlike the growth, the development (appearance of new organs as leaves or ramifications on the radicle or lateral roots) is independent of the weight of the originating seed. It expresses the physiological age of the seedling, according to the time passed, as the sum of temperature (table II). The ratio of reserves used when a seedling starts an autotrophic life is about 65%, showing little variation. At that time, seedlings have passed the 3rd leaf stage, the advent of ramifications on lateral roots, and we observe a temporary decrease in root weight as described by Bourdu and Gregory (1983) (table III). On the deep sowing treatments, the seedlings use seed reserves more rapidly and as a consequence of this, grow faster (table IV). In these situations the emergence is comparatively delayed with shallow sowings, but this difference of stage disappears after the advent of the 1st leaf. When the ermergence takes place, the seedlings originating from light seeds sowed deeply dispose of only 45% of maternal reserves (ie 75 mg) in contrast with those from heavy seeds and shallow sowing which still dispose of 71% (ie 235 mg). In the first situation, the safety margins, in the event of adverse conditions (cold weather, excess of water, coarse seed-bed, surface crust) are restricted. On the other hand, the weight of chlorophyllian organs, when efficient photosynthesis takes place, is much higher in seedlings originating from heavy seeds rather than from light seeds: the initial state of the exponential growth is much better, and, as a consequence, this fact could induce better progress at all stages.