The evaluation of an 8 × 8 diallel cross revealed that the mean length of the growing period was the same for inbred lines and their hybrids. However, the vegetative phase was 7-8% shorter and the generative phase 7-8% longer for the hybrids than for the inbred lines. The generative phase of the growing period, unlike the vegetative phase, proved to be extremely variable. Under dry conditions the length of the generative phase was negatively correlated with the length of the vegetative phase. The later a genotype flowered the less time remained for grain filling, due to the stress which curtailed the growing period. The shorter generative phase of late-flowering hybrids was the reason for the weakening of the growing period-yield correlation under dry conditions. This vulnerability of late-flowering hybrids makes selection for stress tolerance particularly important.
An improvement in the early spring cold tolerance of maize would allow it to be grown in more northern areas with a cooler climate, while on traditional maize-growing areas the profitability of maize production could be improved by earlier sowing, leading to a reduction in transportation and drying costs. The cold tolerance of crosses between inbred lines and sister line crosses belonging to three related groups that combine well with each other (BSSS, Iodent, Lancaster) was tested in the Martonvásár phytotron. The results confirmed those of earlier experiments and led to the following new conclusions: - the average emergence time of the tested Iodent inbred lines was longer than that of the BSSS and Lancaster groups, - all three groups contained inbred lines with significantly earlier emergence than the others, - the average emergence percentage and individual shoot dry matter production in the Iodent group were also lower than in the other two groups, - a close negative correlation (r = -0.70) was found between the number of days to emergence and the individual dry shoot mass. The results were used to select inbred lines and sister line crosses with various genetic backgrounds that could be used in crosses aimed at improving the resistance of hybrids to cold stress in early spring.