Alteration in the timing of vegetative phase change associated with nine cycles of divergent selection for rind penetrometer resistance in Missouri Stiff Stalk Synthetic
--Abedon, BG; Darrah, LL and Tracy, WF

Vegetative development in maize can be divided into juvenile (basal) and adult (distal) phases, each with distinct morphology and physiology (Poethig, Science 250:923-930). Juvenile leaves lack trichomes and are covered with an epicuticular waxy bloom, giving the leaves a grayish appearance. At juvenile nodes, adventitious roots are produced and axillary buds develop into tillers. Adult leaves have three types of trichomes (macrohairs, bicellular, and prickle) and are covered with glossy wax, which gives them a green appearance. Adult nodes do not produce adventitious roots. Axillary buds from adult nodes either develop into ears or are suppressed. The existence of heterochronic mutants (Corn-grass1, (Cg1), glossy15 (gl15), Teopod1, Teopod2), which alter the timing of vegetative phases, suggests that heterochrony has a strong genetic basis in maize. Studies involving these mutants indicate that the juvenile and adult-vegetative phases are regulated independently of each other (and of reproductive initiation) but overlap in a transition zone that normally occurs between leaves five and eight in most US field corn backgrounds.

Variation in the timing of developmental phases (heterochrony) has adaptive value and evolutionary importance in a number of plant species (Lord, EM and Hill, JP, p. 47-70 in Development as an Evolutionary Process, Alan R. Liss, New York, 1987). Until recently, there was little evidence of an adaptive value for heterochrony in maize. Abedon and Tracy (J. Hered., in press) found that adult resistance to common rust (Puccinia sorghi Schw.) and European corn borer (Ostrinia nubilalis Hubn.) is delayed in Cg1, which has an extended juvenile-vegetative phase. Passas and Poethig (p. 83 in Abstr. 37th Annu. Maize Genet. Conf.) found that an accelerated transition to an adult epidermis in leaves of gl15 mutants resulted in increased resistance to European corn borer relative to wild type sibs. These results suggest that heterochrony may have adaptive value in normal populations of maize and be a source of variability for agronomic performance.

Stalk lodging can cause substantial yield losses in maize production fields. Efforts to develop stalk lodging resistant germplasm at the University of Missouri have focused on recurrent selection for rind penetrometer resistance (RPR) in Missouri Stiff Stalk Synthetic (MoSSS). RPR is measured at the middle of the internode below the ear node. Two populations, divergently selected for high and low RPR, have been developed. Previous studies indicate that selection for high and low RPR has resulted in increased and reduced stalk lodging resistance, respectively. Our objectives were to investigate heterochrony and other developmental changes associated with nine cycles of divergent selection for RPR in MoSSS.

In 1995, five cycles (C9high, C4high, C0, C4low, C9low) were grown in randomized complete blocks with three replications at the West Madison Agricultural Experiment Station, Madison, WI. Two row plots were overplanted and thinned to 15 plants per row. Data were collected on 20 plants per plot. The duration of the juvenile-vegetative phase was determined based on the last leaf with juvenile wax, last node with adventitious roots, and tiller number. Ear leaf and total leaf number were also determined because some heterochronic mutants that affect the duration of the juvenile-vegetative phase also affect leaf number. The timing of adult-vegetative phase initiation was determined based on the first leaf with adult wax and first leaf with pubescence. The timing of reproductive phase initiation was estimated based on total leaf number and days to 50% anthesis. Ear height, plant height, and days to 50% silk emergence were also recorded. Data were analyzed by analysis of variance (data not shown) and LSD (p<0.05) was used for means comparisons.

Significant differences among cycles were observed for many traits (Table 1). Last leaf with juvenile wax showed an increasing trend while tiller number increased significantly between C9high and C9low, indicating that selection for high RPR truncated the juvenile phase while selection for low RPR elongated the juvenile phase. First leaf with adult wax increased significantly between C9high and C0 but did not change between C0 and C9low. First leaf with pubescence increased significantly from C9high to C9low. These results indicate the selection for high RPR resulted in a faster initiation of the adult-vegetative phase, while selection for low RPR delayed the onset of pubescence without affecting the first leaf with adult wax (suggesting that these traits are regulated independently).

Variation in the timing of vegetative phases was not associated with changes in the timing of reproductive initiation since total leaf number was not significantly different for any cycle (although a trend toward lower total leaf number was observed between C0 and C9high). Ear leaf number decreased from C0 to C9high, indicating that selection for high RPR resulted in a shift of ear placement downward on the plant. Ear leaf number was unchanged between C0 and C9low. Flowering time became earlier in both directions of selection. This may have been an artifact of the recurrent selection program since recombination was stopped in each cycle before the latest plants had flowered. These results indicate that no relationship exists between the timing of vegetative and reproductive phases in these populations. Ear and plant height decreased significantly in both directions of selection while no significant difference was observed for last node with adventitious roots.

The heterochronic effects due to selection that were observed in this study suggest that a faster transition to the adult-vegetative phase is associated with increased RPR in MoSSS. Physiological differences between vegetative phases may contribute to variation in stalk strength. We plan to replicate this experiment in 1996 in order to confirm these results. Further investigations are also being initiated in order to determine the relationship between heterochrony and agronomic traits in other populations.

Table 1. Developmental trait means in MoSSS divergently selected for rind penetrometer resistance.

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