QTL controlling root and shoot traits of maize seedlings under cold stress.

The improvement of early vigour is crucial for the adaptation of maize (Zea mays L.) to the climatic conditions of central Europe and the northern Mediterranean, where early sowing is an important strategy for avoiding the effect of summer drought. The objectives of this study were to identify quantitative trait loci (QTL) controlling cold-related traits and to investigate the relationships among them. A set of 168 F2:4 families of the Lo964 x Lo1016 cross was grown in a sand-vermiculite substrate at 15/13 degrees C (day/night) until the one-leaf stage. Twenty QTL were identified for the four shoot and two seed traits examined. Analysis of root weight and digital measurements of the length and diameter of primary and seminal roots led to the identification of 40 QTL. The operating efficiency of photosystem II (PhiPSII) was related to seedling dry weight at both the phenotypic and genetic level (r = 0.46, two matching loci, respectively) but was not related to root traits. Cluster analysis and QTL association revealed that the different root traits were largely independently inherited and that root lengths and diameters were mostly negatively correlated. The major QTL for root traits detected in an earlier study in hydroponics were confirmed in this study. The length of the primary lateral roots was negatively associated with the germination index (r = -0.38, two matching loci). Therefore, we found a large number of independently inherited loci suitable for the improvement of early seedling growth through better seed vigour and/or a higher rate of photosynthesis.

Haplo-diploid gene expression and pollen selection for tolerance to acetochlor in maize.

The objectives of this research were to determine if genes controlling the reaction to the herbicide acetochlor in maize (Zea mays L.) are active during both the haploid and the diploid phases of the life cycle and if pollen selection can be utilized for improving sporophytic resistance. Pollen of eight inbred lines, previously characterized through sporophytic analysis for the level of tolerance to acetochlor, showed a differential reaction to the herbicide forin vitro tube length; moreover, such pollen reactions proved to be significantly correlated (r =0.786(*),df=6) with those of the sporophytes producing the pollen. Pollen analysis of two inbred lines (i.e. Mo17, tolerant, and B79, susceptible) and their single cross showed that thein vitro pollen-tube length reaction of the hybrid was intermediate between those of two parents. An experiment on pollen selection was then performed by growing tassels of Mo17xB79 in the presence of the herbicide. Pollen obtained from treated tassels showed a greater tolerance to acetochlor, assessed asin vitro tube length reaction, than pollen obtained from control tassels. Moreover, the backcross [B79 (Mo17xB79)] sporophytic population obtained using pollen from the treated tassels was more tolerant (as indicated by the fresh weight of plants grown in the presence of the herbicide) than was the control backcross population. The two populations did not differ when grown without the herbicide. These findings indicate that genes controlling the reaction to acetochlor in maize have haplodiploid expression; consequently, pollen selection can be applied for improving plant tolerance.

Sporophytic response to pollen selection for Alachlor tolerance in maize.

In order to assess the efficiency of male gametophytic selection (MGS) for crop improvement, pollen selection for tolerance to herbicide was applied in maize. The experiment was designed to test the parallel reactivity to Alachlor of pollen and plants grown in controlled conditions or in the field, the response to pollen selection in the sporophytic progeny, the response to a second cycle of MGS, and the transmission of the selected trait to the following generations. The results demonstrated that pollen assay can be used to predict Alachlor tolerance under field conditions and to monitor the response to selection. A positive response to selection applied to pollen in the sporophytic progeny was obtained in diverse genetic backgrounds, indicating that the technique can be generally included in standard breeding programs; the analysis of the data produced in a second selection cycle indicated that the selected trait is maintained in the next generation.

Comparison between responses to gametophytic and sporophytic recurrent selection in maize (Zea mays L.).

In order to evaluate the response at both the gametophytic and sporophytic level of a selection based on the pollen competitive ability and to compare its effects with those obtainable from a conventional sporophytic procedure, three recurrent selection plans were developed in maize starting from the same F2 population. Two gametophytic recurrent selection procedures at high (GSH) and low (GSL) selection intensity were performed by utilizing, to advance the populations, kernels taken from the base (GSH) or apex (GSL) of ears obtained from pair-crosses of randomly chosen plants. The third scheme was a sporophytic full-sib recurrent selection procedure (SS); the only selection criterion was the machine-harvestable grain yield of the families. In a sixyear period of selection, six cycles of both GSH and GSL and three cycles of SS were performed. The source and the selected populations (16 entries) were tested for pollen performance and for sporophytic traits. The selection cycles advanced through GSH showed a progressive increase, as compared to GSL, in pollen tube length measured at 4 h of in vitro culture. The SS cycles response was intermediate at 4 h whereas at 2 h it exceeded both GSH and GSL. A slight decrease in pollen diameter was evidenced in populations advanced with GSL procedure. The SS selection caused a marked increase for grain yield, lateness, leaves per plant and plant height. No response was shown by gametophytic selection for grain yield. The GSH procedure, however, led to an increase in kernel weight and to a decrease in kernel moisture, leaf number and plant height, as compared to GSL. Though gametophytic selection showed limited effects on sporophytic traits, it can be considered an efficient tool to supplement conventional sporophytic selection.